# Determine Charge with Spring between Particles

1. Feb 5, 2012

### kgal

1. The problem statement, all variables and given/known data
Two initially uncharged identical metal spheres, 1 and 2, are connected by an insulating spring (unstreched length L0 = 1.55 m, spring constant k=30.0 N/m).

Charges +q and -q are then placed on the spheres and the spring contracts to length L = 1.25 m.

a. Determine the charge q.
b. If the spring is coated with metal to make it conducting, what is the new length of the spring?

2. Relevant equations

Fs = -kΔx
Fe = 1 / 4∏ε0 * (+q)(-q) / r^2
m1 = m2

3. The attempt at a solution

I attached my work.
for a. what I basically did was set Fe = Fs and solved for q.
I got 3.16*10^-5 C, but the answer should be 3.96*10^-5 C. Could it be because of truncated significant figures?

for b. How can I show that coating the spring in metal cause the metal ball to return to L0?

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2. Feb 5, 2012

### Spinnor

You dropped r^2.

1.25^2 * your result is what you want?

3. Feb 5, 2012