BATTERIES STORE CHARGE, AND THIS CHARGE FLOWS IN WIRES? (William J. Beaty)
November 15, 2010 Leave a comment
No. The word “charge” has more than one meaning, and the meanings contradict each other. The “charge” in a battery is energy (chemical energy), while the “charge” that flows in wires is electron particles. The term “charge” refers to several different things: to net-charge, to quantities of charged particles, and to “charges” of energy. If you are not very careful while using the word “charge” in teaching, you might be spreading misconceptions. For example, even when metals are totally neutral, they contain vast quantities of movable electrons. So, should we say that they contain zero charge because they are neutral? Or, should we say that they contain a very large amount of electric charge, because they are filled with electrons? Don’t answer yet, because your answer might be inconsistent with how we describe capacitors (further below.) Another: if I place an electron and a proton together, do I have twice as much charge as before, or do I have a neutral hydrogen atom with no charge at all? What I DO have is confusion. Misuse of “charge” makes descriptions of electric circuits seem complex and abstract, when the explanations are really just wrong. Another: electric currents in wires are actually a motion of “neutralized” charge, where every electron has a proton nearby. If we teach that a wire is uncharged, and we ALSO teach that electric current is a flow of charge, how can anyone make sense of a situation where a wire has no charge at all, yet contains an enormous flow of charge? We could say “Oh, but electric currents are usually a flow of Uncharged Charge.” WHAT? What would a student make of THAT statement? Can you see the problems that arise because of the word “charge?” Another one: as you “charge” a battery, you cause an electric current to appear in the electrolyte, and this motion of electric charges causes chemical reactions to occur upon the surfaces of the battery’s plates. Chemical “fuel” accumulates, but charge does not: the charges flow into (or out of) the surfaces of the plates and do not accumulate there. Chemical energy is stored in the battery, but electrical charge is not. When a battery is being “discharged”, it’s chemical fuel drives a process which pumps charge through the battery. The fuel will eventually be exhausted, but the total electric charge within the battery will never change! Here’s a way to imagine the process: a battery is like a spring-driven “wind up” water pump. Send water backwards through this pump, and you wind up the spring. Then, provide a pathway between the inlet and the outlet of the pump, and the spring-motor will pump the water in a circle. But now think for a moment: the water is the charge, yet our wind-up pump does not store water! When we “charge” our wind-up pump, we send the charge (water) THROUGH THE PUMP, and this stores energy by winding up the spring. Same with a battery: to “charge” a battery, we send electrical charges THROUGH THE BATTERY and back out again. This causes the chemicals on the battery plates to store energy, like winding up the spring in our spring-powered water pump. See how “charging” and “charges” can create a horrible mess of misunderstandings? When this mess gets into the textbooks, and educators start teaching it to kids, the kids end up believing that Electricity is too complicated for them to understand. Yet the fault does not lie with the students!!!! Another one: if you “charge” a capacitor, you move charges from one plate to the other, and the number of charges within the device as a whole do not change. Or from an engineer’s perspective, you drive charge THROUGH the capacitor, which causes potential across the plates to rise. But capacitors have exactly the same total charge within them whether they are “charged” or not! Whenever we take an electron from one plate, we put an electron onto the other plate. When we speak of “charging” capacitors, we’ve suddenly stopped talking about charge, and started talking about electrical energy. A “charged” capacitor has quite a bit more energy than an “uncharged” one (but exactly the same net-charge, and the same quantity of + and – particles inside it.) This basic concept is very important in understanding simple circuitry, yet it is rarely taught. The misleading term “charge” stands in the way of understanding. I suspect that students are not the only ones being misled. Many teachers misunderstand simple physics, and they believe that the purpose of a capacitor is to store electric charge. Think like this: both capacitors and inductors (coils) store ENERGY, and neither one stores charge. Yet electric charge is the medium of energy storage in both coils and capacitors. In capacitors, energy is stored in the form of “stretched charge”, or potential energy, while coils store energy in the form of moving charge which contains kinetic energy. However, we don’t put any charge into a capacitor when we “charge” it, any more than we put charge into a superconductor ring-inductor when we give the ring a “charge” of electromagnetic energy.