# Homework Help: Electromagnetism: Possible Trick Question?

1. Feb 11, 2012

### Xyius

The nature of this problem seems like it is elementary E&M, but it is in my graduate electromagnetism class so I didn't know the proper section to post it.

1. The problem statement, all variables and given/known data
Two small conducting spheres have charges of 2x10^-9 C and -0.4x10^-9 C.

a.) When they are placed 4 cm apart what is the force between them?
b.) If they are brought into contact, and then separated by 4 cm, what is the force between them? [Note: Indicate attractive or repulsive force.]

2. Relevant equations
Coulombs Law I assume.

3. The attempt at a solution
It seems to me that you should treat these small spheres as point charges and simply use coulombs law to find the force. The tricky part is, how would bringing them together and then separating them change the force between them if they are the same distance apart?

The only thing I can think of is that for part A, you simply calculate the electrostatic force between the spheres, treating them as point charges. For part B I am thinking that the force between them might be zero because you would have to be pulling them apart with an equal an opposite force to keep them from coming into contact with each other. Does this seem too abstract? I can't imagine the answer to B being the same as the answer to A unless its a trick question!

2. Feb 11, 2012

### technician

When they touch charge will be transferred (the spheres are conducting)

3. Feb 11, 2012

### Xyius

Excellent point! I knew there was something to this problem..

Thanks!