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I do apologize for the ignorance that I'm sure is imbedded in this question, but I'd like to understand the exact point at which the following argument goes wrong:

1) A battery (let's say an ordinary flashlight battery) maintains a voltage between its positive and negative terminals.

2) The only way to maintain a voltage is by maintaining a charge distribution. Therefore, at least one of the terminals on that battery carries a non-zero net charge.

3) If a terminal carries a non-zero net charge, I ought to be able to use it to pick up a paper clip.

Nevertheless, my flashlight batteries do not pick up paper clips. Is this because the charge is too small or because (at least) one of my three points is dreadfully wrong?

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  • $\begingroup$ Your paperclip is electrically neutral. $\endgroup$
    – Richard Terrett
    Commented Jul 13, 2012 at 0:55
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    $\begingroup$ Richard Terrett: Yes, my paper clip is electrically neutral, but I'd have thought that a (say) positively charged terminal would draw electrons toward the close end of the paper clip, whereupon that end would acquire a slight negative charge, leading it to stick to the terminal. $\endgroup$
    – WillO
    Commented Jul 13, 2012 at 1:03
  • $\begingroup$ Richard Terrett: If nevertheless you are saying that I am wrong about point 3), I'd still like to know whether it is true that the terminals on my battery carry non-zero net charges (when, say, the battery is stored in a drawer). $\endgroup$
    – WillO
    Commented Jul 13, 2012 at 1:04
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    $\begingroup$ WillO is right, @Richard is wrong. There will be a surface charge induced on the paper clip to "shield" the inside of the paper clip from the electric field of the battery. And the paper clip will be attracted to the battery terminal. The force must be very small. You can move particles around with static charges, see sciencefair-projects.org/physics-projects/… for example. $\endgroup$
    – mwengler
    Commented Jul 13, 2012 at 1:38
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    $\begingroup$ @AlfredCentauri no motion necessary. In electrostatics, a surface charge goes to the surface of a metal whenever it is exposed to an electric field. This needs to happen so that the net electric field inside the metal, a conductor, is zero. The field from the surface charge exactly cancels the external field, otherwise the field inside the conductor would be the external field, non-zero, and that field would cause the charges in the conductor to move around... until they found a configuration with no net field in the conductor. $\endgroup$
    – mwengler
    Commented Jul 13, 2012 at 17:40

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Richard Terrett's comment gives the correct answer: Richard, you should post it as an answer so people can upvote it.

A battery does indeed have excess charge at it's terminals, and the charge is simply given by the usual equation Q = CV, where C is the capacitance of the battery and V the voltage. However both the capacitance and the voltage of a typical domestic battery are small so the net charge is negligable.

However the reason a battery won't pick up scraps of paper is that the voltage is small. If you do the usual party trick of rubbing a ballon on a pullover you can charge the balloon to several thousand volts. If you only charged the ballon to 1.5V it wouldn't pick up small bits of paper let alone a paperclip.

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  • $\begingroup$ @Richard no Richard's answer is not right. The Debye force is an example of how an induced dipole moment (moving + and - charges apart in an on-balance electrically neutral object) allow a neutral object to be attracted to a charge. $\endgroup$
    – mwengler
    Commented Jul 13, 2012 at 18:10
  • $\begingroup$ mwengler: I took John Rennie to mean that you (or perhaps Richard after you corrected him) were right and assumed he'd just mistyped. His answer does indicate that a) a battery does have excess charge at its terminals, b) that excess charge should in principle be able to move a piece of metal but c) the amount of excess charge is in principle too small to move anything the size of a paper clip. I think everyone now agrees on all of this. Yes? $\endgroup$
    – WillO
    Commented Jul 13, 2012 at 20:14
  • $\begingroup$ Yes, I think we're all agreed :-) My point about Richard's comment was that he correctly identified it was the kilovolt potential that allows charged balloons etc to pick up objects. $\endgroup$
    – John Rennie
    Commented Jul 14, 2012 at 6:16

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