# Homework Help: Max speed -> electron

1. Apr 14, 2010

### trignum07

max speed --> electron

1. An electron starts from rest 15.0 cm from the center of a uniformly charged insulating sphere of radius 10.0 cm and total negative charge -3.00 μC.What is the maximum speed (in m/s) the electron can reach after releasing it? (Note: Assume a reference level of the potential V = 0 at r = ∞)

2. Relevant equations
ke = 8.99e9 Nm^2/C^2
k = 1/2 mv^2
E = k*Q/r^2
e = - 1.6e-19 C
me = 9.11e-31 kg

3. The attempt at a solution
I can't figure out what my Q will be in this problem, because I believe Q refers to the total charge of the sphere, not total negative charge of the sphere.

2. Apr 14, 2010

### collinsmark

Re: max speed --> electron

If I had to guess, I would agree with you. I'm guessing that what is meant is that both the electron and the sphere are negatively charged. So I'm guessing what the problem means to say is, "total charge [of sphere] -3.00 μC." I'm guessing the word 'negative' was put in there just to emphasize that sphere is negatively charged.

On the other hand, there is enough information given in the problem to work it out the other way, with the sphere being positively charged. I sort of doubt this is what the problem's author meant, but there is enough information to work it out both ways if you wanted to.

Asking your instructor might be the best way to clear things up for sure. But if you're pressed on time, I'd initially assume the problem means "total charge of sphere is -3.00 μC."