# Homework Help: Touching Conducting Spheres of Different Size

1. Sep 25, 2011

### royblaze

1. The problem statement, all variables and given/known data

Two uncharged conductors A and B are of different sizes. They are charged as follows:

1. A is charged via an electrostatic generator to charge q

2. A is briefly touched to B.

3. Steps 1 and 2 are repeated until the charge on B reaches a maximum value.

If the final charge on B is 3q, what was the charge on A after the first time it touched B?

2. Relevant equations

Charge is conserved, and must be distributed evenly between conducting material (?).

3. The attempt at a solution

I know that when you have two conductors touch, the net charge between them should be shared. I used something along the lines of

(Ainitial + Binitial) / 2

To get what to "share" between the two. I'm just unsure if that's how to approach it.

2. Sep 25, 2011

### ehild

You are right, the charge will be shared. There is something that drives the charges from one sphere to the other one, till this something becomes equal for both spheres. What is that?

ehild

3. Sep 25, 2011

### royblaze

What is it? Um... the intrinsic nature of conductors to allow the free movement of electrons??

4. Sep 25, 2011

### ehild

Let's try the opposite question. When will not go any charge from one conductor to the other when they touch each other?

ehild

5. Sep 25, 2011

### royblaze

When they are of equal charge

6. Sep 25, 2011

### ehild

What do you think, will not charge flow from a very small sphere to a very big one when they have equal charge?

ehild

7. Sep 26, 2011

### royblaze

When they have equal charge... more charge will go into the larger one?? Because it can accommodate it??

8. Sep 26, 2011

### ehild

Yes, the bigger sphere can accommodate more charge, but why?

Have you studied Electrostatics? Have you heard about Coulomb's Law, electric field, electric potential, capacitors, how is the charge distributed on a conductor?
I suggest to read your notes and books. I can not help more.

ehild

9. Sep 26, 2011

### royblaze

Ah, I read up on conductors and equipotentials... the potentials are equal on both when they touch and so I can set up an equation equating them, taking the "loss" of q that is "put" on to the other conducting sphere.

Thanks! I got the answer. :D

10. Sep 26, 2011

### ehild

I am relieved ...

ehild