Electric Field/Centripital Force/Gauss law question?

[momatik]

Homework Statement

A proton with speed v = 3.42 x 106 m/s orbits just outside a charged sphere of radius r = 0.862 cm. What is the charge in coulombs on the sphere?

Homework Equations

(k*q1*q2)/r^2

F = mv^2/r

Gauss Law for sphere, possibly? I'm not sure if I need this.

The Attempt at a Solution

setting the equations equal to each other and then solving for q2

kq1q2/r2 = mv^2/r

rearranged...

q2= mv^2r/kq1

m = mass of proton = 1.672e-27
q1 = charge of proton = 1.602e-19
v = 3.42e6
r = .00862 m

Values subbed in...

q2 = [(1.672e-27)*(3.42e6^2)*(.00862)] / [(9e9)*(1.602e-19)]I get 1.1696e-7 which is incorrect.

 

[gabbagabbahey]

I haven't checked your numbers, but you seem to be forgetting that force is a vector, not a scalar. You need to consider the direction of both the centripetal and Coulomb forces...what sign must q2 be for the Coulomb force to be responsible for the centripetal acceleration of the proton?

 

[momatik]

Wow Yeah, the answer simply had to be negative. I justifiably feel pretty stupid right now.

Thanks for your help.