What is the surface charge density sigma?

1. Mar 2, 2008

dgoudie

[SOLVED] Electric Potential

1. The problem statement, all variables and given/known data
20. [1pt]
A 30.6- cm-diameter conducting sphere is charged to 538 V relative to V = 0 at r =infinity. What is the surface charge density sigma?

Correct, computer gets: 3.11E-08 C/m^2 = Sigma, r=.153

21. [1pt]
At what distance will the potential due to the sphere be only 11.0 V?

2. Relevant equations
$$E=\sigma / \epsilon_{0} and E=V/r$$

3. The attempt at a solution
I already found sigma for Question number 20, and for 21 I am trying the same method, but am just solving for r.

so I got $$r=(V/\sigma) *\epsilon_{0}$$
and everytime I do it I come out with r=0.00313 m which is smaller than the initial r, even though I know from this statement "538 V relative to V = 0 at r =infinity" that if V is smaller r must be bigger.

What am I missing? Thanks in advance

2. Mar 2, 2008

Shooting Star

I'm not very sure what you mean by saying "computer gets". Do you know the concept you're applying?

The next question has nothing to do with sigma, but to do with the total charge. Find that.