# Finding Magnetic Field from a Box

1. Apr 9, 2015

### Shinwasha

1. The problem statement, all variables and given/known data
A uniform magnetic field exists in a cubic volume of space with a
50-mm side length.
If the magnetic energy stored in this volume is
19J , what is the magnetic field magnitude?
(This is all the information I got. Been a bad week for since it just seem information for my homework is missing)

2. Relevant equations
This is where things are a little shaky. 0Al

UB=1/2*B^2/μ

3. The attempt at a solution
Taking the length I used it to find the area of the cube. Which is what I take A to be, which I find weird considering I needed l as well.
Setting it up I got

(19*2*4piE-17)/(0.015)(5E-4) = B^2

B = 2.51E-5 T

2. Apr 9, 2015

### deedsy

I believe the equation you started with is the energy density, not just the Energy (check the units)

$\frac{U}{Volume of cube} = \frac{1}{2} \frac{B^2}{\mu}$

3. Apr 9, 2015

### deedsy

actually, it looks like you did divide by the volume in your calculation, but the volume of the cube should just be (side length)^3.