MadSci Network: Physics |
It's a pleasure to deal with questions so well-expressed; a credit to you and your teachers. First, let's agree on what "electricity" is. For our purposes, we are talking of moving charges. This is called "current." A little more than 100 years ago, the electron was discovered and, in metals, moving electrons make up current. Benjamin Franklin happened to get positive and negative wrong when charge was thought of as a fluid, and we now know that metals have outer orbit electrons that are free to move under influence of an electric field. Let's call these "free electrons." And "conventional current" moves in a direction opposite that of the electron flow. Now moving electrons can do work -- that is, transfer energy. When an electromotive force -- voltage difference -- is applied to the ends of a path filled with free electrons, they move. Moving electrons can transfer energy through their magnetic field (as with relays and motors), or through collisions with stationary atoms (as with electric heaters). But the ultimate source of the energy is the voltage source. The moving electrons are the means for energy transfer. If you pull on a rope over a ceiling-mounted pulley to lift a bucket of nails, you are doing the work and the rope is the means of energy transfer. The means for creating a voltage difference, whether by an electrochemical cell or turning a DC generator or alternator, is important for us only in the fact they convert one form of energy (in these cases, chemical and rotational-mechanical) to electric. So let's just take our ability to create a voltage difference as a given. I want to set up a thought experiment. Let's take a 10 centimeter piece of resistance wire that measures 0.15 ohms and place it across our 1.5 volt cell. We notice the wire gets hot -- 10 amperes of current is flowing. One model we can use for this is the circuit theory model. We represent the cell as an ideal voltage source of 1.5 volts and the resistance wire as a 0.15 ohm resistance. The lines we draw from the source to the resistance ARE NOT WIRES! They merely indicate connections between two ideal entities. This model allows us to calculate that 10 amperes come from the source and that 15 watts are dissipated in the resistance. This model says nothing about electrons or the mechanism the resistance uses to change electric power into heat; it just describes power (rate of energy production) out of the source and power (rate of energy dissipation) into the resistance. But this first model does not satisfy you. You want to talk about electrons. OK -- we'll use a second model. We draw a 1-quadrant graph with 0 to 10 centimeters on the horizontal scale, and 0 to 1.5 electron- volts on the vertical scale. Electron-volts (eV) are energy units. We draw a straight line from (0,1.5) to (10, 0). We can think of this graph as the energy each electron has as it flows through the resistance wire. As it "rolls down the potential hill," it loses energy. This model ignores the atoms the electrons that bump into and says nothing about quantum states. But what would be gained if we did include these? The rule is, we use as simple a model as possible to approximate the real world. In the second model electrons give up their energy as they travel through the wire. Electrons never get used up. But their ability to do work does get used up. And, in this thought experiment, second model, 6.25E19 electrons are passing through the resistance wire every second. Think of the free electrons in a conductor as an incompressible fluid. Try Bill Beaty's site: http://www.eskimo.com/~billb/ Choose 'My "Electricity" Articles'. Then choose 'What is "Electricity?" ' Bill has done a wonderful job on this (and other issues). I think you'll want to bookmark his site. Finally, your sentence mentioning "...'flow of electrons' just an analogy to make it easier for high school kids like me to understand its nature?" shows a sensitivity to the learning situation that is somewhat rare. The analogy is for humans of all ages. And I think you'd make a fine teacher. Best of luck, and cheers. Larry Skarin
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