# Calculating capacitance -Got wrong answer

## Homework Statement

Find the capacitance of a capacitor that stores 320 MJ(MegaJoules) when the potential difference across its plates is 170V.

So basically q: is 320MJ
V: is 170V
I need to find C the capacitance.

## Homework Equations

To find the capacitance is q=CV.

## The Attempt at a Solution

I converted 320MJ to 320* 10 ^ 6 Joules for q. 320* 10 ^6 J is also 320 * 10 ^ 6 Coulombs. Let V the way it is 170V.

Divide (320 * 10 ^ 6 Coulombs) / (170 V) = 1882352.941 Farads. to convert farad to nanofarad I multiply the answer by 10^ 9 and got 1.882352941 * 10 ^ 15 nanofarads.
I put 1.88 * 10 ^ 15 nanofarads but apparently it is the wrong answer.

## The Attempt at a Solution

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rock.freak667
Homework Helper
In your equation Q=CV, Q is the charge stored.

You are given 320 MJ which is a unit of energy. What is equation for the energy stored by a capacitor?

In your equation Q=CV, Q is the charge stored.

You are given 320 MJ which is a unit of energy. What is equation for the energy stored by a capacitor?
that equation would be (1/2)C(V^2). but i do not see your point. i should be using that equation.

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you know the energy and you know the voltage so using this equation will give you the value of C.
Have you read the question carefully? You have the energy stored as 320 megaJoules.....that sounds totally unreasonable !!! it is about 100kWhr!!!!!