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OK OK, enough with the definitions!

Here is the simple answer. There are two main things that flow along wires:

  • Electric Charge
  • Electric Energy
There are several other things that flow as well, but to keep it simple, we'll ignore them.

Because there are *two* things flowing, we cannot call them both by the name "electricity." And for this reason, we cannot ask "what is electricity?" (Which one are we asking about?!) So, instead we have to ask more specific questions, like these below:

  1. What is the stuff that flows through a light bulb and comes back out again through the other wire?
  2. What is the stuff that flows into a light bulb and gets changed entirely into light and heat?
The answer to question #1 is ELECTRIC CHARGE. Charge is a "stuff" that flows through lightbulbs, and it flows very slow around a circuit. Normally no charge is lost during the operation of a circuit, and no charge is gained. It just goes around and around, like a rubber belt or a steel flywheel. Also, charge flows very slowly, and it can even stop flowing and just sit there inside the wires. In an AC circuit, charge does not flow forwards at all, instead it sits in one place and wiggles forwards and back.

The answer to question #2 is ELECTRICAL ENERGY. It's also called "electromagnetic energy". This energy is also like a "stuff" and it can flow from place to place. It always flows very fast; almost at the speed of light. It can be gained and lost from circuits, such as when a light bulb changes the flow of electrical energy into a flow of light and heat. Also, this energy flows entirely outside the copper wires, and it's made of photons (made of EM fields only.)

Here is a list of different behaviors of these two kinds of "stuff":

Flows very slowly, and can even stop. ----- Always flows incredibly fast, almost at the speed of light.
The flow is called "electric current," measured in Amps. ----- The flow is called "electric power," measured in Watts.
Flows through light bulbs --- Consumed by light bulbs (and converted into light)
In AC cables, it wiggles back and forth --- In AC cables, it flows continuously forwards
Supplied by metals (and by all other conductors) --- Supplied by generators, batteries, etc.
It's a component of matter --- A form of energy
Doesn't usually leave a circuit. --- A "Source" injects it into a circuit, while a "load" removes it again.
Composed of movable charges from conductor atoms --- Composed of electromagnetic fields
Electrons and protons are particles of CHARGE --- Photons are particles of E.M. energy
Flows inside of wires --- Flows in the space adjacent to wires
Generators pump it through themselves --- Generators create it
Circular flow. It flows around and around the circuit, and never leaves it. --- One-way flow, from a "source" to a "load".
VISIBLE: it is the silvery part of a metal --- INVISIBLE: the EM energy can only be seen if you use iron filings, etc.
Measured in units called Coulombs --- Measured in units called Joules
Occurs naturally --- Produced and sold by electric companies
Scientists of old called it "electricity." --- Today, electric companies call it "electricity."


Perhaps some pictures would help? Here are two diagrams below. More can be found at Where does the Energy flow?


CHARGE flows in a closed circle.
The flow is measured in Amperes
It flows inside the wires.
It's provided by the metal.
Batteries pump it through themselves.
None is created, none is ever lost.


ENERGY flows from the battery to the resistor.
The flow is measured in Watts
It flows in the space outside the wires.
It's made of invisible fields.
It's the same as light and radio, but lower in frequency.
Batteries create it. Electric heaters consume it.

Electric charge flows along wires. So does electromagnetic energy. If you ask "what is electricity", the answer is which one?

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How do we make electricity? This question is impossible to answer, since the word "electricity" has no clear meaning.

OK, how about this. I'll answer the question, but I'll use the scientific definition for the word "electricity" employed by Faraday, Einstein, Maxwell, etc. You probably won't like this, since most K-12 textbooks define Electricity very differently than scientists do. My answer is going to sound weird. Scientists say that electricity is the quantity of electric charge. Grade-school textbooks disagree; these textbooks instead define electricity as the quantity of electrical energy. But charge and energy are two completely different things! They're as different as air versus sound, or as steel drive shafts versus HP-hours. For a list of the many differences between electric charge and electric energy, see above.)

OK, forward to the answer!

"Electricity" means charge. Electricity is a fundamental property of matter, so in order to create electricity, we have to create matter. The positive and negative charges of electricity are permanently attached to the electrons and protons in atoms. To make electricity we'd have to create protons, or create electrons! There is no easy way to make electric charge out of thin air. It's not impossible though. If you have a gigantic particle accelerator at a physics laboratory then you can create new charged particles. The same thing happens naturally in certain radioactive materials, and when cosmic rays from space strike atoms down here on earth. But other than that, it's not possible to "generate" any electricity.

If a textbook says that electric generators make electricity, that textbook is using the word "electricity" in an unscientific way.

  • Generators don't make any electricity
  • Batteries don't make any electricity
  • Solar cells don't make any electricity
  • Fuel cells don't make any electricity
  • Rubbing fur on plastic doesn't make any electricity
  • VandeGraaff machines don't make any electricity
If all of the above devices don't make electricity, then what DO they do? Simple. All of them are electricity pumps. As with any pump, the pump isn't a source of liquid, the pump merely moves the liquid. (Your heart doesn't create blood, it only pumps it in a circuit; a complete circle.) An electric generator doesn't supply electricity, it only pumps it. The movable electricity was already in the wires, and the generators (or batteries, etc.) just pump it. But how did the electricity get into the wires? I answer that question over here: WHAT DOES 'CONDUCTOR' REALLY MEAN? But breifly, any "conductor" is a substance which contains an enormous amount of movable electricity, sort of like a pre-filled water pipe.

So, when the electric company says that they're selling electricity, what's really going on? Simple: they're using the unscientific definition of the word "electricity." They really don't sell any electricity. Instead they sell a pumping service. Instead they're just pumping electricity back and forth inside the wires. That's what AC "alternating current" means. The electricity just sits in the wires and wiggles 60 times per second. The electric company sells a pumping service, and you can use their service to run motors and heaters and light bulbs. They sell energy, and then send the energy to you along some long columns of electrons, but they don't sell you any of the electrons. The electrons don't even really flow at all, they just vibrate. It's still like a drive-belt, but one where the belt gets cranked back and forth instead of rotating constantly in one direction.

Is this all too confusing? Maybe you'd like the answer to a different question: "HOW CAN WE MAKE ELECTRIC CURRENT?" See below.

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All conductors contain some movable charges, some movable "electricity." We never have to make electricity, since electricity is already there. We just have to move it somehow.

So how can we move it? Exaclty how can we pump the "electricity" and create some electrical currents? Brief answer: create voltage. Voltage is something like electrical pressure. To make a conductor's charges start moving, just apply some voltage-pressure across that conductor.

There are three common ways to create voltages which can push electric charges along:

  1. Move a magnet past a conductor.
  2. Place two different conductors into some salt water.
  3. Touch two different conductors together, then shine some light on them.
It's also a list of the three common kinds of electrical power supply:
  1. ELECTRIC COMPANY GENERATOR: Move a magnet past a conductor.
  2. BATTERY: Place two different conductors into some salt water.
  3. SOLAR CELL: Touch two different conductors together, then shine light on them.
There are more than just three ways to move charges, but the other ways are more exotic and not often used for powering everyday products.
4.   ANTENNA: Shine some radio waves on a short metal wire.

5.   PIEZOELECTRIC CRYSTAL: Place a special crystal between two metal plates, then squeeze or bend the crystal.

6.   THERMOCOUPLE OR T-E MODULE: Touch two different conductors together, then heat them.

7.  ELECTROSTATIC GENERATOR: Touch two different insulators together, then pull them apart again.

8.  THERMIONIC CELL: Place two metal plates in a vacuum, heat one plate white hot so it spews out electrons.

9.  NUCLEAR ELECTROSTATIC BATTERY: Collect the charged alpha or beta particles emitted by a radioactive material.

10. PHOTOSYNTHESIS SOLAR CELL: Place some nano-sized biological proton pumps on a membrane, shine light on them.

11. TOLMAN DEVICE: choose a conductor which has fixed positive charges and movable negative charges, then vibrate the conductor

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In an AC system, the wires are filled with vibrating charges. In a DC system, the wires are also full of charges, but the charges flow constantly forward, like a rubber drive-belt. (And when everything is turned off, the wires are still full of charges, but they aren't flowing.)

Here is an analogy for understanding AC and DC. Get a bicycle wheel. Fill the wheel with mechanical energy by spinning it fast. Now put your finger against the spinning tire. The tire slows down, and your finger gets hot! The rubber tire acts like the charge inside the wires of an electric circuit. It moves in a single direction, and that's what "Direct current" means. DC appears whenever the invisible belt inside the wire circuit is rotating smoothly.

OK, now take the same bicycle wheel and have a friend start turning it back and forth, back and forth. Have them do this very fast, so the "turning" is more like a wiggling. Now put your thumb on the tire so the tire rubs back and forth upon your skin. Your thumb gets hot! You have just demonstrated "alternating current."

In both of the above examples, your thumb represents an electric heater, The rubber bicycle tire represents the charges flowing inside the wires of an electric circuit. We can pump them in a single direction, and this creates "DC". Or we can use a different kind of "pump", and force them to all move back and forth. This is "AC".

One last thing. It is very important that you clearly understand that batteries and generators never create any flowing charges. All wires are full of charges, all the time. All metals are full of movable charges. So are all conductors, including battery acid, human flesh, glowing plasma, etc. Batteries and generators are "electricity pumps", but they don't create the stuff that they pump. A circle of wires contains something like an "invisible bicycle wheel," and if you push its charges along, then all the charges in the entire circle would move forward, just like the solid rubber of a drive belt. We can only create a flow, an electric current, if the charges are already there. Fortunately, wires are full of movable charges. They are like pipes which are always pre-filled with water.

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In DC circuits and in 60Hz AC circuits, the current exists all through the entire wire. The charge doesn't flow only on the surface. (If it did, then we could replace all our expensive copper wires with cheap plastic... just give the plastic a very thin copper coating.)

But the question brings up some important ideas. For example, when we place an electrostatic charge on a wire, the charge spreads out and occupies only the surface of the metal. It does not go inside. But this makes no sense! After all, an electric current is a charge flow. If charge exists only on the surface, how can electric current be deep inside the metal? Yet the currents really are deep inside, while electrostatic charge appears only on the surface.

Here's the solution... really it's only the *excess* charge which exists on the surface of the conductor, while the charge itself, the electrons and the protons, they exist all throughout the metal. Remember that metal wires are already made of charge; they contain a sea of movable electrons. This is always true, even when the metal is "uncharged." Heh, metal with no charge is always full of charge! In other words, metals are always full of "uncharged charge" because every movable negative electron is near a positive proton, and the opposite charges cancel out. Yet still the "electron sea" can flow along through the metal as if electrons were a kind of a liquid. The liquid is made of charge, but it's cancelled-out charge; it's "uncharged" charge. This electric flow is not on the surface.

But suppose we give the wires some excess positive charge by removing some electrons. This "excess charge" will migrate almost instantly to the surface of the metal. It's all very confusing, no? The confusion occurs because the word "charge" has two separate meanings. It means "a glob of charged particles." Copper is full of movable electrons, so it is full of "charge." But Charge also means net-charge, or negatives subtracted from positives. Inside copper, the number of electrons and protons are equal, so copper contains no "charge" at all. Yet copper is full of charge all the time, charges which can be pumped along by generators and batteries. It's all screwy! The twisted terminology misleads beginners and causes all kinds of misconceptions. See my stuff about the word 'charge.' And, these misconceptions make people argue over whether electric currents are deep inside wires or only on the surface. Answer: currents are deep inside, yet wires may or may not have a "surface charge," and this causes confusion.

To make matters even more confusing, there is another phenomenon here called...


The skin effect causes electric currents to avoid the middle of wires and only appear on the surface. (GAH!!!!!!) But fortunately the Skin Effect only applies to AC. Also, skin effect is mostly significant for frequencies far higher than the 60Hz of household AC circuits. It's usually OK to ignore the Skin Effect unless you're involved with audio cables, antennas and transmitters, electromagnetism theory, pulses and lightning strikes, etc.

The skin effect occurs because metals act as electromagnetic shields, and because electrical energy always travels as electromagnetic (EM) fields across circuits. When a generator sends electrical energy to your home, the energy travels as EM fields surrounding the wires, and this flowing energy is solidly coupled to the electrons and protons in the metal wires. (Most people assume that electrical energy travels inside the wires. Not so.)

When pulses of electrical energy travel along a wire, they produce an excess charge on the surface of a wire, and they cause an electric current inside the wire. But because the metal acts like an EM shield, at first the path for electric current only exists on the surface. As the millionths of seconds pass by, more and more electric current appears deep inside the wire. Finally after a fraction of a second the current is everywhere inside the wire. But what if we're dealing with Alternating Current? Then the process has to re-start for every pulse of current. If the frequency of the AC is low, then the current path has plenty of time to migrate everywhere inside the wire. But if the wire is very thick (many cm across,) or if the frequency is very high, then the current-path never migrates very far from the surface before it has to reverse and start over.

Because of the Skin Effect, we can save money in high-frequency circuits by replacing the expensive solid cables with cheaper hollow pipes. This mostly applies to high-power radio transmitters. And with UHF radio and microwave circuits, the "skin of current" is so thin that we can give the copper conductors a plating of silver, and the entire current will exist only in the high-conductivity silver... as if we were using all-silver conductors. (In many circuits it would be best to use silver wires rather than copper, but that stuff is too damn expensive.)

The Skin Effect also makes people argue over whether the currents are inside the wires, or only on the surface. Answer: for DC and 60HZ AC circuits, the skin effect can almost always be ignored. But the higher the frequency, and the thicker the conductor, the worse the Skin Effect becomes.

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This question has an easy answer: the lights turn on instantly because wires are already packed full of movable electrons. So if the battery or generator tries to pull some electrons out of one end of a wire, it has to suck all the electrons forward into the battery, and this creates a current in the entire circuit.

Or, imagine a drive belt with two pulleys. When you turn one pulley, the whole belt moves instantly, and the distant pulley turns too. Yet the belt itself didn't move very fast. The electrons inside the wires are like the circular drive belt.

Here are other similar questions:

  • When I pull on a chain, why does the far end of the chain start moving instantly?
  • When I push on the end of a long stick, why does the other end of the stick suddenly move?
  • When I blow a puff of air into a long hollow tube, why does air instantly come out of the far end?
  • When I push on the rim of a bicycle wheel, why does the whole wheel start moving?
  • When I turn the roller at one end of a conveyor belt, why does the roller at the other end move too?
  • When I talk, the air just wiggles back and forth. How can sound fly in just ONE direction, outwards from my mouth?
  • When I turn on a garden hose, and the hose was already full of water, why does water squirt out of the far end instantly?
See what's happening? It's waves. If you pull on a chain, the chain moves towards you, yet something else is moving away from you very quickly. For chains, this "something" is mechanical energy. The first link in the chain pulls on the second link, which pulls on the third link, etc. Each link moves towards your hand, yet each link delivers energy to a link farther out. Each link moves slowly, but the "wave" moves very fast. This wave is like the electrical energy in a circuit. The links in the metal chain are like the electrons inside a wire. Or in other words, electrical energy is the waves, and electricity is the medium through which the waves flow.

There's a big problem here. The word "electricity" is the problem. Science books in elementary school correctly teach us that electrons are particles of electricity, and that electric current is a flow of electricity. In other words, they teach that electricity is like the metal part of that chain we yanked upon. Or in a pipe full of tennis balls, the electricity is the balls. But then the books contradict themselves... they also tell us that electricity is... a form of energy that travels almost instantly along the wires! WHAT?! In other words, they tell us that electricity is supposed to be the electrons themselves, and also electricity is supposed to be the wave that moved along the chain of electrons? Well, which is it? If "electricity" is the wave, it can't be the medium, it cannot be electrons in the wire.

The books are wrong. They're screwed up. Their authors don't understand the difference between a wave and its medium. In particular, they don't understand charge versus energy. They don't grasp basic electricity at all. They teach that electricity is like air flowing inside a tube, but they also teach us that electricity is like sound waves in a tube. But ... sound is not air. No wonder we don't understand electricity. Yet these authors are being paid to be the experts that our teachers rely upon. The end result is that our teachers don't understand electricity at all, and it's because they trust grade-school textbooks which are wrong.

I suspect that nobody wants to fix the books, since all these grade school science books have the same mistake. To fix the error, first the K-6 book publishers would have to be honest and take responsibility for such a huge problem. All the teachers would have to admit that they're wrong. This hasn't happened yet. Professional scientists have been complaining about this same problem at least since the 1960s, and still it hasn't happened yet. But the internet lets us expose the problem for all to see.


The plague of errors in K-6 grade textbooks

Typical textbook errors about electricity

A huge source of misunderstanding: the "freight cars" analogy

What *IS* Electricity?!

How fast is an electric current?

Central organization battling the plague of errors

Here's a way to understand how electric circuits work. Get a long chain and hook its ends together to form a loop. Wrap this chain around two separate pulleys so the chain is like a conveyor belt. Now if you turn one pulley, what happens? The other pulley turns at almost exactly the same time.

The chain is like the electrons inside a wire. The chain flows slowly in a circle. That's how electrons flow too. However, energy flows very fast. When you turn one pulley, the links of the chain yank on their neighbors, and waves of energy flow down both halves of the chain. (Both halves: a wave of "push" on one side, and a wave of "yank" on the other.) The distant pulley turns almost instantly. And, (Ta Dah!) the first pulley is like a DC generator, while the distant pulley is like a DC motor. The circle of chain is like an electric circuit. The links of the chain are like the electrons inside a wire.

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Yes, DC batteries are fairly safe, but the wires within AC wall outlets are not. However, this has little to do with AC versus DC. Electric wall outlets would be dangerous even if they were DC. This danger is caused by two main things:

  1. high voltage
  2. an ability to pump a large electric current

In the case of wall outlets versus batteries, it's the voltage of the power supply that makes the difference.

Electric currents cause harm when the charges in your body are forced to flow. Yet both batteries and wall outlets can pump a large electric current. But it's not their current-making ability that causes electrocution. Flashlight batteries can put out several amperes, yet batteries are safe because human skin is a relatively bad conductor. It takes a fair amount of electric "pressure" (or voltage) in order to force the charges within your body to start flowing. Touch both terminals of a D-cell, and the electric current in your skin will be so tiny that you can't feel anything. On the other hand, metal wires aren't like skin, and it only takes a tiny voltage to pump electric charge through a flashlight bulb. Because the voltage of a D-cell is very low, it can only create large currents in wires and in light bulbs, but not in people.

OK, if 1.5 volts from batteries is safe, then what level of voltage is "dangerous?" The answer:

it varies from person to person, but serious danger only appears when the voltage is higher than about 40 volts.
The voltage of a typical battery is far below the 40 volts needed to electrocute you. AC wall outlets are 120V, which is far higher than the 40-volt threshold. 120 volts can force a large electric current through your skin, and therefore wall outlets are dangerous. The "AC" is not the problem, since an AC 12-volt power supply (such as the type used with laptop computers) is not dangerous, even though it is AC. The 12v computer supply DOES have the ability to produce large currents in wires, but its voltage is too low, and it can't produce a large current in a human body because the skin is too resistive.

Humans are electrically protected by their skin. Here's a disgusting thought: remove your skin, and even a battery becomes a danger! If you have a big cut in your chest, don't go sticking a 9-volt battery into it. If you have huge cuts on your hands, then don't grab the terminals of a car battery. It could stop your heart! (I guess it's fortunate that most people don't stick electric wires into large open chest wounds. Yeesh!) It's especially dangerous when the path for current is through your heart. If you have a big open wound on both your hands, don't grab the terminals of a power supply, because the path for charges would lead into one wound, through your arm, through your chest, then out through the other wound and back to the battery.

Flowing charge inside your body is dangerous, but it takes a significant voltage to create the charge-flow. A flashlight battery is rarely dangerous because the 1.5 volts can't create a large current in your skin (or in your heart.) On the other hand, high voltage by itself is not dangerous. For example, if you slide across a car seat and then climb out of the car, 20,000 volts can appear between your body and the car! Touch the car, and you feel a painful spark, but you certainly aren't in danger of dying. High voltage was present, but there weren't any continuous electric currents. You can scuff your shoes on the rug and zap doorknobs all day with little harmful effect, even though the voltage occasionally approaches 10,000 volts. Everyday "static" sparks are not very dangerous, since the high voltage instantly vanishes when the spark occurs, and it cannot produce a large, continuing flow of charge through your body. To be dangerous, an electrical energy source needs to be above 40 volts so it can get through your skin. Also the energy source needs to be able to supply a large current for a long time (for at least a few seconds.)

OK, what about AC versus DC? What if the battery and the wall outlet both were 120 volts? Would one be safer than the other? Both can supply a large current, and both have dangerously high voltage. If we compare an AC high-voltage power supply with a DC supply of identical characteristics, here's one answer I've heard:

All else being equal, AC is somewhat more dangerous than DC because AC has a slightly greater effect upon your heart.
If an AC or a DC 120-volt power supply should shock you, and if the path for current should go across your chest, then the AC has a greater chance of triggering fibrillation and stopping your heart. Make no mistake, the 120V DC supply is nearly as painful and nearly as dangerous. But if everything else is equal, a 60Hz AC high voltage cable is slightly more dangerous than a DC high voltage cable as far as your heart is concerned.

Another interesting tidbit: Very high voltage power supplies can actually be less dangerous than the medium-high voltage used in wall outlets. By "very high", I mean voltages well over 500 volts. High voltage can be less dangerous because high voltage can act as a natural heart-defibrillator. It re-starts your heart at the same time as it stops your heart. High voltage also tends to create very high currents, which force your arm or leg muscles to contract, which can throw your body away from the live conductors. If given the choice, I might prefer to touch a 1,000 volt wire than a 120 volt wire. With the 120 volts, my hands could latch onto the wire and I wouldn't be able to let go. With the 1,000 volt wire there would be a big flash and a loud bang, and I could be thrown across the room. (The energy didn't throw me, instead the current made my arm and leg muscles do the work.)

On the other hand, very high voltage has its down side. It can rapidly heat flesh and cause internal burns, whereas medium-high voltage would take much longer to cause this sort of 'cooking.' In the previous paragraph, I might receive severe burns from touching that 1,000-volt wire, and maybe loose a finger or hand, but I'd still be alive. (But if I grabbed tightly to 1000 volts and couldn't let go, I'd quickly be roasted into charcoal. No fun at all!)


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I remember arguing about this with other kids in elementary school. My books and teachers were no help in answering it. Maybe this mystery is one of things that attracted me to electronics in the first place.

So, if I answer your question and destroy the mystery, will you lose your fascination with this field of science? (grin!)

People are harmed by electric current mostly because the current can stop your heart. High current can also cook your body or cause lethal chemical changes in your muscles. But human skin is a poor conductor. It takes a fairly high voltage in order to push a fast flow of charges through a human body. Voltage is like a "push". Voltage causes current. Voltage alone cannot hurt you. However without high voltage, electrocution could not occur. The voltage is the "pressure" that causes charges in your body to flow along, and it takes more than about 40 volts in order to push a big enough current through your body to severely shock you.

High current is never dangerous as long as it remains contained inside a wire. In order to cause problems, the path of the charge-flow must go through your body and not just through a wire. A one-ampere current can kill you, but suppose that 1-ampere current is inside a 3-volt flashlight circuit? You can grab the bare flashlight wires without danger, and the large current will stay within the metal. Three volts is too weak to push a dangerous level of current through your skin. If the voltage of the flashlight batteries were 120 volts, things would be different, and there might be a dangerous current in your body if you grabbed the bare wires. (Still, you'd have to grab them in such a way that your body became part of the circuit.)

So, if a power supply is rated in volts and amps, which one is the danger? both. In order to be dangerous, the power supply voltage must be higher than 40 volts, and the current rating must be higher than about ten milliamps (1/100 ampere.) At a much lower current than this, even a high voltage power supply cannot electrocute you. And if the power supply voltage is well below 40V, it's not dangerous even if the current rating is very high.

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Wires are always full of movable electrons (all metals are.) The electrons act something like a liquid or fluid: they act like a substance. Electrical energy is less like a substance. Instead the energy is waves which travel through this "electric fluid" or "charge-substance" within the wires.

This topic can be confusing because some books tell us that the electrons are the electrical energy. Or perhaps they'll say that a current is a flow of energy. Those books are simply wrong.

Here are some similar questions which might help to clarify things:

  1. When I pull on a long cable, why does the other end move at the same time?
  2. When I push upon a stick, why does the other end move too?
  3. When I pour water into a bathtub, why does the water level rise everywhere at the same time?
  4. When I blow air into a balloon, why does every part of the entire balloon get larger?
  5. How can sound move fast, even though the wind moves slow?

Electrical energy can move quickly along a column of electrons inside a wire, even though the electrons themselves move slowly. All metals are always full of electrons. Wires are like pipes, but these "pipes" are always filled with "water" all the time.

If something pushes the electrons forward into one end of a wire, all of the electrons in the entire wire will try to move forwards, and energy appears at the other end *almost* instantly. It's just like pushing on the end of a stick: the whole stick moves forward, even if the stick is very very long.

If you form the wire into a circle, then the movable electron-stuff inside the wire can act like a drive-belt. If you force the electrons in one part of the circle of wire to move along, ALL the electrons must flow in a circle (just like a moving drive belt.) This is true even if the drive belt circle is miles across.

So let's get back to our original question. The question is the same as this one: "How can a drive belt move quite slowly across a pair of pulleys, yet it still delivers mechanical energy almost instantly from the first pulley to the second?"

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Why do electric outlets have three holes?


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Voltage is "Potential", and potential is not really a pressure, even though a potential-difference can "push" upon the electrical charges. Electric potential is closely associated with electric force, with pushes and pulls. But potential and force aren't the same thing.

Here's one way to imagine it. Suppose we roll a boulder up a hill. This stores potential energy, and we get the energy back if the boulder rolls back down. Electrostatic fields are like gravity, and voltage is like the height of the hill. The higher we go, the more "gravitational potential" we put into the boulder. But height is not pressure, and even when the boulder is gone, the hill and the height-difference is still there. Voltage is like altitude.

In a similar way, we need both voltage and charges before there can be any "electrical pressure." The voltage only causes a "push" when the charges are present. Voltage can appear in space, but if there are no charges, then no pushing-force or "pressure" exists. This is very different than, say, water pressure. Water can push on the surface of a submarine, but the pressure doesn't go away when there's no submarine present. With voltage, the "pressure" DOES go away, so voltage is not exactly like a physical pressure. (Specifically, voltage is distance through an electrical field, times the strength of the field.)

Also see: What is "voltage?"

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How are Watts different from Amps?

Amps and Watts are not the same, because charge is not energy. Huh? It's because Amps are a measure of the flow rate of charge, while Watts are a measure of the flow rate of energy. Amps and Watts are both flow rates, but they measure two different things which are flowing.

First the watts. Watts are a measure of energy flow, and a "watt" is just a shorthand name for "Joules of energy per second." Keep in mind that Watts themselves are not like a stuff, watts do not flow, instead the watts are a measurement of the flow-rate of something else: flowing electrical energy. Joules of electrical energy can flow along, and their flow rate is called "Watts." If you have twenty Joules of energy flowing across a circuit per second, then that's a flow of twenty Joules/second, also called twenty Watts. (Maybe it would be less confusing if we stopped using the word "watts" entirely, and just said "joules per second" all the time, and never mentioned "watts.")

Amperes are a measure of charge flow, and an "amp" is just a shorthand name for "Coulombs of charge flowing per second." Keep in mind that Amps are not like a stuff, amps do not flow, instead the amps are a measurement of the flow-rate of something else. Coulombs of charge can flow along inside of wires, and their rate of flow is called "Amps." If you have twenty Coulombs of charge flowing in a circuit per second, then that's a flow of twenty Coulombs/second, also called twenty Amps.

Another way to think about it: In power lines and in AC cords, "amps" are a wiggling flow, while "watts" are a fast one-way flow. The charge within an AC wire is 'alternating', or wiggling back and forth while sitting in place. The back-and-forth wiggling is measured in terms of amperes. On the other hand, electrical energy in an AC cord does not wiggle, and it does not sit in place. Instead it flows from the source to the load at almost the speed of light. This fast energy flow is measured in terms of Watts. Amps are the wiggle, and watts are the waves zooming along. Also see:

  1. Charge vs. Energy, two things are flowing
  2. Abbott and Costello get very confused (vid)

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What is Electric Charge?

Try these links:

The simple, very brief answer: "Charge" is the stuff that flows during an electric current.

Charge is not a form of energy, instead it is a component of all everyday matter. All atoms are made of positive charges, negative charges, and a few other things.

Copper wires are always full of large amounts of movable charge, but so are all conductive materials. Electrical conductors behave as water-filled tanks and pipes, with the water taking the place of the quantities of movable charge. Insulating materials are also made of charge, but in their case the charge is frozen in place and cannot flow around. Your body is full of movable charge in the form of sodium, chloride, and potassium ions, and whenever you experience an electric shock, it's these bits of charge which flow along through your flesh.

But take note: normally all these charge-filled materials are uncharged and electrically neutral. They are filled with charged particles, yet they have no total charge on average. They are filled with equal amounts of positives and of negatives, and the two kinds of charge end up cancelling each other out to give zero total charge. For every positive particle, there is a negative particle somewhere nearby. Yet if either the positives or the negatives should alone start flowing, that's a genuine electric current. Most electric currents are a flow of "uncharged charge," where each moving charged particle has a nearby non-moving neighbor particle with opposite charge.

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What's the difference between AC and DC?

This answer is about the letters "AC" and "DC". If you want to know about Alternating Current, see above

"AC" originally meant "Alternating Current", while D.C. meant "direct current". Over the years the meanings have changed. AC has come to mean "vibrating electrical signals." For example:

  • AC is vibration, DC is flow
  • AC is dynamics, DC is statics
  • AC is like sound, DC is like wind
  • AC is like ocean waves, DC is like rivers
  • AC moves back and forth like a piston, DC moves continuously forward, like a drive belt.

If you hear people talking about "AC voltage", you need to realize that they are not saying "alternating current voltage". Instead they are saying "vibrating voltage".

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How can we convert AC into DC, and vice versa?

To convert AC to DC, we can use an "electrical ratchet" which only allows the charges to move in one direction. These "ratchets" are called Diodes or Rectifiers. They act like a one-way valve for flowing charges in wires. To change the vibrations of AC into one-way DC, just add a diode to the circuit. Or, if you need a device which takes in AC and spits out DC, then hook four diodes together (this is called a "full wave bridge rectifier.)

Converting DC to AC is more difficult. Some sort of "electrical wiggler" is required. The circuit is not simple, and must contain transistors or other types of electronic switching. This type of device is called a "DC to AC inverter."

An older "one way ratchet" existed long before diodes were invented. This device is called the Commutator, or better known as carbon brushes on DC motors and generators. All motors and generators are actually AC devices. To turn them DC, we have to add a rotating switch which reverses the AC every time the AC tries to reverse itself. So, when the voltage tries to make a backwards flow of charge, the Commutator reverses the connections, and the pulse of current is forced to go in the same direction as before. Motor brushes are also called "synchronous mechanical rectifiers." Synchronous, because they have to switch the AC wave backwards at just the right time, i.e. stay in synch with the AC being "rectified" into DC.

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What happens during a "static" shock?

It's painful to get "zapped" by the car door. What exactly is going on?

Most of the interesting phenomenon during a static shock cannot be seen by humans: they're either invisible, or they're microscopic. First, the imbalance of surface charges on a human body are totally invisible. No matter how long you scuff your shoes on a rug, you cannot build up enough charge to change your skin color! After all, your body is already made of charge (made of protons and electrons,) and even the strongest surface charge is just a teacup in the ocean when compared to the charge which is already there.

Besides the invisible charges on your skin, the volume of space around your body becomes filled by an invisible electric field. This is where the electrical energy is stored. This field is very much like the invisible field surrounding a magnet, but in this case it's an electric field rather than a magnetic field. The field sprays outwards from your entire body surface, then the flux-lines arc downwards to meet the floor. They also bend around to meet the surface of any nearby metal objects such as a car. The field concentrates itself on the pointy parts of your body: fingers, elbows, ears, nose, the top of your head, etc. If the floor is slightly conductive, or if you've been scuffing on the carpet, then much of the field collects under your feet. The "field lines" connect with the charges on your skin surface, and the other end of these "lines" connect with opposite charges in the floor and surrounding conductors. If there weren't any imbalanced charges in those other surfaces, your charged body will create them by "induction," by pushing away alike surface-charges while attracting opposite charges.

As you reach for a metal object such as a car door, the e-field becomes concentrated at the ends of your fingers, and an intense patch of opposite surface charge begins to gather on the car near your hand. The total energy becomes slightly less as your hand approaches.

When your fingers are close enough to the door handle, a spark jumps. Or in other words, the intense electric field in the space between your fingers and the metal handle will tear the air molecules apart. First the field stretches the molecules by attracting their alike charges while repelling the unlike ones. Then finally an electron pulls loose from one molecule. This electron takes off at extremely high speed, driven by the e-field. It quickly strikes another air molecule, which liberates more electrons, which then repeat the process. It resembles a landslide, where one pebble strikes another, freeing it to strike others. This "electron avalanche" glows violet, since some electrons are recaptured by air molecules, and they emit violet light typical of ionized nitrogen/oxygen mixtures. Some of the light is ultraviolet, and this light knocks electrons off neighboring air molecules. Also, the region of space that's filled with electrons and positive ions is a conductor, a plasma, and so it distorts the flux lines of the electric field. Plasmas typically take the shape of a long filament, a "lightning leader," since the tip of the plasma filament is somewhat sharp, and it causes the e-field to concentrate there (which promotes faster creation of plasma.)

The electron avalanche and the plasma filament can start out on the car door, then reach outwards toward your fingers. Or it can start out on your fingers and leap towards the door. Or it can be triggered by dust motes in the space between the two, and then leap in both directions. Charge polarity doesn't make too much difference, and the visible "leaping" of sparks is not a motion of charges, it's not a visible current. Instead it's an outbreak of glowing plasma, and this outbreak can go in either direction.

Finally the plasma filament touches your finger and the car door. It's a conductor with a typical resistance of a few tens of ohms. This conductor explodes. It has shorted out the capacitor plates formed by your body and the metal car. The e-field in the space between you and the car then collapses inwards towards the spark. Electrical energy that was in the space near your hand is flowing inwards towards the spark. (Energy doesn't flow across the spark, instead the energy behaves like a cylinder shape that surrounds the spark and shrinks inwards.)

A huge electric current appears in the spark, and temperatures in the air (and in the dead skin surrounding your salty conductive flesh) rise to immense values. The air emits sound and bright light, while your dead skin is cooked or even vaporized by the electrical energy pouring into the spark. The pain you experience is not necessarily electrical, it's similar to having your finger poked by a white-hot needle. If you grasp a metal coin or some keys, and let the spark jump to the metal, you'll feel almost nothing. The metal prevents the burn while doing little to stop the current.

Painful finger-sparks can measure a few amperes, but they only last for a hundredth of a microsecond. The worst ones can range up to many tens of amperes, with peak energy flow up in the megawatts. But these sparks last for incredibly brief times. Your nervous system only responds on time scales of a tenth or a hundredth of a second. Your nervous system "blurs" the energy and charge flows, and it "thinks" that the wattage and current of the spark is roughly a million times weaker than it actually is. Hurts though.

For more info about the typical "human finger sparks" used by manufacturers for stress-testing new appliances, search for HBM or "Human Body Model": Google: ESD, HBM, CDM, MM

ENGINEERING VERSION BELOW, some crude rule-of-thumb figures:

A typical tiny spark, too small to see:

  • Human body capacitance to ground: 100pF, 1e-10 farads
  • Body's potential to ground: 400 volts
  • Stored energy: U = 1/2 C * V^2 = 8e-6J, eight microjoules
  • Arc resistance: R= 100 ohms
  • Discharge time constant: Tc = R * C = 100 * 1e-10 = 10 nanoseconds
  • Peak current: Ip = V / R = 400/1000 = 4 amperes
  • Peak wattage: W = U / Tc = 8e-6 / 1e-8 = 800 watts
Worst case zap: touching metal inside a truck in the winter after sliding across the long bench-seat:
  • Human body capacitance to ground: 300pF, 3e-10 farads
  • Body's potential to ground: 30,000 volts
  • Stored energy: U = 1/2 C * V^2 = .14 joules
  • Arc resistance: R= 100 ohms
  • Discharge time constant: Tc = R * C = 300 * 1e-10 = 30 nanoseconds
  • Peak current: Ip = V / R = 30000/100 = 300 amperes
  • Peak wattage: W = U / Tc = .14 / 3e-8 = 4.5e6 = 4.5 megawatts
Notice the interesting numbers? A tiny spark is 800 watts and four amps! But it only lasts for ten billionths of a second, and the total energy that flows into the spark is very low. But if you slide your butt along a truck seat during a Michigan winter, the energy is almost high enough to stop your heart (a tenth of a joule!) Also, the four megawatts peak power looks very impressive, even though it only lasts for billionths of a second and does little damage.

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What is Static Electricity?

Static electricity is NOT electricity which is static or unmoving. See Static Electricity Misconceptions

Instead, "static electricity" is more properly known either as "High Voltage" or "charge-imbalance." We could also call it "separated electricity" or "contact electrification."

Interesting things only happen whenever a large amount of positive charge is separated from a large amount of negative charge, and it doesn't matter if the charges are moving. It's the separation or imbalance which is important. The stillness or static-ness of the charge has nothing to do with it.

To learn something about separated charge, see: explaining electricity with colored plastic sheets.

Suppose you rub a balloon upon your arm. Hold it near your arm, and your arm hair stands up. You probably don't realize that it requires around 100,000 volts to make your arm hair rise like that. Rubbing balloons upon arms can easily create potential difference of 100,000 volts or even more. For more on this, see: "Static" actually means "high voltage".

Also, "Static electricity" is not the opposite of electric current. Suppose you have some "static electricity" on a wire, and suppose you use a power supply to make it flow along. What happens? You'll find that the wire still attracts lint. The wire still can cause your hair stand on end. It still creates sparks and crackling noises and purple corona discharges. In fact, all of the usual "static" effects will continue, even when the electricity starts flowing. But how can "static" electricity flow along? It's not static anymore!! True, but remember, "static" electricity is just separated opposite charges. It's not electricity which is "static." As long as the positives remain separated from the negatives, all the usual electrostatic effects will continue.

Here's another way of saying it: flowing charges are NOT the opposite of separated charges, so "static electricity" is not the opposite of electric current. Or say it like this: voltage is not the opposite of current, and we can have both "static" and current in the same circuit because the true name of "static" is Voltage.

Have you ever seen a Wimshurst machine or a VandeGraaff generator? Or a Topler-Holtz device, or any of the other electrical machines invented throughout the 1800s? Many people call these by the name "static electric generators." This isn't correct. These machines are meant to produce high voltage at low current. They are mechanically-driven voltage generators. If you study "static electricity", you are really studying voltage itself.

Elementary school textbooks teach us that there are two different kinds of electricity: "current" electricity and "static" electricity. This is wrong. These books are trying to teach a very important concept, but the authors don't understand electricity enough to explain it. The real concept is this: electricity has two main characteristics: the electric current and the electric voltage.

Whenever you mess with "static electricity," you are actually playing with pure voltage. "Static electricity" gives us some hands-on experience with basic voltage concepts. But K-6 grade textbooks say nothing about this! Instead they tell us that "static electricity" is unmoving charges. They convince us that "static" is just obsolete Ben-franklinish stuff; ideas which are only important for explaining dryer-cling and photocopiers. As a result, we all learn something about electric current, but when it comes to electric voltage we haven't the foggiest notion.

So go take a look at "What is Voltage?"

Also: What Is Static Electricity, another version.

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What voltage is considered "High Voltage?"

When someone says "high voltage," what do they mean? Is 120V high voltage, or is it low? There's no single answer, since the answer depends on the situation.

As far as human safety is concerned, "High voltage" is any voltage which can injure or kill. Some safety organizations consider 60V to be dangerous, and everything above 60V is called "high voltage." Others put the threshold at 40V. If you soak your skin with salt water and then solidly connect yourself to a DC circuit by grabbing some metal bars, you can probably injure yourself with forty volts.

There is another meaning for "high voltage:" any voltage which can cause sparks to jump through air. The tiniest sparks begin to be seen at voltages between 500V and 700V, so anything above these values can be considered as "high voltage."

There is another safety issue: at very high voltage levels you don't have to touch the wires to be electrocuted, instead a flaming electric arc can cross the air if you bring your hand too close. Significant sparks become a problem above voltages of several thousand volts. Electrical workers may consider 120V to be low voltage (and even 220V or 440V is often called low voltage,) while the many thousands of volts in outdoor power lines is far more lethal. It requires workers to use all sorts of safety procedures when dealing with live circuits. In this case, "high voltage" is something over 1000V or 2000V or so.

Finally, there is so-called "static" electricity. To create tiny but visible sparks we need at least 1000V. To attract lint and to create painful "doorknob sparks" we need more than several thousand volts. To create hissing crackling noises and purple corona discharges from sharp points we need even higher voltages. In other words, "static electricity" involves High Voltage at tens of thousands of volts. Don't forget: a small tabletop VandeGraaff machine can easily generate 50,000 volts. Rubbing a balloon on your arm-hair can do the same. The larger classroom-style VDG machines can approach one million volts: high voltage by almost any definition.

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Do light bulbs consume huge energy when first turned on?

There is a myth going around which says we should leave our lights on all the time. The myth says that, each time we suddenly turn on our lights, this will consume a huge amount of energy. But it's a myth.

You can prove this yourself. Go outside and find the electric utility meter. See the little wheel which slowly turns? OK, now go indoors and turn off everything in your house (including the furnace and water heater.) Verify that the little wheel has stopped turning.

Next, turn on a single 100-watt light bulb in your house, then use a wristwatch to time how long it takes the wheel to rotate once. This gives you a rough idea of how much energy that light bulb is using every minute. Now turn off the lamp.

Finally, have someone stand next to the lamp while you stay outside and watch the electric meter. When you yell "start," have them turn on the lamp, and at the same time start timing the little wheel. See how long it takes the wheel to make one complete revolution when the bulb has been suddenly turned on. (You timed the wheel earlier when the bulb was already running normally, not when it was suddenly turned on.)

You'll find that it doesn't matter much whether you turn on a bulb and run it for a minute or so... or whether you simply leave the bulb on for the same minute or so. The wheel in the energy meter gives about the same measurement in both instances, proving that light bulbs don't consume vast amounts of energy when first turned on.

On the other hand, incandescent light bulbs tend to burn out when first turned on. The higher initial current can oveheat a weak spot. Also, the sudden heat will expand and flex the filament, and if the filament is about to break, turning it on can break it. So, if you leave the lights on all the time, you'll pay for wasted energy... but if you turn them on and off all the time, you'll shorten their lives. Which is more expensive? (If you want to get around this problem, then install "light dimmers" in place of your wall switches. This avoids the sudden stress of turn-on, and lets your incandescent bulbs last longer.)

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Why is Static Electricity invisible?

Whenever we create "static electricity" (whenever we create a "charge-imbalance"), the imbalance of charge is very small. Compared to the amount of electric charge already inside everyday matter, the imbalance is too small to make a visible difference.

Everyday objects already contain many coulombs of cancelled-out charge in each cubic centimeter of their substance. We could create a visible change if We could add or remove a coulomb's worth of charge. However, a typical imbalance of charge is incredibly tiny. For example, rubbing a balloon upon your head involves millionths of millionths of coulombs (a decimal point with twelve zeros.) It's quite literally like a teaspoon of water added or subtracted from an ocean. Pouring a teaspoon of water into the ocean does not make the ocean look different. And if you rub a balloon on your hair, the surface of the balloon doesn't look any different.

Here's a different way to explain it. Suppose we rub some hair upon some styrofoam under low-humidity conditions. The voltage between the charged objects might be as much as 30,000 volts, and it creates a crackling sound of tiny sparks. (You can see these sparks if the room is dark.) So here's the question: what voltage would be needed before the hair or styrofoam looked different? We can figure this out. If we steal one electron from every single atom on the plastic surface, that should make a visible difference. Atoms are spaced about 0.1nM apart, so there are 100 billion per cm. If the charged spot is say sixty square cm in area, then that's 60*(100 billion)^2 = 60*10^16. Each atom lacks one electron, and the charge of one electron is 1.6*10-19 coulombs, so the surface lacks 0.096 coulomb. From this calculation we know that a capacitor with 60cm^2 plates spaced at 1cm has a value of 5pF. For capacitors, volts V equals charge Q divided by capacitance C, V=Q/C. So to create a visible difference in the plastic and hair, we need to generate .096 coul / 5e-12 = 1.9e10 or ONE POINT NINE TRILLION VOLTS?!!! That's only seven hundred thousand times higher than the voltage created by lots of rubbing with fur. So the answer is this: if we charge some fur and plastic by rubbing it, the voltage will only rise to a certain amount before corona discharges and little sparks will halt the rise. The discharges will allow the opposite charges to start rejoining each other, and when the voltage can rise no further, the amount of surface charge still remains far too low to cause a visible difference in the materials.

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Why is electricity visible in sparks, but invisible when it's inside the wires?

A spark is not electricity.

A spark is nitrogen/oxygen plasma. Plasma is related to fire. The plasma is created when some gas (some air) and some high voltage is present. High voltage causes air molecules to be torn apart, and as they hit other molecules or fall back together, they give off light. Plasma is conductive, so once it has formed between two wires, it connectes the wires together electrically, and charges can flow through it. It might seem as if "electricity" has jumped through the air. In reality, a sort of "glowing wire" has formed in the air, and this "wire" is made of plasma. Note that we cannot see the flowing charges or the flowing electrical energy. We can only see the plasma jumping between the ends of the wires.

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What's the difference between static and current?

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Why does the electric company bill us, since it takes back all of the electrons it gives us?

The electric company does NOT sell electrons. Instead, it only pumps electrons. It pumps the movable electrons which fill the wires. The electrons are provided by the atoms of the copper. You pay for a pumping service!

Also, since power lines use AC, the electrons really don't move much at all. Instead they sit in one place inside the wires and vibrate back and forth. (It's somewhat like sound: the sound waves move fast, but the air molecules just vibrate back and forth without flowing forwards.)

Imagine this: if electric motors and generators had never been invented, then the "Power Company" could use water instead. The water would be inside a long, long loop of hose, and when the "Hydricity Company" pumped the water, you could attach their hose to a water motor, and the motor would turn. The water inside the hose would serve as a long drive belt. The water would stay inside the circle of hose, and it would be pumped around the loop over and over again. And when you opened the valve, your motor would turn on instantly, even though the water might be flowing quite slowly. (When you remove the blockage, the whole loop of water starts flowing at once.)

Many years ago, before motors and generators were invented, "power companies" used leather drive belts and rotating drive shafts to send energy to their customers. This really happened, although their customers were not way out in the suburbs. Instead their customers were all in the same area, and the "power company" was just a huge steam engine in the middle of a factory. Energy was sent to all of the factory machines using long leather belts and metal drive shafts. I guess you could say that these old factories ran on "Mechanicity" instead of "Electricity". Today we still use steam engines, although they're powered by nuclear reactors as well as coal or oil. Electric wires and electric motors aren't so incredible, they are really just a way to hide the leather belts that connect all the machines to the distant steam engine!

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When electricity is sent to homes, how does it 'know' if no appliances are connected? Does it go back to the generators again?

Great question!

(And when you say 'electricity' I'll assume that you mean electrical energy.)

Whenever the electric company sends electromagnetic energy to your home, and when you don't have any appliances plugged in, something interesting occurs. The energy bounces! It reflects from the open ends of the wires and travels back to the big generators, where it's automatically used to keep them spinning. Because this occurs, the generators won't slow down much. And that means the electric company won't have to burn much fuel at all to keep the giant rotors going. But if you turn on all your lights and run all your appliances, then some of the energy stops bouncing when it gets to your house. The big generators start to slow down, so more fuel must be burned to run the steam turbines which keeps the rotors going at their original speed.

Here is another way to say the same thing:

If you unplug all of your appliances, less energy gets used.

Isn't this cool? I was fairly amazed to discover how electricity really works. I learned that the above question is not nearly as silly as most educators believe. In truth, those big electric generators can reach out through the wires and feel your appliances. The generators "know" what's connected. Whenever you plug in a light bulb, the electric company's generators feel it almost instantly. They feel the extra friction (the electrical friction, not mechanical). Your light bulb uses up some energy, and this means that some of the energy doesn't get reflected back to the generators. As a result, the generators start to slow down a bit, and more fuel must be burned in order to prevent this. By turning on a light bulb, you can cause a distant nuclear reactor to eat more U-235, or cause a coal-fired boiler to grind up a bit more coal into powder for burning.

On the other hand, when you suddenly turn off a light, you create a "dead end" in the energy system. The energy that was sent to your home starts being reflected back to the big generators, and it makes them spin a tiny bit faster. The electric company must then turn down the fires which run the steam turbines to keep the generators from speeding up. They do this quickly, and the changes in generator speed are extremely tiny.

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The filament inside a light bulb is much thinner than the wires that lead up to the bulb. The charges flow slowly in thick wires, but they must flow fast in the thin filament. Charges experience a kind of "electrical friction", and when they flow faster, more heat appears. This friction experienced by the fast charges heats up the filament.

The same kind of "friction" heats up all wires, but the charges flow slowly in thick wires, so this heating is usually not enough to even notice.

The same kind of friction heats up the wires inside of toasters and electric heaters. In that case, the heating isn't enough to make the wires glow white hot like a light bulb filament. Instead they just glow red or orange.

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The charges would rather go straight through the wire, rather than taking a detour through the bird! Bird skin is a conductor, but copper has thousands of times more conductivity. If a robot bird made of metal landed on a power line, then there would be charges flowing through the metal bird.

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Two answers: wasted energy, and motor brush wear.

AC and DC are not that different. If your electric outlets were DC. then light bulbs and electric heaters would still work fine. Many motors would still work. So what's the big deal?

DC motors require sliding brushes. Unfortunately, carbon brushes wear out. If your fridge, fans and furnace contained DC motors, you'd have to open them up about once a year to replace the worn brushes. This would be even more inconvenient than replacing light bulbs. But what if someone invented a special kind of motor which never needed new brushes? Nikola Tesla solved the problem by inventing the magnetic vortex motor (commonly known as the AC Induction motor.) These motors have no brushes to reverse the current. Instead they rotate because a magnetic vortex pulls them along. However, these motors require AC. Their operation is based on AC electrical waves. If you never want to replace motor brushes, then you need AC outlets to run all of your brushless "Tesla motors."

Second answer: Nikola Tesla discovered how to make cross-country electrical grids possible. If electric companies use AC, then there is a simple way to greatly reduce the electrical friction in every cross-country power line. Just transport the energy at low current and extremely high voltage. It's easy to change low-voltage AC into high voltage. Just use a "transformer"; a pair of electromagnet coils.

But Transformers require AC. If DC was used, then either the cross-country power lines would be too expensive (they'd have to be immensely thick cables,) or the electric generators would have to be built right in your neighborhood. With DC, cities would need thousands of small generators instead of one huge generator at a dam or nuclear plant.

But why does AC make a difference? It's because electrical energy is made of voltage and current, but only the current can waste energy by heating up the cross-country power lines. If we could convert the energy into high voltage and low current, then we could send it across hundreds of miles of thin wire, and the electrical friction of the copper metal wouldn't absorb all the energy. Unfortunately, electrical generators can't directly produce a high enough voltage. However, there is a simple device which can. It's called the AC Transformer, and it can convert low voltage electrical energy into high voltage electrical energy. At the same time, it converts high current into low current. If "transformers" are used on both ends of a long power line, then that power line can be hundreds of miles long, yet most of the electrical energy won't be absorbed by the copper. But transformers only work on AC. They can't change the voltage of DC. And so electric companies use Tesla's patents: low voltage generators with transformers and high-voltage transmission lines.

If we had some other simple way of stepping the voltage and current up and down, then maybe we could use DC instead. DC works fine for running motors, heaters, and light bulbs. But if you want to send electrical energy through very long wires, you need the AC so you can convert it into high voltage at low current. (If the current is low, then the long wires won't get hot, yet you still can send just as much energy as when the voltage is low and the current is high.)

Some people do use DC electrical outlets. Boats and campers frequently have them. People living "off the grid", using solar or hydro power, often use DC instead of AC. These people have nearby generators, so they don't have to send energy through very long power lines. Some appliances aren't compatible with both AC and DC, so anyone who has DC outlets instead of AC outlets usually has to buy an entire set of DC-only appliances.

Also, some electric companies use DC cross-country power lines. They do this because high voltage DC has less energy loss than the equivalent AC energy. (You see, high voltage DC works better, it's just very hard to create it.) Electric companies use gigantic expensive transistor devices to convert DC into AC and AC into DC. They mostly use these specialized DC high voltage systems for very long cross-country transmission lines, and also to connect between statewide AC power grids.

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Batteries are chemically-powered charge pumps. They contain "fuel" in the form of chemicals (these chemicals are usually metals in the form of metal plates.) When the chemical fuel becomes exhausted, the battery has "gone dead". No chemicals ever leave the battery, so what happens to the fuel? It turns into waste products.

If you have a rechargeable battery, then you can "recycle" the waste products. By pumping charges backwards through the battery, you force the chemical waste to turn back into fuel. This is a bit like pumping some exhaust into your car engine, pushing your car backwards, and having the tank slowly fill up with gasoline! The chemical reactions inside of rechargeable batteries are reversible, while the burning of gasoline is not. The "waste" really does turn back into "fuel" when we force charges back through the battery.

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Batteries are chemically-powered charge pumps which create voltage. The location of the actual charge pump is at the place where the liquid electrolyte is touching the metal. All batteries actually contain two charge pumps: one on the surface of each metal plate. These are called the "half-cell reaction sites."

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In other words, even a single dry cell actually contains two separate "cells," one on each plate. The liquid electrolyte connects the two cells together. These cells are wired in series, but pointed in opposite directions, so their voltages subtract. The surfaces of the metal plates act as the true energy-producing "batteries." The conductive liquid acts as a wire. It connects these two "batteries" together.

                       The location of the actual
                         | "voltage generator"
         METAL           V
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    ||||||||||||||||||||||       LIQUID       |
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               ONE OF TWO "HALF-CELLS"
So, rather than being a short circuit, the conductive liquid is part of the battery's internal "wiring," it's part of the complete circuit.

If you really wanted to "short out" the innards of a battery, you would have to somehow disrupt the thin surface layers of the battery plates. The charge-pump mechanism is inside the surface layer. If part of those layers were destroyed, this would let the charges on one side of the charge-pump go directly back to the other side without having to flow through the battery's outside terminals. (And this is one reason that batteries have a "shelf life;" it's because part of the battery plates stop acting like charge pumps, and in that case some charge does leak backwards across the pump, causing the battery to eventually go dead.)

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All conductors are full of movable charges. Conductors are like water tanks, and wires are like pipes which are pre-filled with water. In order for charges to flow continuously within wires, the charges must be within a circular path. Why? Because all wires are always packed full of charge, and it takes an immense force to make the charges leave the material. In most situations the movable charges cannot leave the metal. If you want to push more charge into one end of a wire, the charge at the other end must have some conductive material to go into. And the charges in that other material must have a place to go as well. If they don't, then no charges can flow. What to do? Just connect a wire in a circle! That way the charge within the wire is able to flow anywhere within the circle.

In a metal circle, when some charge tries to flow forward, it pushes all the charge in the circle ahead of itself. The charges move along like a solid wheel. A circle of wire contains a sort of "drive belt" made out of movable electric charges. Push the charges along in one spot, and the whole "belt" will start going.

An "open circuit" is like a stuck drive belt. An "open circuit" has a blockage made of empty space, while a "closed circuit" has a complete circular metal path with no blockages. If you break the circuit, this is the same as inserting a blockage. It's like grabbing the drive-belt so it cannot move. If one part of the belt is stopped, then the entire belt cannot move either. For electric circuits to operate, the "electron drive-belt" inside the wires must form a complete circle with no air-gaps to "put on the brakes" and halt the motion of the invisible "belt."

In order to operate, most electric devices require complete circuits. However, there are some situations where this rule is not obeyed. The most common examples are radio antennas, and in buildups of excess charge (or "static electricity.")

If conductive objects are like tanks full of electric liquid... what happens if the liquid should start sloshing back and forth? In that case the liquid does not leave the tank. In that case there are no circular flows; the flow is back and forth. This is just how radio antennas work: radio waves cause the sea of charge inside the wires to slosh back and forth. A bit of excess charge flows into the far end of the antenna wire, then it flows back out again almost instantly. Then it repeats. In other words, the "electric liquid" inside a conductor can vibrate at high frequencies, and no complete circuit is needed.

There is another situation where no complete circuits are required. If conductors are like tanks of water... what if we have TWO separate tanks? In that case we could take some water out of one tank and dump it into the other one. This is possible with electricity, but it requires a relatively enormous force to accomplish it. Thousands of volts are involved (or even tens of thousands of volts.) We can pull charges out of one conductor and dump them onto another one. Then we can touch the two conductors together, allowing the excess to vanish again. When dealing with extremely high voltage, sometimes no complete circuits are required. (for more analogies, see Red and Green Electricity

Here's a less common situation where complete circuits are not required: Tesla Coils! Tesla coils are popular science projects because they do bizarre things. And tesla coils are like "static electricity" in that they involve tens of thousands of volts. Tesla coils are also like radio antennas in that they involve high frequency and charges which slosh back and forth. As a result, Tesla coils violate the rule for complete circuits.

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This was Nikola Tesla's great invention: to use the vibrations of Alternating Current to create a rotating magnetic vortex, then let the magnetic vortex cause a hunk of metal to turn. The rotating magnetic field sweeps through the metal and drags it along. These are called "induction motors."

There's also another way to do it. Suppose we take a little battery powered motor and remove the magnets. Replace them with electromagnet coils. Connect the coils (and the motor's rotor) to AC. Now whenever the current reverses, all of the magnetic poles in the motor reverse too, and the motor still spins in the same direction. The "N" attracts the "S", but when the current reverses direction, the "S" now attracts the "N", and the motor still turns the same. If you feed AC to that motor, it will keep spinning even though the direction of current is flipping back and forth.

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In the USA, the"D" cells, "C" cells", "AA" cells, and "AAA" cells are almost the same except for the amount of chemicals that they contain. The bigger batteries just last longer. Otherwise they are exactly the same

9-volt batteries are different. If we break open a 9-volt battery, we'll find six little 1.5-volt batteries inside of it. The same is true of 6-volt lantern batteries. Open up a 6-volt battery, and you'll find four 1.5-volt batteries inside.

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"Energy" is a stuff that flows, while "power" is the rate of energy flow. If "energy" were like water, then "power" would be the gallons per second.

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They can! But you'd need lots of batteries. Single electric cells are very safe.

Everyday batteries are safe because their voltage is so low.

First, understand that electrocution is not just caused by electric current. Instead it is caused by an electric current inside your body. Currents themselves are not dangerous as long as the path for current is through a wire! Voltage is important here, but voltage is not dangerous unless it causes a current inside your body.

Human skin is electrically conductive, but it is not a good conductor. It takes about 40 volts of electrical "pressure" in order to create a dangerous electric current inside your flesh. 40 volts is the "danger voltage." Anything higher than 40V can shock you. Fortunately, most batteries are way below 40 volts (most are below 12 volts.) Batteries lack the pumping-force needed to create dangerous currents in humans. Another way to say it: our skin is insulating enough to keep us safe from batteries, but it cannot protect us against the high voltage of a 120V AC outlet.

Batteries can electrocute people if we connect a large number of batteries in series. Put eighty D-cells in series, and that gives you 120 volts DC, which can run a normal house lamp and is more than enough to kill you if you touch the wrong wires.

Batteries can electrocute people if the path of the current somehow goes through the skin and into the salty meat inside. For example, if you made some big bloody cuts in your hands, then even a 6V flashlight battery might kill you if you placed those cuts against the battery terminals.

And if people were made of metal, then even a single D-cell would be dangerous!

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I will answer this question with a question. When a circular belt is passing over two pulleys, why are two belts needed? The answer: There are not two belts! There is only one belt, and the belt forms a circular loop. It looks like there are two belts, with one of them flowing leftwards and the other one flowing right. But in truth, there is just one belt, and it is rotating.

So, why are two wires needed? The answer: There are not two wires! Instead there is only one wire, but it is connected in a circle. All metals are full of movable electrons, so when we connect a wire in a circle, we are forming a kind of "electric drive-belt" which can move inside the wire.

But household electric outlets have three prongs! Yes, but only two of them are used. The third one is only used for safety purposes. See WHY THREE PRONGS

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Voltage is sort of like electrical pressure.

A current is a flow of electric charge.

It's best to think of it like this: voltage causes electric current, just like water pressure causes water to flow.

You can have a voltage without a current: when a battery is sitting on a shelf, it is creating a voltage between its terminals, but there is no current. It's like a force without a motion. It's like a pressurized balloon, but without any leaks.

You can also have a current without a voltage: a ring of 'Superconductor' can contain a loop of flowing charge that flows inside it forever. It's like frictionless motion (with no force needed to keep it going.) It's like a flywheel which keeps spinning forever.

Voltage is associated with electrostatic fields in space. Whenever you have a voltage, you also have an electric field.

Current is associated with magnetic fields in space. Whenever you have an electric current, you also have a magnetic field.

Also see:

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Ooooo, good question! It ties in with " what is charge" and "what is electricity"

Here's the very briefest answer: Electrical energy (also called electromagnetic energy) is the stuff sold by electric companies. It is produced by electric generators, and it gets used up by lights and appliances. Electric energy is not made of electrons or other charges. Instead it is made of electric fields, and also is made of magnetic fields. (That's why it's also called "Electromagnetic Energy.") If you have a bar magnet, the invisible "stuff" that surrounds the magnet is the electrical energy. If you have a charged, fur-rubbed balloon, the invisible "stuff" that surrounds the balloon is the electrical energy. And if you have an electric circuit, the electrical energy can be found in the invisible fields that surround the wires. Electrical energy has two faces: magnetism and "electricism" (magnetic fields and electrostatic fields.)

Where do voltage and current come in? Easy: the voltage is part of electric fields, and the current is part of magnetic fields. For example, whenever the charges in a coil of wire are forced to flow along, a magnetic field appears around the coil, and energy is stored in the magnetic field. Even if the wire is straight and is not wound into a coil, there is still a magnetic field surrounding the electric current in the wire. We could almost say that electric current is the energy, since whenever a current exists, there must be a magnetic field and there must be energy present in that field. (We could almost say that, but not quite, since all the energy is sitting in the fields, and it's not moving along with the flowing charges inside the wires.)

In a similar way, voltage is profoundly connected with electric fields. Whenever we "charge" up a capacitor, energy is stored in the electrostatic field between the capacitor plates. The wires of an electric circuit can also act like capacitor plates, and energy will be stored in the voltage-fields that surround the circuit. If we have voltage, then we must have an e-field, so we must have some electrical energy present.

Whenever a battery powers a light bulb, where is the energy flowing? Does it flow inside the wires where the current is located? Nope. It flows in the space outside the wires. Here's a way to think about it:

Electrons and protons are not particles of energy (they are matter.)

Electrons and protons can store energy. Pull an electron away from a proton and you store energy in the space between them. Let the electron fall back towards the proton again, and you get the energy back.

As the electron is pulled far from the proton, does the electron change? Nope. If not, then where is the energy being stored? It's stored in the fields! When you pull the electron away from the proton, the electrostatic fields around them do change: they get much bigger. When we pull an electron far away from a proton, the fields between them will expand like an inflating balloon. Whenever we use electrons and protons to store energy, the energy is actually stored in the fields in the space around them.

When we put electrical energy into an electric circuit, do the electrons and protons of the wires become "energized?" Do they change somehow? No, only the fields surrounding them change. The fields extend outside the wires. The sored electrical energy is out there, outside the wires.

Whenever the electric company sends energy to our homes, does the energy flow inside the wires? Is the voltage and current and part of the energy? Nope. The current and the voltage are connected to the flowing energy, but all the energy flows outside of the wires where the electromagnetic fields are. It goes like this: CURRENT/INDUCTOR/M-FIELD, and VOLTAGE/CAPACITOR/E-FIELD. Because even the simplest electric circuit is like a coil and a capacitor, we have no choice but to say that the energy is stored in the fields surrounding the wires, and is not stored inside the wires.

Get an AC generator. Connect it to a long cable, and put a light bulb on the end of the cable. Crank the generator. It creates voltage and current which lights up the light bulb. At the same time, the generator emits electromagnetic fields which run along the cable and dive into the light bulb. Since the generator is AC, these fields which run along the wires are the same as electromagnetic waves. The light bulb absorbs the energy of the waves, and this lights up the bulb.

Get a radio transmitter. Connect it to a long cable, and put a light bulb on the end of the cable. Turn on the transmitter, and electromagnetic waves flow along the cable and dive into the light bulb. It lights up. Another way to say it: the radio transmitter behaves as an AC power supply which supplies voltage and current to light the bulb. Even though we're using a radio transmitter, the voltage and current in the cable is just the same as any other voltage and current. There's no real difference between the radio transmitter and any other AC generator

Take the same radio transmitter, connect it to a long cable, and put an antenna on the end of the cable. Now the radio waves come out of the transmitter, flow along the cable, and spew right out into space! The antenna lets the electromagnetic field-energy escape from the cable and fly outwards as radio waves. The electromagnetic energy is flying outwards, but no voltage or current is needed out in empty space. (The fields break loose from the antenna, and they start causing each other, with no charges or charge motion involved. Very weird.)

And finally, take the normal AC generator, connect it to a cable, then connect it to a very large antenna. What will happen when we crank the generator? The generator creates voltage and current in the cable and in the antenna. The generator also emits electromagnetic waves which move along the cable, then they hit the antenna and fly right out into space. Weeeeird! The electrical energy from the generator has escaped from the wires. (But if you know that electrical energy *is* electromagnetic fields, then it's not so weird that the energy is the same as radio waves.)

Can AC generators really make radio waves? Yep. However, in order to get the waves out into space, the antenna needs to be about the same size as the waves. At sixty cycles per second, you'd need an antenna that was many hundreds of miles long. At the turn of the century, radio pioneers actually used AC generators to create radio waves. They called these "alternators", and they ran at extremely high frequencies, 50KHz and up. Since electrical energy is electromagnetic fields, and since electromagnetic fields are the same "stuff" as radio waves, it makes sense that the energy in an electric circuit can also escape from the circuit and fly through empty space all by itself.

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Amazingly, the answer is yes. However, this can only be done with AC and not with DC. Also, the frequency of the AC must be very high, much higher than 60Hz.

If we want to send electrical energy using one wire, then rather than using a single straight wire, instead we must use a very long hollow coil (like wrapping a single wire around a very long rod.) Nikola Tesla invented this single-wire energy transmission scheme. It uses standing waves to transmit energy from one end to the other. Amazingly enough, no complete circuit is needed. The coil acts a bit like an organ-pipe, but the waves are not sound waves, they are electromagnetism. If a generator is connected in series with this coil, and a light bulb is connected in series too, then the generator can light the bulb. His invention played a big role in the early development of radio, but it was never used for commercial energy distribution.

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Why doesn't the incoming electricity run out onto the carpet or something?

Once I thought that this was a silly question. Today I realize that it is a very sensible one. It exposes some profound aspects of electricity. People who ask this question are on the right track.

Wires are full of movable charges, and when these charges are moving, we call this an "electric current." What keeps the charges inside the wires? Why don't the charges just fly out into the air?

The answer is: static electricity!

Metal wires, as well as everything else, contain an equal amount of positive and negative charge. If we try to take some charges out of the wire, the negative attracts the positive, and the charges are pulled back inside. It takes a lot of work to pull charges out of a wire, and this doesn't just happen by itself.

It takes a very strong push to push charges out of a wire ...in other words, it takes a high voltage. 120volts is far too little to do this, but it can be done with 10,000 volts. If electrical outlets used 10,000 volts instead of just 120V, charges would constantly leak out of the wires and be pushed right out into the air! If you stood too close to 10,000V wires for too long a time, you might begin to feel some "static cling" on your clothes. No lie! The charges in the wires will spew out into the air, and end up on nearby surfaces.

On the other hand, even 10,000V can't push very much charge out of the wires. For example, to remove all the free electrons from a piece of #18 lamp cord and place them in the ground a few feet away requires really high voltage: approximately 1,000,000,000,000,000,000 volts (10^18 V)

So that's why electricity remains trapped inside the wires: the electrons and protons in the metal are being pulled together quite strongly. A metal wire is like a tank of water: the water can easily swirl around, but it can't leave the tank. A metal wire is like a river in a deep canyon, and the water cannot leave the canyon because the walls of the canyon are millions of miles tall.

Notice that this means that all electric circuitry actually runs on static electricity. (If you don't believe it, read this extensive pdf article by college physics authors Chabay & Sherwood, and their MI textbook which explains circuits from a properly electrostatic viewpoint.)

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What the heck is "imaginary power" and Real Power? PF Power Factor?

No, there is no such thing as "imaginary" amperes or power, see [*2] below. Instead call it Apparent Power versus Actual Power. But even that's not completely right, since really this is all about one-way energy-flow versus "sloshing energy," where electrical energy doesn't flow in just one direction.

In normal AC systems, the electrical energy comes out of the distant generator and flows one-way, going into all of the electrical devices plugged into the power line. But sometimes this doesn't happen. If we plug a capacitor into a wall outlet, the capacitor does draw a current, but it remains ice cold. Hmmm. No watts? And if we plug in a large coil, the coil may get very slightly warm, yet it could be drawing a huge current. Watts are disappearing? What the heck!

What's happening here is that the AC generator charges up the capacitor, storing energy as e-fields in the dielectric between the capacitor plates ...but then the AC voltage reverses. The capacitor dumps all the stored energy back to the AC generator. Then it happens again, but with the voltage backwards. Over and over, the cap charges and discharges with the energy flow-direction reversing, and ideally the capacitor isn't using up any energy or getting hot.

Remember that this isn't usual behavior for AC lines. It's "alternating" current, but usually the **CHARGE** is reversing during the AC alternations, while the energy-pulses always go in one direction, from dynamo to resistor, and then out and away as heat. But capacitors don't emit any work or light or heating. They can temporarily store up energy, but then they dump it all out again. So if we plug a capacitor into the wall outlet, the average energy flow ...is ZERO. Yep, it goes in, but then it all goes back out again. We can measure energy-flow as usual by multiplying the volts times amps. But it's not normal wattage, instead the energy is sloshing back and forth without being used. This "sloshing energy" is the Apparent Power, or what some people insist on calling Imaginary Power. But it's not fake, it's just not our usual one-way energy-flow like the electric heaters use.

And yes, if we plug a coil into the AC wall outlet, it too will draw a current, but if its wire is thick enough, it won't use up any energy, it will only store it. Like with the capacitor, the coil gets "charged up" and briefly stores energy in the strong magnetic field. But then the AC reverses direction, so the field collapses and the coil dumps all its electrical energy back into the power line; back to the distant generator. Then it happens all over again, but with the polarity reversed during the second half of the AC sine wave.

To see what's actually going on here, we'd need to plug in our high-volt wall-outlet capacitor, then drag out a two-channel Oscilloscope and display the sine waves of the AC voltage and AC current. What we find is that the waves are not in synch. The current is off by 90 degrees. This makes sense, since for resistor loads like heaters and incandescent bulbs, the current goes positive when the voltage goes positive. And also, if the position of generator and resistor are swapped, then the current goes negative when the voltage goes positive. That's 180 degrees, and just means that we hooked up the meter the other way, and the energy is flowing continously leftwards, rather than continously to the right. So with the capacitors and inductors, the only weird thing they could do would be to NOT be in-phase, NOT be reverse phase, but somewhere in between. What's between 0deg and 180deg? Ninety. Positive 90deg or negative 90deg. And that's exactly what we find: with inductors the peak of current occurs after the peak of voltage; lagging by positive 90deg. And with capacitors, it's lagging by negative 90deg (or leading, not lagging.) And that's what the 90-degree stuff is all about. Perfect synch is normal, and perfect reverse-polarity 180degrees is also normal. Both mean that the electrical energy is flowing in a single direction. But the point of maximum weirdness is at 90deg phase difference between amps and volts, and that's where the capacitors and coils live.

PF POWER FACTOR? Very briefly, PF tells us the mixture between the "sloshing energy" versus the actually-used one-way energy flow. A Power Factor of 1.0 means that an AC line is supplying pure normal one-way energy. On the other hand, a PF of zero means that an AC cable contains no one-way energy-flow at all, and the energy is entirely made of reversing-direction coil/capacitor energy, where the amps and volts may still be large.

Finally: the millions of POWER-FACTOR ENERGY SAVERS all over eBay? Yes, those are scams. They're magic boxes that pretend to reduce your electric bills. A great discovery, recommeded by six moms! (These device might actually reduce your power factor. But this has no effect on your electric bill, since residential utility meters are designed to read only the one-way energy flow, and they perfectly cancel out any effects of leading/lagging currents!)

[*2] When people start talking about "imaginary" watts or current, they're really talking about mismatched phase of the AC voltage versus the AC current ...and an energy flow which reverses direction. There's nothing fake or imaginary about it, it's perfectly Real. It's a matter of constant forward energy flow, versus "sloshing" energy flow which returns to the power plant and gets sent out again. Better terms for this might be "apparent power" versus "actual power." Or less formally, "sloshing wattage" versus "one-way wattage." The word "imaginary" comes from a mathematical method of describing the phase of AC waves and the direction of the flowing energy. But if you use another math method to describe these, then the word "imaginary" has no connection at all. [The usual method: a phase-shifted sine wave can be plotted as a vector on a graph of "Complex Numbers," where the X axis is in Real numbers, and the Y axis is in Imaginaries all multiplied by the square root of negative one. That way you're avoiding use of vector math and the trig rules for sine and cosine. Instead you're writing equations in "rectangular coordinates" filled with the letter lower-case i. Or I guess lower-case j, if you're a double-E, and you want to avoid confusion with "i" meaning signal amperes.]

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