# Homework Help: Calculate capacitance of conductors

1. Jul 25, 2012

### s-f

Problem: (a) What is the capacitance of the 2 conductors +20nC and -20nC? (b) If the voltage difference between the 2 conductors increase to 250 V, then what else changes?

a). I calculated that ΔV = 100 V. So then I plugged into the formula C = Q/ΔV. I thought 20 nC = 2 x 10^-8 C, which divided by 100 V = 2 x 10^-9C but the answer is 2 x 10^-10 C. What am I doing wrong? I know this is very simple but I have never studied physics before and am terrible at math.

b). charge of the conductors increases to keep capacitance constant, since capacitance is a property of area and distance between plates (I think this is right but please tell me if it's wrong!)

2. Jul 25, 2012

### Staff: Mentor

Welcome to the PF.

Where did the 100V come from? Is it given for part a)?

3. Jul 25, 2012

### s-f

Thanks. Actually the problem had more steps before this and Ohm's Law was used to calculate Voltage = 100 V (I have the answers so I know it's correct). The part I'm unsure of is how C = 2 x 10^-10 Farads (not Coulombs - I accidentally wrote C instead of F earlier!).

4. Jul 30, 2012

### s-f

Anyone? I would really appreciate it : )