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WHAT IS VOLTAGE?

1998 William J. Beaty

[magnetic field around bar magnet, electric field around electric charges]

Of several electricity concepts, the idea of "voltage" or "electrical potential" is probably the hardest to understand.

It's also really tough to explain. It's a headache for both the student and the teacher. <GRIN!>     To understand voltage, it helps if you first understand a little about its nearest relative, magnetism.


Most of us are familiar with magnetic fields. Small magnets are surrounded with an invisible "field" which pulls upon iron, and which can attract or repel other magnets. The magnetic field can force any oblong magnetic objects (such as iron rods, or bits of iron powder) to twist and align to follow particular directions. Put a bar magnet under a piece of paper, sprinkle on some iron filings, and the filings all line up and show the general shape of the invisible field. Obtain a small compass, and you'll see the little compass pointer twists and aligns with the magnetic field of the earth. That's magnetism.

There is another type of invisible field besides magnetism. It is called the "electric field" or "electrostatic field" or "e-field." This second kind of field is much like magnetism. It's invisible, it has lines of flux, and it can attract and repel objects. However, it is not magnetism, it is something separate. It is voltage.

Most people know about magnetic fields but not about e-fields or "voltage fields." In part, this is because magnetism is explained in school, but for some reason voltage fields are hidden away under the name "static electricity." E-fields are never mentioned in beginner's science textbooks. This is odd, since voltage and "static electricity" go together. Whenever a negative charge attracts a positive charge, invisible fields of voltage must exist between the charges. Voltage causes the attraction between opposite charges; the voltage fields reach across space. In reality, "static" electricity has nothing to do with motion (or with being static.) Instead static electricity involves high voltage. Scuff across a rug, and you charge your body to several thousand volts. When you remove a wool sock from your clothes dryer, and all the fibers stand outwards, the fibers are following the invisible lines of voltage in the air. Fibers are the "iron filings" that make the voltage patterns visible. And whenever the charges within a conductor are forced to flow, they only move because they're being driven along by a voltage-field which runs through the length of the wire. Voltage causes current. Voltage causes dryer-cling, but it also causes electric currents in wires. Another way to say it: electric currents are caused by "static electricity," and "static electricity" is not necessarily static. The connection between voltage and "static" electricity is poorly explained in the books, and that's one main reason why voltage seems so complicated and mysterious.

The Simple Math Behind "Voltage"

To be a bit more specific, Voltage is a way of using numbers to describe an electric field. Electric fields or "E-fields" are measured in volts over a distance; volts per centimeter for example. A stronger e-field has more volts per centimeter than a weaker one. Voltage and e-fields are basically the same thing: if e-fields are like the slope of a mountainside, then the volts are like the various heights of each different spot on the mountain. The slope of a mountainside can make a boulder start rolling. So can the differing heights of the different points on the mountain, it's just another way to describe the same thing. "Voltage" and "e-fields" are two ways to describe the same basic concept.

When you have e-fields, you have voltage. E-fields can exist in the air, and so can voltage. Whenever you have a high voltage across a short distance, then you have strong e-fields. Whenever an e-field is attracting or repelling an object, instead we could say that the object is being driven by the voltage in the space around the object.

How High is my Voltage?

Can an object have a certain voltage? No. Why not?

Well, please tell what my distance is. What is my distance? That's a ridiculous question, because I didn't tell you my distance FROM WHAT. Voltage is a bit like length; it is a measurement made BETWEEN two things. My distance is 300ft above sea level, but simultaneously my distance is also 1cm from the floor (since I'm not barefoot,) and it's also 93 million miles from the sun. My voltage might be -250 Volts in relation to the earth, but it also might be billions of volts when compared to the moon. Volts are always measured along the flux lines of electric field, therefore voltage is always measured between two charged objects. If I start at the negative end of my flashlight battery, I can call that end "zero volts", and so the other end must be positive 1.5 volts. However, if I start at the POSITIVE end instead, then the positive battery terminal is zero volts, and the other terminal is negative 1.5 volts. Or, if I start half way between the battery terminals, then one terminal is -.75 volts, and the other terminal is +.75 volts. OK, what is the REAL voltage of the positive battery terminal? Is it actually zero, or actually +1.5, or is it +.75 volts? Nobody can say. The terminal can have several voltages at the same time. But this is no big deal, because neither can anyone tell you the battery's distance! We can easily imagine the distance between two points, and we can also imagine the voltage between two points. But single objects don't "have distance", and single objects also don't "have voltage."

Un-twisting the Terminology

You've probably heard of electromagnetic fields and electromagnetism. In the word "Electromagnetism," the term "electro" does not refer to electricity. Instead it refers... to voltage! Electromagnetism is the study of e-fields and magnetic fields: electro/magnetism. The charge flow (electric current) is intimately associated with magnetism, while the separated opposite charges are intimately associated with voltage. A flow of electromagnetic energy along a cable is composed half of electric current, and half of voltage. It is "voltagecurrent," it is electrostatic/magnetostatic, it's electro-magnetism. Electromagnetism is a two-sided coin, so what is voltage? It's one side of EM (the other side being magnetism.)

Besides not being found in elementary school science books, Voltage is also missing from our everyday language. If we have no common words to describe something, we tend to never talk about it. We have trouble even thinking about it, or believing it exists. For example, we have the word "magnetism", and most people have heard of magnetic fields. ELECTRIC fields exist too, but unfortunately "electri-cism" is not an English word. Everyone can discuss magnetism, but nobody ever talks about "electricism." Without the word "electricism," we have a tough time talking about electric fields, or about electric attraction/repulsion forces, and we tend not to realize that they are important in electric circuits. Yet there's a word we could use instead of "Electricizm." We don't have to coin some weird new term.

If magnetism is "that which involves magnetic fields", then what is "that which involves electric fields?"

Voltage!
Pick up some nails with a magnet, and that's an example of magnetism, then pick up some bits of paper with a fur-rubbed balloon, and that's an example of voltage. What are the three kinds of invisible field? Gravity, magnetism... and voltage!

Perhaps we should change the word "Electromagnetism" into "Voltagemagnetism?" (grin!)


[magnetic field 
around bar 
magnet, electric field around electric charges]
VOLTAGE SURROUNDS TWO ELECTRIC CHARGES MAGNETISM SURROUNDS A MAGNET'S POLES

Electromagnetic Duality

Voltage and magnetism form a pair of twins; they are two halves of a duality. Physicists and engineers even use the word "dual" to describe them: voltage is the "dual" of magnetism, and magnetism is the "dual" of voltage. This duality raises its head in many places in the physical sciences. One small analogy: A spinning flywheel can store energy. So can a compressed spring. In electrical physics, a superconductor ring can store energy in the form of magnetism, and a capacitor can store energy in the form of voltage. A coil of wire and a capacitor are the "duals" of each other, since one involves magnetism, and the other is based on voltage.

Voltage Energy

Voltage is intimately connected with electrical energy. So is magnetism. We can even say that electrical energy is the fundamental object of our study, while voltage and magnetism are the two faces it displays to the outside world. Another analogy: in mechanical physics, both the Kinetic energy (KE) and the Potential energy (PE) are part of matter: relative motion of an object has Kinetic Energy, and stretched or compressed objects (e.g. springs or rubber bands) have Potential Energy. In a similar way, electrical kinetic energy appears whenever positive charges flow through negative charges. We call this "electric current," and it causes magnetism. On the other hand, electrical potential energy appears whenever positive charges are yanked away to a distance from their corresponding negative charges. We call this "net electrostatic charge," and it causes voltage. Electrical KE is associated with current, and electrical PE is associated with voltage. If electrical energy is the same as Electromagnetism, then maybe we should be more sensible and name it "VoltageCurrent-ism."

Potential Energy vs. "Potential"

Voltage is also called "electrical potential."

So... is voltage a type of potential energy? Close, but not totally accurate. Confusion between voltage and potential energy is a common mistake. Think of it like this. If you roll a big boulder to the top of a hill, you have stored some potential energy. But after the boulder has rolled back down, THE HILL IS STILL THERE. The hill is like voltage: the height of the hill has "Gravitational Potential." But the hill is not *made* of Potential Energy, since we need both the hill *and* the boulder before we can create potential energy. The situation with voltage is similar. Before we can store any ELECTRICAL potential energy, we need some charges, but we also need some voltage-field through which to push our charges. The charges are like the boulder, while the voltage is like the hill (volts are like height in feet. Well, sort of...) But we wouldn't say that the Potential Energy is the boulder, or we wouldn't say the hill is the PE. In the same way, we should not say that electric charges are Potential Energy, neither should we say that voltage is Potential Energy. However, there is a close connection between them. Voltage is "electric potential" in approximately the same way that the height of a hill is connected with "gravitational potential." You can push an electron up a voltage-hill, and if you let it go it will race back down again.

Currents don't have Voltage

Voltage is not a characteristic of electric current. It's a common mistake to believe that a current "has a voltage" (and this mistake is probably associated with the 'current electricity' misconception, where people believe that 'current' is a kind of substance that flows). Voltage and current are two independent things. It is easy to create a current which lacks a voltage: just short out an electromagnet coil. It is also easy to create a voltage without a current: flashlight batteries and charged capacitors maintain their voltage even when they are sitting on a shelf. Water analogy: Think of pressurized water without a flow. That's like voltage alone. Now think of water that's coasting along; a water flow without a pressure. That's like electric current alone.

"Kinds" of Electricity?

Grade-school textbooks wrongly teach that electricity comes in two types: static electricity and current electricity. These textbooks would be much closer to the truth if they instead said this:

The two halves of Electricity are "voltage electricity" and "current electricity."

Still a bit misleading, since the meaning of the word "electricity" is not clearly defined. It would be better if they said that electrical energy has two main characteristics: voltage and current. But the above statement is not nearly as bad as the stuff they usually teach us about "static versus current."

For one thing, the static stillness of the charges is not important. For example, if we view a frozen "snapshot" of a dynamic electrical phenomenon, we'd be seeing an electrostatic situation. Current that is static? Yep, and it's because "Static" electricity is NOT electricity which is static. The screwy terminology fouls things up. Instead, "static charge" really means "separated opposite charges". We should not be surprised to learn that "static electricity" is able to flow from place to place without losing any of its characteristics. Maybe it's not "static" anymore, but it's still, ahem, Static Electricity. Meaning charge-separation. The lack of motion doesn't matter, since a separation of charge can move along. It's the IMBALANCE between opposite charges that's important, and their "static-ness" is not.

NOTE: Do you see how K-6 textbook authors could be playing a game of 'telephone?' In this "game," words are progressively distorted by errors in communication. In K-6 textbooks the science concepts become more and more distorted over the years. Authors are taught from earlier textbooks, and often they get their information directly from modern textbooks. Then they write new ones. If authors make mistakes, what will happen? Start out by saying "electromagnetism has two complimentary halves, voltage and current". Decades later we end up with books which are teaching kids something like this: "the two forms of electricity are static electricity and current electricity." Wrong. Yet we can see where the crazy stuff originally came from.

Seeing the Invisible Voltage

Magnetic fields are invisible, and so is voltage. Both can be made visible. Iron filings let us see magnetic fields. To see voltage, suspend some metal or plastic fibers in oil, or sprinkle grass seeds on a pool of glycerine. If we then expose the oil to the strong voltage-field surrounding a charged object, the fibers or grass seeds will line up and show the shape of the field. Rub a balloon on your head, hold it near the suspended fibers, and you'll "see" the lines of e-field flux.

Measuring Voltage

To measure current, we allow the magnetism around a coil of wire to deflect a compass needle. To measure voltage, we allow the "electricism" between a pair of delicately suspended metal plates to deflect one of those plates. The simplest voltmeter is called a "foil-leaf electroscope." Electroscopes are simple versions of zero-current voltmeters. find such things in books about "static electricity", when they really should be in all electronics books. A more complicated version of the foil-leaf electroscope is called a "quadrant electrometer." These two devices can measure voltage directly, without creating any electric current at all. Besides the moving capacitor plates, there are a few other ways to measure voltage too.

The Voltage of Light

Here's a strange idea: Flowing Electromagnetic energy always involves voltage. For example, if you touch the antenna of a powerful radio transmitter, you can receive an electric shock because of the high voltage at the antenna. Radio waves are electromagnetism, and the intense waves surrounding a radio transmitter's antenna will have a high voltage-field. So, radio waves can be measured in terms of voltage. Even the brightness of the light from the sun can be measured in terms of volts per meter. So can the energy which comes from the utility company's generators and flows along wires to your 120v table lamp. All of these involve electric fields (and voltage), and magnetic fields (and current.) Power lines deal with voltage, but in the same way, so do light beams and optical fibers.

Expose All Students to High Voltage! :)

"High voltage." Might you already know what that is? It's not just the dangerous devices behind the electric company fence. High voltage is also balloons rubbed upon your hair, and "static electric generators" and their very long sparks. You might be interested to know that ALL voltage creates the same effects as "High Voltage." The effects are just weaker when voltage isn't high. Understand "high voltage," and you'll understand voltage itself. High voltage devices are not just toys, they're educational: they let us experience voltage directly. If you want to understand magnetism, then play with strong electromagnet coils and strong magnets. If you want to understand voltage, then get yourself a VandeGraaff generator.

Voltage has wrongly been hidden within "static electricity" and declared to be an obsolete and useless science, important only for historical reasons. But in a certain sense, "static electricity" *IS* voltage. Static electricity is a high-voltage phenomena. If we stop teaching about "static electricity," and regard it as ancient and useless "Ben-Franklinish" stuff, then we also stop teaching about voltage. Can you see why voltage has become such a mystery? We've nearly eliminated "static electricity" from high school science classes, and so we've also throw away our basic voltage concepts.

LINKS


MISC. NOTES

Imagine a waterwheel being turned by a stream of water pouring from above. If the water is like the flowing electric charge, and the waterwheel is like an electric motor, then what is voltage? Voltage is like the height of the stream at the top of the wheel, or like its slope from the top of the wheel to the pool below. Without a height difference, there can be no water current and no work done by the waterwheel. Without a voltage difference across an electric motor, there can be no electric current and no work done by the motor.

What if a situation is changing slowly and isn't really static, yet it doesn't involve electrodynamics? No EM waves or magnetism? Ah, that's called "Pseudostatic." If it's changing, yet it's still basically electrostatic, then it's a pseudostatic situation

Voltage is like an electrical pressure or push, it can cause electric charges to flow. Or, if flowing charge is suddenly blocked, this can cause a momentary voltage to appear. But current can exist without voltage, and voltage can exist without current.

Voltage exists in space, not just on surfaces. Rub an inflated balloon on your arm hairs, then wave the balloon around so it makes the hairs stand up. You are seeing and feeling voltage in the space between the balloon and your arm. Think about a 9v battery. The 9 volts aren't on the surface of the battery terminal, they are in the space between the terminals, like the magnetic field between a north and a south pole. A 9v battery is like an "electret", the electric version of a bar magnet.

An inductor (an electromagnet coil) is an electric current device. A capacitor is an electric voltage device. If energy is stored in a shorted coil, the energy is in the surrounding magnetic field, and there must be an electric current circulating in the coil. If energy is stored in a non-shorted capacitor, the energy is all found in the voltage field between the plates. If the shorting bar is suddenly removed from the inductor, there is a loud bang, and a huge voltage briefly appears. If a short is suddenly connected to a capacitor, there is a loud bang and a huge current briefly appears. Both components can produce a violent discharge, yet the process for one is the reverse of the process for the other. Capacitor, coil. Electro, magnetism. "EM" energy.

Voltage is the stuff that connects the protons and electrons of atoms to each other, and it connects atoms together to form objects. Pull on your finger, and you are feeling the microscopic voltage between and within the atoms. Without voltage, there would be no solids or liquids in the universe, just gas. When you break a solid object, you are defeating the attractive microscopic voltages which were binding its atoms together.

The bonds between atoms are often associated with a constant voltage. If one atom is positive and the other negative, then there is a voltage between them. If billions of atoms could be line up in parallel, the voltage of the atoms could be easily measured. What would happen if we could align billions of atoms in parallel? We've just re-invented the battery. A battery is a couple of metal plates immersed in liquid. At the surface of the liquid where it touches each plate, all the atoms line up in parallel, and a voltage appears between the liquid and the metal. That's what causes the voltage of any battery: the micro-thin layer of atoms (ions) at the surface of the metal plates inside the battery. Everything else in the battery is just there to provide the electrical connections and the chemical fuel supply. Ideally, a flashlight battery could be three atoms thick (a thin film of liquid sandwiched between two thin metal films,) and it would still put out 1.5 volts.

Everyday electric motors operate by magnetic forces surrounding a coil, with electric current in the windings of the coil. Let's call this sort of device by the name "current motor". Electric motors in everyday life are invariably "current motors", but "voltage motors" exist too. They operate because of voltage-forces between charged objects. The microscopic motors used in cutting-edge nanotechnology are voltage motors. The linear chemical-motors inside your muscles are voltage motors. The spinning cilia on the tail ends of bacteria are little voltage motors. The mechanical enzymes which assemble ATP molecules (the 'energy molecules' of the cell) are voltage motors. The tiny microscopic parts inside a living cell are like little robots. They all rely on voltage motors, none use coils or magnetic motors.

Potential energy involves stretching, squeezing, pressure and forces. Voltage is associated with electric charge which has been "stretched" or "pressurized." Spin a flywheel, that's an analogy for electric current and magnetism. Stretch a rubber band, that's an analogy for voltage and charge separation.

Is magnetism like a warping of space? Then so is voltage. Voltage and magnetism can be combined to become a traveling wave of warped electromagnetism. We call these waves "light," or "radio," or "electrical energy." When the Electric Utility Companies sell you some "electricity", they're really selling you pulses of "EM field-warp," waves which are guided to you by a pair of copper wires. (After all, the electrons inside the wires are just wiggling back and forth.) They didn't sell you any electrons, instead they're selling you a combination of voltage and current. When voltage and current are there, electromagnetic energy is flowing down the wires.



 

Still Under Construction

Voltage is sort of like "electrical pressure", but it's not really pressure. Real electrical pressure is an attraction/repulsion force which is felt by physical objects. Electrical forces only exist when charged matter is being attracted or repelled, and if charged matter is absent, there can be no pressure, yet there can still be voltage. Analogy: if e-fields are like gravity, then voltage is like height above the earth. Voltage is not really force or pressure, in the same way that height above the earth is not a kind of "weight." Without the Earth and without height above it, "weight" cannot exist. Without electric charge and without voltage, electric attraction/repulsion cannot exist. Voltage is always measured between two points. Voltage is like distance in this respect. Analogy: At what height is your body right now? Well, your feet are on a floor that is a particular height above the ground. But you are a different height above sea level. And you are yet another height above the center of the earth. If a huge sinkhole suddenly opens up next to you, you will have yet another height, the height above the bottom of the hole. In each case your height is measured between two points: between the level of the dirt outside and the bottom of your shoes, between your shoes and the earth's center, etc. And the upshot of this is that your location does not "have" height, because it has several heights at once, depending on what two points you measure from. Voltage is very similar. A single wire cannot "have" a voltage. There can be a particular value of voltage between two wires. But the voltage between one of those wires and the surface of the earth will be a different value. And the voltage between that wire and the surface of some other planet will be different. One wire cannot "have" a voltage, because it has many different levels of voltage at the same time, depending on the two points you measure from. Suggestion: if you catch yourself talking about the voltage a wire has, it might be wise to entirely stop using the word "voltage" and instead say "potential difference". This forces you to think in the correct terms, since it is silly to say that a single wire has a "potential difference." Difference compared to what?! Exactly. What is voltage NOT? Voltage is not energy. It is closely connected with two classes of electrical energy: the energy of e-field flux, and the energy of charges in a potential field. However, voltage itself is not energy. Voltage is a physics concept called "potential", but voltage is not Potential Energy.
 

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[1] The magnet/charges GIF diagram kindly supplied by David Lewis

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