Hey Alex,
I had a question regarding one of the problems in the Physics section. On page 186, question #3 (Chpt 6 Electrostatics), it asked "If an e- were placed midway between R and S above, the resultant electric force on the e- would be..."
I was wondering why the answer is B "towards S". Am I missing something because how would you tell which one is positive and which one is negative? I know that since the force is towards each other, that one is positive and one is negative but is there any other information that tells us S is positive?
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You are correct in feeling that something is missing here. This must be a piece of errata that has not yet been posted -- I am submitting it on your behalf.
In general, the only way to know that a point charge is positive or negative is in context. Either we would have to be told the charges directly, or they could tell us that S repels a known +q charge. In this case, we aren't given that info; thus, we can determine the magnitude of the force, but not the direction (sign).
Hope this helps!
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