# why do we test electric fields with positive charges? not negative ones?

Is there any difference between using a positive vs a negative charge to test an electric field? If so, please explain.

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 Are you asking about the definition "The electric field intensity is defined as the force per unit positive charge that would be experienced by a stationary point charge, or "test charge", at a given location in the field" en.wikipedia.org/wiki/Electric_field#Definition ? – anna v Sep 13 '11 at 7:47

## 3 Answers

You can use a negative charge to test an electric field. You just have to remember that the electric field points opposite to the force on the charge, rather than parallel to it. That's just a convention, though; we could have defined the electric field to point parallel to the force on a negative charge, and physics would work the same, except for a couple of negative signs in some formulas.

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 This is actually the answer I was looking for... – wizlog Sep 13 '11 at 6:30 @wizlog: the system allows you to change which answer you've accepted if you think that's appropriate. – David Zaslavsky♦ Sep 13 '11 at 7:11 Thanks, I thought there was a time limit. – wizlog Sep 13 '11 at 7:42

There is no fundamental difference between positive and negative charges. Anything that couples to positive electric charge couples to negative charge and vice versa. Practically, the main difference is that negative charge is on electrons and positive charge is in protons in atomic nuclei, so on a day-to-day basis they play different roles. However, these signs are reversed for antimatter, so we see there really is nothing special about the sign of a charge.

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You can also use the positive charges. what happens is that usually the negative charges are used because, by convention, these were the charges that they decided to use to determine the direction of the electric current. besides, you can see it from this perspective: negative charges can move on the "lattice" and they are more "fun", while the protons are "boring", there are always "stuck" in the atomic nucleus.

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