Question on Passage in Six Easy Pieces

[LarryOnline]

Question on Passage in "Six Easy Pieces"

I am reading Feynman's Six Easy Pieces. In chapter 2, Basic Physics, he describes electric charge attraction/repulsion.

Here's an excerpt (from the bottom of page 28 in my edition):

"Suppose that we have two unlikes that attract each other, a plus and a minus, and that they stick very close together. Suppose we have another charge some distance away. Would it feel any attraction? It would feel practically none, because if the first two are equal in size, the attraction for the one and the repulsion for the other balance out. On the other hand, if we get very close with the extra charge, attraction arises, because the repulsion of likes and attraction of unlikes will tend to bring unlikes closer together and push likes further apart. Then the repulsion will be less than the attraction. "

My question is this: If electric charges are elemental, with the plus and minus being equal but opposite to the minus, how can the two unlike charges end up attracting another charge? This implies that one charge is slightly different than another, no?

 

[Vanadium 50]

Force depends on the distance as well as the charge magnitude.

 

[LarryOnline]

I understand. However, no matter the distance, the "like" and "unlike" charges should completely cancel and therefore provide no attraction to another charge.

 

[Vanadium 50]

"stick very close" does not mean "are in exactly the same spot".

 

[LarryOnline]

Ah, so if a third charge nears the other +/- pair, it could get relatively closer to an opposite charge and be captured by it. A different, oppositely-charged particle might also take a similar path towards the +/- pair, but in this case be repulsed. So over time, the +/- pair will tend to attract. Correct?

 

[The_Duck]

The lone charge will feel an attraction regardless of whether it is positive or negative.

Here is what Feynman is trying to say: Suppose I have a + charge and a - charge attached by a spring or something. I bring this contraption near a lone positive charge. What happens? The + charge gets pushed away from the lone positive charge, while the - charge gets pulled towards it, so the - charge will end up somewhat closer to the lone positive charge (the + and - can't get too far apart from each other because they are attached by the spring). Now what force does the lone positive charge feel? It is attracted by the - charge and repelled by the + charge; since the - charge is closer, attraction wins out and the lone positive charge feels a slight attraction.

Now suppose instead I bring a lone negative charge close to the +/- pair. In this case the + charge will get pulled closer, and the - charge will get pushed farther away. Since the lone positive charge is attracted to the - and repelled by the +, it will again feel a slight net attraction since this time the - is closer.