# Three Charges and a Spring

1. Oct 15, 2011

### Lunar_Lander

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

Charge Q1 is fixed to the ceiling by means of a spring with the spring constant D and is moved from its rest position (which is at y=h) by the two charges Q2. The gravitational force may be neglected.

a) Determine the resulting force on Q1 in dependency of y.

b) The distance of the two charges Q2 should now be zero (a=0). Determine the charge Q2 as a function of y so that the resulting force on the charge Q1 becomes zero.

2. Relevant equations

Field equations of radial electric fields, force of a spring.

3. The attempt at a solution

I do not really have an idea for a), for b) however I tried to equalize the electric force and the spring force and then I solved for Q2. That looked like this:
$Q_2=-\frac{D*4*\pi*\epsilon_0*y^3}{2*Q_1}$. Can this be right?

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