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ericmazur / PnPbook

Tracking of typos, errors, and improvements for "The Principles and Practice of Physics"
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batteries and "charge elevator" #61

Open ericmazur opened 9 years ago

ericmazur commented 9 years ago

From @JohnDenker: The "charge elevator" shown in figure 31.2 is not a good model of how a battery works. It seems to be a recent invention, popular in the "research based" textbooks, but it's the sort of thing that gives "research" a bad name.

For one thing, according to the only way I can see to interpret the diagram, it puts out both constant voltage and constant current. A real device that did that would be extremely dangerous, for example if somebody accidentally open-circuited it. Real batteries are not remotely like that.

In any case, the "charge elevator" is unnecessary, to a rather comical degree. For 200 years or so, sensible people have been modeling batteries as high-value capacitors. That makes a fine model, good to first order in charge and zeroth order in current. It's better than the "charge elevator" in every way. If you add a little bit of series resistance, it becomes even better, i.e. good to first order in current.

ericmazur commented 9 years ago

Good point. I guess I do want to lay the 'ground work' for the "pipe-and-ball" model I use for junctions etc. I will rethink.

JohnDenker commented 9 years ago

However ... the pipe-and-ball model is predicated on some wrong physics. A discussion of this point is in the email queue, and should appear on the tracker eventually. In the meantime, you can watch the movie: https://www.av8n.com/physics/conservation-continuity.htm#sec-speedup

ericmazur commented 9 years ago

That's true, but so much is "wrong" if taken literally. It does make the continuity point well.

JohnDenker commented 9 years ago

Still, though, the incompressible-balls model is completely unnecessary. You can just say that (a) there are no sources and there are no sinks, because charge is conserved, and (b) in the steady state, charge isn't accumulating anywhere, therefore (c) what goes in must come out. That's all that is needed for chapter 31. Short and sweet and correct.

Anything students learn about incompressible balls will have to be unlearned almost immediately. See e.g. the discussion of the Drude model later in chapter 31. See also the discussion of semiconductors in section 32.3. See also issue #66.

For chapter 32, you need (b') usually rather little stray charge is accumulating anywhere, because the stray capacitances are usually rather small relative to things you care about. People like for circuits to behave this way, and generally design things accordingly. However, this is an engineering goal, not a law of nature. Beware that there can be significant exceptions. This is not a question of taking things over-literally. For example: https://www.youtube.com/watch?v=9tzga6qAaBA&t=0m52s