Spherical Capacitor with Charge, Potential Difference and More

1. Oct 2, 2012

Northbysouth

1. The problem statement, all variables and given/known data
A spherical capacitor contains a charge of 3.10nC when connected to a potential difference of 220V . If its plates are separated by vacuum and the inner radius of the outer shell is 5.00cm .

Part A) Calculate the capacitance.

2. Relevant equations
C = Q/V

C = Aε0/d

3. The attempt at a solution

I tried plugging in the values into

C = Q/V

C = (3.1x10^-9 C)/220V
C = 1.409x10^-11 F

But it didn't accept this. Any suggestions?

2. Oct 2, 2012

nasu

Maybe a matter of significant digits?

3. Oct 2, 2012

4. Oct 2, 2012

Northbysouth

I'm stuck on the second part as well, it asks:

B) Calculate the Radius of the inner sphere

C = Aε0/d

where I believe that A is the area of a capacitor and d is the distance between them.

C = 4πr^3*ε0/d = 14.1

So I solved for r and I got

r = Cuberoot[(14.1x10^-12 F)*0.05 m/8.85x10^-12]

r = 0.2937 m = 29.37 cm

It says this is wrong and I don't know where to go from here.

5. Oct 2, 2012

nasu

This formula is valid for capacitors with plane, parallel plates.
For spherical capacitor there is a different formula.
You can look it up on Google if you don't have it.