# Homework Help: Electric potential

1. Sep 24, 2010

### popo902

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

What are (a) the charge (in C) and (b) the charge density on the surface of a conducting sphere of radius 0.21 m whose potential is 290 V (with V = 0 at infinity)?

2. Relevant equations

v = kq/r
$$\sigma$$ = q/SA
SA of a sphere = 4$$\pi$$r^2

3. The attempt at a solution
so this is what i did
i used the first equation and solved for q = Vr/k
i plugged in the numbers
v= 290, r is the same as the radius because its only the surface r = 0.21,
k = 1/(4$$\pi\epsilon$$)
I got 5.5e11

Then for the charge density,
i put the numbers in too
but this is also wrong for sure because it uses my q value
I got 9.9e11

I don't know how i got that far off

2. Sep 24, 2010

### ehild

How did you get that value for q?

ehild

3. Sep 24, 2010

### popo902

yeh sorry i realized i divided by 1/k istead of multiplying by k/1 :S