Potential energy chapter

1. Feb 9, 2010

gwenle1

1. Four charges are placed at the corners of the square where the opp diagonals have +2microcoulombs and -2 coulombs. what is the electrostatic energy of the system? ANS: -4.7 J

2. Two conducting spheres that carry equal charges are very far from each other and from all other charges. One of these spheres has twice the radius of the other one. If the potential of the smaller sphere (relative to infinity) is 100V, the potential of the arger sphere (relative to infinity) is closest to.... ANS: 100V, 200V, 50V, 25V, or 400V

3. three conducting spheres of radii 1m, 2m and 3m are connected by wires 50 meters long in a triangle with the angles 60 degrees each. a charge of 4x10^-4 C is initially placed on teh large sphere. determine the charge on teh large sphere in equilibrium. ANS 2.0 x 10^-4

The attempt at a solution
1) I tried using PE = kq1q1/R... found the sides and the diagonals but I got an answer of zero, which is wrong.

2) I found the net energy of both spheres, and then used this energy with the given potential to find the potential of the larger sphere, but i think im rearranging wrong?

3) i drew a FBD diagram and did horizontal and vertical components on teh larger sphere, but I got a much larger number as my answer.

2. Feb 9, 2010

sArGe99

You will have 4C2 combinations to consider.
Two of these will be diagonally opposite pairs and all others will be adjacent pairs.

3. Feb 10, 2010

gwenle1

I have no idea why you would use combinations for my problems

4. Feb 11, 2010

sArGe99

Was just telling you there would be 6 pairs to consider.