In watching those light bulb energy theater episodes, I realized I needed to do some thinking about energy transfers and transformations in a light bulb that is burning steadily. Here's what I feel clear on: Energy, which I will call electrical energy, enters the bulb. Electrical energy is transformed into thermal and light energy in the filament. Thermal and light energy leave the bulb. Check.
How does the electrical energy enter the bulb? By means of the current, right? Energy is carried by objects, and electrons are the objects that are going into the bulb. That's why I called it "electrical" energy in the first place. But, here is the issue that Russell brings up in the first episode of my earlier post: The electrons leave the bulb, too. And the current is exactly the same when it goes out as when it came in. How can a current lose energy (as it must, if thermal and light energy are going to be produced at the filament) and yet be the same afterwards?
Over lunch, Amy and I did our darndest to think of what could be different that we might not be thinking of. Could the electrons have less kinetic energy after the filament, and perhaps be differently dispersed in the wire to make up for it? We think the answer is no. We think that anything we could measure about the charges in the wire would be the same before the filament as after it.
My physics brain, meanwhile, was reminding me that there is something different about the electrons before the filament than after it, which is that they are at a "higher voltage." That's just dead knowledge when I say it that way, a vocabulary word rather than an explanation; but it got me thinking. I thought about the electrons as blocks sliding downhill. Or actually, sliding frictionlessly on a horizontal surface (the wire), then sliding down a carpeted hill (the filament) so that the speed on the hill is constant, then sliding horizontally again (the other wire), and then being lifted back to their starting point by an elevator (the battery). The blocks are the same before and after the hill – anything I cared to measure about any one block is the same, and a movie of the set of blocks on the lower level looks the same as a movie of the set of blocks on the higher level. Yet they "generated" thermal energy on the way, when they rubbed on the carpet. So the upper blocks must have had some energy not associated with an observable property of each block itself. The only thing that changed is the height. On that basis, I hereby invent "height energy" in the context of electric circuits. Except it's not height; it's position relative to the battery. So let's call it "position energy," or "arrangement energy," or "configuration energy."
Position energy is tricky, because you can't see anything about the object itself, in isolation, that tells you that it has more energy. You know that the object has more energy because it can make more of something happen - it can warm the carpet more, or light the filament more. But you can't see that (ahem) potential directly.
I'm also ready to define a "field" now, and my definition is: A set of different positions associated with different energies.
Brian Frank (and possibly also Leslie Atkins is involved with this too) explained a model for energy in an electric circuit that had the current remaining the same *in quantity* on either side of the light bulb, but changing *in composition,* in the sense that you can make the same current with a different charge density and a different velocity. So, the electrons can change their spacing relative to each other, and change their speed (both of which, funly enough, correspond to changes in energy), without changing the current. So the changing relative position might not be in relation to the battery but in relation to each other. It reminds me of when I first felt like I understood chemical potential. We use chemical potential like it is a field of a certain height in which the particles reside, but it might be a possession that each particle has and can give away, so the chemical potentials for different molecules in the same vicinity might be different because they contain different amounts of energy. Not exactly the same as with the electrons, but both possibly involving a figure-ground reversal, or a re-conception of the energy as being more local than we originally thought.
ReplyDeleteHunter, are you saying that the electrons change their spacing _and_ their speed? That's the only way I can think of that the current would stay the same.
ReplyDeleteHere's my naive question (?)/thought: I imagine that they would _slow down_ after going through the light bulb if they do change their speed, because they're "giving away" some of their energy. But to keep the same current if they've slowed down, they'd have to be closer together. How can they be closer together and slowed down at the same time?
I'm sure I'm missing something -- can you clarify?
If each electron slowed down when it got to the filament, they would get closer together, because the ones behind them that hadn't slowed down yet would catch up to them, like a traffic jam!
ReplyDeleteAlso note that if they are closer together they will have a greater electrostatic potential energy. So, if you want the electrons to lose energy, they can either increase their spacing or slow down, except that these are mutually exclusive options, since the spacing and speed must vary in inverse proportion to keep the current the same.
ReplyDeleteSheesh, I was all ready to relax with my model and a glass of wine for a while, but no, I gotta get up and do business with another model. I remember Brian's model with the hula hoop and I remember not really engaging with it. Fortunately Benedikt blogged about his conversation with Brian, and Brian prepared visuals for his model. New post.
ReplyDelete