Determining charge of q and the spring coated with metal?

[Richard Ros]

Homework Statement

Two initially uncharged identical metal spheres, 1 and 2, are connected by an insulating spring (unstretched length L0 = 1.55 m, spring constant k = 30.0 N/m), as shown in the figure.
Charges +q and –q are then placed on the spheres, and the spring contracts to length L = 1.250 m. Recall that the force exerted by a spring is Fs = kΔx, where Δx is the change in the spring's length from its equilibrium length. {Correct Answer: a. 3.96e-05 C, b. 1.55 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?

Homework Equations

1/4πε0 * q1*q2/r^2
f = -kΔx

The Attempt at a Solution

I set fe = fs and got my answer for letter a as 3.16*10^-5 C, which is incorrect from the actual answer. Not sure why it's .8 difference in value. For letter b, I have no idea how to do b.

 

[Haye]

Your method for a is correct, you just forgot to write the 1/r².

For question b, you just need to think about what happens to the charges. What changes when you go from an insulated spring to a conducting one?