# Energy density of a capacitor

## Homework Statement

A parallel-plate capacitor with C = 16 mu or micro FF is charged so as to contain 1.7 J of energy. If the capacitor has a vacuum between plates that are spaced by 0.35 mm, what is the energy density (the energy per unit volume)?

I am a little lost of this problem and have no idea if I'm doing it correctly. Am I ?

v = A*d
E,density=U/v

## The Attempt at a Solution

v=16e-3/.35e-3=45.71
E=1.7/45.71=.0371 J/m^3

ehild
Homework Helper

## Homework Statement

A parallel-plate capacitor with C = 16 mu or micro FF is charged so as to contain 1.7 J of energy. If the capacitor has a vacuum between plates that are spaced by 0.35 mm, what is the energy density (the energy per unit volume)?

I am a little lost of this problem and have no idea if I'm doing it correctly. Am I ?

v = A*d
E,density=U/v

## The Attempt at a Solution

v=16e-3/.35e-3=45.71 ????
E=1.7/45.71=.0371 J/m^3

Is the capacitance 16 μF (microfarad)?

How do you calculate the volume?

ehild

Yes, I believe that is (microfarad); it doesn't say specifically.
I was very unsure on that step and wasn't quite sure how to compute the volume. Do I even to find the volume?

ehild
Homework Helper
As yo have written, energy density is energy divided by volume. So you need it.

ehild

Okay so how do I find the area in order to find volume.

ehild
Homework Helper
What is the formula for the capacitance?

ehild

C=(k* epsilon_0*A)/d

The volume between the plates is given by Ad where 'A' is area of the plates and 'd' is separation between them . 'd' is given whereas 'A' is unknown .

Using C= ε0A/d ,find the value of A.

Now,energy density = Energy stored/volume .