Re: Neutrality of a battery; was Capacitor or condenser?

[LUDWIK KOWALSKI <kowalskil@alpha.montclair.edu>]

1) Roger Pruitt defends the term capacitor. I was not trying to suggest
   that we replace it; it was just a follow up for an earlier observation
   the name we use can lead to a misleading analogy. My argument would be
   valid some 40 years ago when the word condenser was started to be
   eliminated. And everything can lead to misconceptions.

2) How to explain the equality of charges to a student? 

a) Don Simanek made an interesting observations and predicted what should 
   be expected (on the basis of Gauss's law) but he did not come with an 
   explanation. He did not say "Gauss" but I could read it between lines.

b) Mark Shapiro said (see below) that the conservation of charges should 
   be used. This would be OK if the electric neutrality of a battery could 
   be taken for granted. I suspect that explaining what is going on in a 
   battery, or in a power supply, is harder than explaining a capacitor.
   In fact, I usually treat the equality of charges as a "self-evident
   truth" and students accept this without questionning. But this time
   the why-this-happens question was asked and I must answer it tomorrow.

   Unless something better is suggested I will talk about the conservation
   of charges (a never contradicted law of nature) hoping that nobody will 
   challenge the remark that the electric neutrality of a source can be 
   taken for granted. I know this to be true but I can not say that this 
   is also a law of nature. If I am really pressed I will say "ask your 
   chemistry professor". I know, however, that the topic is usually not 
   covered in the introductory chemistry courses, except in the form of 
   "as you know from physics, ...."

c) William Beaty made a very convincing argument for the neutrality of
   a source (see below) but I can not use it because I teach electricity
   at rest before the electricity in motion. The idea of a constant 
   current loop appears in chapter 17; while we are in chapter 16.

                                            Ludwik Kowalski
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> From Mark Shapiro
> Have you discussed conservation of charge in the course? If you have then 
> an explanation like "the battery moves electrons from the plate connected 
> to the positive terminal to the plate connected to the negative terminal...  
> since the plate connected to the positive terminal was initially neutral 
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From: William Beaty
> You could point out that a capacitor in a circuit acts like a resistor
> in the sense that the current in one lead of a capacitor equals the 
> current in the other lead during charge and discharge. When in a circuit, 
> every time you push a charge into one end of a capacitor, the electrostatic
> force pushes one charge out the other end, and the net charge Q1-Q2 stays
> zero. 
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