ELECTRIC ENERGY IS CARRIED BY INDIVIDUAL ELECTRONS?
November 15, 2010 Leave a comment
Wrong. Some books teach that, in a simple battery/bulb circuit, each electron carries energy to the bulb, is emptied of energy, and then returns to the battery where it’s re-filled with energy. Some books give an analogy with a circular track full of freight cars waiting to be filled. This is wrong. The energy in electric circuits is not carried by individual electrons, it is carried by the circuit as a whole. Here’s an analogy which may help explain it: imagine a wheel that’s free to spin. For example, turn a bicycle upside-down in your mind. Give the front tire a spin. When you spin the tire, your hand injects energy into the whole wheel all at once. Now put your hand lightly against some part of the tire so the spinning wheel is slowed and stopped by friction. Your hand gets hot. Your hand extracts energy from the entire wheel, all at once, and the whole wheel slows down. Finally, put one hand lightly against the tire while you use your other hand to keep the wheel spinning. Would it be right to tell students that the “Power” hand fills the rubber molecules with energy, that the molecules travel to the “Friction” hand and dump their energy, then they return empty to the “power” hand and get refilled? No, of course not! If this were true then your “friction” hand wouldn’t experience any friction until the magically-energized rubber molecules made their way around the rim. Part of the wheel would be spinning while part would be de-energized and unmoving, and it would be really a strange sight to see! A flashlight circuit is like our bicycle wheel. The electrons in the copper circuit are like the rim of the wheel. The battery causes ALL the electrons in the loop of wire to begin moving, and so it injects energy into the WHOLE CIRCUIT all at once. As soon as the battery moves the electrons, the distant lightbulb lights up. The electrons moving into the bulb’s filament are exactly the same as the ones moving out; the bulb doesn’t change them or extract stored energy from them. Did your hand alter the rubber tire as it rubbed on the bicycle wheel? No, it slowed the whole wheel down, it extracted energy from the whole wheel, and was heated by friction. Same thing with the bulb, it slows ALL the electrons down throughout the entire circuit, and in this way extracts energy from the whole circuit as it lights up. In discussing this misconception with teachers, I find that they see nothing wrong with it. The kids instantly grasp it since it is very visible, and it offers a sensible explanation. What more can we ask? Yet there is a problem: in order to understand electricity, a student must UNLEARN the incorrect freight-cars analogy. “Unlearning” rarely happens, and so the analogy forms a learning barrier which can forever prevent any further progress. It freezes their understanding of electricity at the elementary-school stage. Yes, if the kids will never have any need to understand how electricity REALLY works, then the freight-cars analogy is fine. But if the kids grow up to become scientists and engineers and technical people, then the freight-cars analogy causes harm. (Unfortunately, it causes FUTURE harm, so K-6 educators never see the effects of the misconception that they’ve installed in the kids minds.) The “filled freightcars” analogy seems seductively appropriate when used to explain Direct Current. However, when explaining Alternating Current the analogy breaks down completely. Each freight car wiggles back and forth, so how can those energy-filled buckets move from the “battery” to the “light bulb?” They cannot. The analogy doesn’t work, and students who have learned the analogy will find it impossible to understand AC. Again, this is fine if the kids have no hopes of entering any kind of technical career, and their science learning will cease after fifth grade… An analogy regarding this analogy (grin!) How do sound waves work? Would it be OK to teach kids that your vocal chords place energy into air molecules, then the air molecules zoom out of your mouth at 720MPH and crash into the ears of distant listeners? I would think that any author who use this kind of explanation should be ashamed. Yes, the explanation “works”, and it is easy for the kids to grasp. But it is wrong. And any kid who believes this explanation will have terrible difficulties should they ever have need to understand how sound REALLY works. This is an analogy for wires, since electrical energy is wave energy, and the electrons in the wires do not move along with the speed-of-light energy waves. The bicycle-wheel analogy has no problem explaining AC. Just wiggle the bicycle wheel back and forth instead of spinning it continuously. The wiggling wheel will rub upon the distant “friction” hand, and heat it up. Energy can travel instantly across the bicycle wheel, even though the wheel itself rotates slowly. Energy can travel instantly between the hands even if the wheel moves back and forth instead of spinning.