Question
Mon June 06, 2011 By: Gaurav Avhad
 

why does opposite charges attract each other and why does the direction of electric field indicate the direction of decreasing potential?

Expert Reply
Thu June 09, 2011

everything is made up of atoms, which have positive and negative charges all throughout them. Certain materials are excellent conductors, which means that positive and negative charges are relatively free to move throughout a material. So if I bring a negatively charged rod close to a neutral conductor, the following happens:

 

 

The positive charges line up on the side closest to the rod (because they’re attracted to it) and the negative charges line up on the side farthest from the rod (because they’re repelled). Since the “opposite” charges are closer and the “like” charges are farther, this means that the force from the opposite charges is slightly stronger, and so overall, the negatively charged rodattracts the neutral conductor.

Now, here’s the weird thing: what if you charged up the metal with some negative charges? Would it repel the rod, since negative charges repel? Or would it still attract the rod, since the “opposite” charges are closer than the “like” charges? Well, the answer is both!

When the distance is large, they repel each other. But when you bring them close enough, the fact that the opposite charges are closer becomes more important than the fact that there are more like charges, and they attract! There’s even one perfectly balanced point where the force is exactly zero!
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