How do they REALLY work?
[NOTES FOR UNWRITTEN ARTICLE]
©2015 W. Beaty
How diodes REALLY work
How resistors REALLY work
How inductors REALLY work
How capacitors REALLY work
Batteries are like pumps
If an electric circuit is like a drive-belt, or like a loop of
water-filled hose, then a battery is just a pump. It's a
constant-pressure charge-pump. Like all pumps, the
battery does not supply the liquid being pumped. Batteries take in
electric charge through one terminal, pump it through themselves, then
force it back out through the other terminal. Note that that's the
"complete circuit." A battery is a good conductor, is part of a
complete circuit, and no
electric charge is building up inside.
The pumps are at the surfaces of the plates!
If batteries are charge-pumps, then exactly what is doing the pumping?
Corrosion! Natural corrosion provides the pumping action. Here's the
one-sentence explanation: Metals in water dissolve as easily as sugar or
salt, but when this metal corrosion starts up, a weird electrical
effect halts it, and we can harness this dissolving-metal effect to
produce a current-loop in a closed circuit. That's batteries.
Here's the longer version.
Dunk copper into water and it dissolves rapidly. But then it instantly
bunch of volts between the copper and the water, since water is pulling
positive-charged atoms away from the metal, leaving behind a sea of
un-cancelled electrons, which leaves the copper strongly negative while
the water becomes strongly positive. This water-copper static-electric
force layer will push any further copper atoms back into the copper, and
totally halts the corrosion process. (Those who are interested could
google "half-cell potential.")
The water is now a "positive terminal" and
the metal a "negative terminal," and if we could somehow short these
together, the voltage which prevents the corrosion would reduce, the
corrosion would kick into high gear, and a huge electric current would
appear as positive copper atoms flooded into the water. The copper would
rapidly dissolve away, its chemical energy becoming electric heating. But
we can't. I mean, we can't connect water to copper and short out the
voltage, since they're already touching.
So what we DO do, is to dunk some zinc
(or any non-copper metal) into the same water. Let it momentarily
dissolve-and-halt, which produces another voltage between water and zinc.
But the zinc-water voltage is higher than the copper-water voltage. If we
now touch the copper to the zinc, the zinc-water voltage gets smaller, and
the copper-water voltage gets higher. A huge current does appear, since
we've reduced the electrical repulsion force which had been stopping the
zinc corrosion. (We've also *raised* the copper-water voltage, which *reverses*
the copper corrosion and drives dissolved copper ions back onto the metal
surface, electroplating it.)
That's it. That's the basic battery. Ah, we
also have to add lots of dissolved copper ions to the water near the
copper surface. Just dump copper chloride or copper sulfate into the
water, both make the water very conductive (full of movable charged
atoms,) and to guarantee that only copper is being plated back onto the
copper surface, and not some other metal contaminant which would alter the
usual copper-water voltage and screw things up. We don't want our copper
plate to get covered with zinc!
If not charge, then what is stored?
What do batteries actually store? Chemical fuel, of course. A "battery" is
just a fuel-cell, but where the fuel is stored inside. (Imagine a
fuel cell, but where the hydrogen was kept inside the cell.) A discharged
contains waste products, and whenever we "charge" a battery, we just push
charges through a battery and back out again. Doing this injects
chemical energy, and turns the waste back into fuel again. Or instead of
using an automotive
battery-charger, we can take out the car-battery plates and directly smear
them with fuel: lead oxide and lead powder. (That's how manufacturers do
Heh, batteries, they work like the gas tank of a car, but only if we
the exhaust gases
into cars' tailpipes while pushing the whole car backwards, in
un-burn the waste products and re-create the gasoline. Not
possible with engines, but perfectly possible with batteries. We can
turn water into H2 O2 and back again. We can also turn metallic zinc
into zinc chloride and back again. Or dissolve and corroed the lead,
then uncorrode it again to make lead metal.
THEY DON'T STORE ANY CHARGE
Batteries don't store charge, inductors don't store charge, capacitors
don't store charge, resistors don't store charge. In ALL of these
components, the net electric charge remains constant, so for every bit of
charge pushed into one terminal, an equal bit of charge pops out the other
one. In this, batteries are no different than resistors. Note that
saying "batteries store charge" is just as wrong as saying "inductors
store charge." Yeah, there's electric charge involved, but it's never
added or removed from these components, on average.
A lot of introductory textbooks get this wrong. They try to convince us
that the plates store charge, and tell us that the current in the
electrolyte is always zero, when actually the
amperes in the battery acid is exactly the same as the amps in the battery
cables. Batteries are very good conductors. Any accumulation of
charges on the plates is immediately shorted out.
A wire is like a
water pipe, but so is an inductor, capacitor, or battery. Or say it this
way: inductors store magnetic energy, capacitors store electrostatic
energy, batteries store chemical fuel(energy,) and none of them
emit or accumulate any electric charge at all. Neither does a resistor,
yet a resistor is able to take in a one-way flow of electrical energy.
the only simple way to "charge" a battery is also the only way to
charge a resistor or inductor: put them on top of a VandeGraaff generator!
Or, inductors spin their internal charges like a massive flywheel, while
capacitors stretch their charges like a spring. (Note that flywheels and
springs store energy, not metal; their amount of metal is constant!) Or:
capacitors store charge in exactly the same way that inductors store
charge: they don't.
THE VOLTAGE-GENERATORS INSIDE BATTERIES
1. The potential-difference, the voltage in batteries is created in two
separate spots: the surfaces where the conductive liquid touches each
The "charge pumps" which create the voltages; they are located in two
microscopic layers, with one layer on the positive battery plate, and the
other layer on the negative plate. In other words, every electrochemical
cell is actually two separate cells in series, with the electro-
lyte liquid acting as a conductive connection between them. They're
called "half-cells," search the www for lots of info on these.
2. Ask yourself what happens when a hunk of metal is placed in water. It
corrodes rapidly, right? The water molecules behave as an extremely
aggressive solvent, and they attack the metal surface quite vigorously.
At the surface of the metal, the water molecules surround individual metal
atoms and pull them loose. But atoms in metals are in an odd situation:
they're not neutral atoms. Long ago they've lost their outer-shell
electrons. Their outer electrons became part of the "electron sea" of the
metal. So, whenever water grabs atoms from the metal surface, it's
actually grabbing positive ions: metal atoms with an excess of positive
charge. These ions, each surrounded by a "shell" of water molecules, then
diffuses away into the liquid. The metal dissolves. In theory, it should
dissolve rapidly, almost as fast as sugar or salt crystals.
But as water molecules pull away these positive-charged atoms, the
neutral-charged battery plate turns overall negative. At the same time,
the neutral-charged water turns equally positive. It's almost like a
VandeGraaff electrostatic generator, where the dissolving positive ions
are the rubber belt. As the metal ions are pulled away, they create a
charge-separation, like a charged-up Leyden jar. A voltage grows between
the water and the metal. This voltage becomes larger and larger as
corrosion proceeds along. Soon the voltage is big enough that it forces
the dissolving positive metal atoms to stop leaving the metal. The
negative-charged metal surface is now attracting the positive-charged
atoms in the water. They're pulled back, and the water fails to yank them
away from the metal surface. And so, the corrosion process halts. It's
halted because the entire metal object has become charged overall
negative, and the water charged equally positive. (And scientists back in
the 1700s discovered this using electrostatic force detectors,
In truth, metal should always dissolve instantly in water; disappearing
almost as fast as sugar or salt. But the "electrostatic generator effect"
stops it. In the everyday world, metal in water doesn't corrode like
this. In a tiny fraction of a second, the electrical attraction forces
become enormous, and the corrosion halts almost instantly. The metal is
protected by an invisible layer of intense voltage-field.
Now the slightly-corroded battery plate has become charged to a few volts
negative, relative to the water which is charged a few volts positive.
Hey, what if we could use this potential-difference as a power supply?!
After all, the dissolving of metal is a genuine electric generator. If we
could somehow connect it to an external circuit, then we could connect a
light bulb. This would slightly lower the potential-difference between
the water and the metal. The metal would again corrode. The chemical
energy would drive an electric current through the outside circuit. The
bulb should light up!
But try it. It doesn't work.
We can easily connect a wire to the piece of metal in the water. But if
we try to make a second connection to the water itself, we just create a
seonc water/metal pair! Doh! Another "electric generator" or "halfcell"
layer appears whenever we try to connect to the water. But this time the
voltage is pointing backwards, and the two voltages cancel out. No
potential difference on the two wires immersed in water. The water is
still positive. But both metals are equally negative.
So we cannot connect wires to water?
Fortunately there's a way around this. Suppose we first placed copper
metal into the water. The corrosion-powered "charge pump" creates roughly
4V between the copper and the water (with the copper being negative.)
What if we try another metal? Not copper? Say, a plate of Zinc? The
voltage between Zinc and water is smaller than copper by about 0.9V. The
two half-cells still oppose each other. But their voltages wrt the water
are not the same. They still subtract. They leave behind a remainder of
just under a volt (which turns out to be the "cell potential" of a
copper/zinc pair in water.) Connect the two different metal plates to
wires, and the 0.9V should be able to run a small incandescent bulb, or a
tiny sensitive DC motor.
But it still doesn't work! The pure water is an insulator! Our magical
device is an open circuit, a turned-off switch. To make water conductive,
it needs to be full of movable charges. Easily remedied: just fill the
water with zinc salts. Oops, but that drives zinc atoms into the copper,
quickly covering it with zinc, so the two surfaces become the same metal.
The battery works for a moment, then stops again.
Well, what if we put one electrode above the other, then fill the jar with
two pools of electrolyte which have different densities? One liquid layer
full of zinc salts, the other full of copper? Yes, works great! We put
our copper on the bottom, our zinc above it, fill the jar half way with
dense copper salt solution, then the rest of the way with lighter zinc
solution. Both liquids are good conductors. Connect it up, and we've got
an electric energy supply, a charge-pump which is fueled by water's
spontaneous corrosion of metal. The zinc dissolves, and it mostly powers
the copper plate, forcing the dissolved copper to un-corrode. A small
remainder of energy will come out through the wires, and power our light
bulb. (Or our miles of Morse-code signaling wires.) This is the "gravity
cell" or "crow-foot battery" used to power all the telegraphs of the late
Ah, one last thing. The two separate corrosions are fighting, right?
One wins. And while one battery plate loses positive atoms and corrodes,
the other metal plate must gain metal atoms. It runs backwards and
"un-corrodes." The zinc plate gives out energy as it dissolves, but the
copper takes it back again. In our completed battery, the zinc electrode
is corroding away; being dissolved to form more zinc salt solution. This
creates a voltage, and also an electric current (made of positive zinc
ions) flows out of the zinc and into the water. But at the same time, the
copper electrode is getting new copper atoms driven into its surface.
Positive copper atoms are going backwards; flowing out of the water and
into the solid copper to build up new metal. And so there's an electric
current in the electrolyte. The amperes of current are exactly the same
as the electric current in the metal plates. It's exactly the same as the
amps in the two wires leading out to our light bulb. The battery is like
a short circuit, forming a complete loop including the tungsten filament
of the bulb. If there's a one-ampere flow of electrons in the metal
connections, then there's always a one-amp flow of metal ions in the
electrolyte solution. Batteries don't store any electrons.
Batteries don't accumulate any electric charge. The path for electric
current is **through** the battery, and back out again. The electrolyte
serves as a conductor. It connects the two micro-thin "charge pumps" on
the plate surfaces. The battery's voltage originates on those thin
REALLY one final last bit. Remember that thing about metals dissolving
as fast as sugar? We can watch that happen. Just connect the two battery
terminals directly together. This reduces the voltage inside the
microscopic layer where the water touches the zinc. Suddenly the natural
"static electricity barrier" is lower, and the positive-charged metal
atoms aren't driven back as they ordinarily would be. Again the water can
attack the zinc, and spontaneously dissolve the metal, dragging the atoms
far away from the surface. Our large zinc plate will corrode away in a
matter of minutes, as if it were a big piece of salt. If metals aren't
allowed to charge up strongly negative, while the water charges up
strongly positive, then any metal placed in water would fast-corrode like
Batteries? That's where we've figured out how to remove the natural
electric barrier from metals immersed in water ...andthen use metals as a
"chemical fuel" which runs a microscopically-thin charge-pump, which
produces a voltage, and thus powers all kinds of electrical devices.