posted
I always thought it was because the current heats up the wire. But today it struck me that the thermal movement of the electrons is much faster than the drift velocity in the current; and for heating, only the speed should matter, not whether the speed is in a specific direction. So perhaps my question is actually, why doesn't the thermal energy of the electrons make the bulb glow?
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Lightbulbs can be modeled as resistors. Resistors generate power in the form of heat; P=I^2R, so the energy produced is related to the current and the resistance. As far as I know, the heat produced in a resistor is due to the electrons smacking into atomic nuclei in the metal. This heats up the nuclei, and the wire in the bulb glows because it is very hot.
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Yes, that's kinda what I said. What I want to know is, why doesn't that work for the thermal movement of the electrons, which is much faster than the drift movement due to an electric field?
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What exactly do you mean by the thermal energy of the electrons? The current? The actual speed they are moving as they bounce around inside the metal? I think that bouncing around happens even when current is not running through the wire, so in that case the thermal energy has already heated up the wire to room temperature or so.
Also, and I'm not sure about this because I haven't studied heat in great detail, I believe that heat is the random motion of particles, so direction does indeed matter.
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Look you : In a wire with no current, the electrons are moving around at random. The speed of their movement is very high, even at room temperature. If you switch on a potential difference, the electrons are accelerated. They now have a net movement; however, that movement is very much slower than their movement from heat.
But, now that I think about it, the reason it is slower is that they are crashing into atoms, thereby losing their energy, and being given it back by the potential difference. So the actual speed doesn't matter : The point is that there's a net transfer of energy into the wire, which doesn't occur with thermal movement.
Thanks, you made it much clearer for me.
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