# Energy in a volume

## Homework Statement

If V=2x^2+6y^2 V in free space, find the energy stored in a volume defined by -1<x<1,-1<y<y, and -1<z<1. (BTW the < are suppose to less than or equal to or greater than or equal to)

## The Attempt at a Solution

I am not really sure of the formula here but I assume it needed at triple integral as it gave me 3 sets of limits and a function. Furthermore it asked for a volume so it was the only thing I could think of I relize it is probably wrong but am I even close? If not what formula should I use?

#### Attachments

• EPSON029.jpg
8.9 KB · Views: 543

mfb
Mentor
A triple integral is good, but you have to find out what to integrate first.

I'm not sure how to interpret "V=2x^2+6y^2 V", but it looks like an electric potential, so we have an electric field in that volume. What is the energy density of an electric field?
Are you sure there are no units involved?

Oh sorry yah the V stands or volts so yes it is electric potential. (1/2)(epsilon)E^2 equals energy density of an electric field

mfb
Mentor
Good, now you can use that to solve the problem.

So what your saying is that E would equal electric potential here or should I use dV=-Es(dS)

Thus I would take the derivative of the voltage

mfb
Mentor
Thus I would take the derivative of the voltage
That's the idea, right.

Well what should I derive the function by x or y I'm guessing x

gneill
Mentor
Well what should I derive the function by x or y I'm guessing x

x,y, and z, actually. You want to determine the electric field, a vector quantity, by finding the gradient of the potential. Presumably you'll then use the magnitude of this vector in your energy density evaluation (triple integral over the volume).

Ah ok man thanks so partial derivative with respect to x y and z then integrate that three times with the limits given got it will do later on

Well the gradient is (4x,12y) if I integrate it straight I get 0 which can't be right . I don't know if it is mathematically legal for me to e the agnitude with the x and y arable still in it but ill try anyway see if that does the trick I gets 101.19288

mfb
Mentor
Remember that you have to square its magnitude:
Oh sorry yah the V stands or volts so yes it is electric potential. (1/2)(epsilon)E^2 equals energy density of an electric field

6e-9

I got the magnitude to be sqrt(4^2+12^2) which is sqrt(160)=E so then E^2=160 which when I integrate I get 6e-9

Where epsilon is 8.85e-12

mfb
Mentor
(4x,12y) depends on x and y, and so does the magnitude. In addition, you are missing the z-component here (it is zero, but you have to include it to get a vector with 3 components).

I get E=sqrt((4x)^2+(12y)^2)=sqrt(16x^2+144y^2) then E^2=16x^2+144y^2 after integrating I get (1/2)epsilon(1280/3)=2e-9

Am I ok not trying to rush you just wondering?

mfb
Mentor
You can check calculations with a computer, so what are you waiting for?
The answer has missing units, but the problem statement has the same problem.

Ok thanks man yah u are right wolfram said it was ok. I guess I said it wrong did I have the integral set up correctly so that when I performed the integration it would be correct?

mfb
Mentor
That looks good.

1 person
Ok thanks man

oh hey I know this post is old but I caught a mistake I believe since dV=-E dot dS then the answer shourld be negative 2E-9 also what should the units be Joules?

mfb
Mentor
You square the electric field, its direction does not matter, and the energy density is always positive.

Ah yah your right didnt consider that it was squared.