# Magnetic energy

1. Nov 21, 2009

### tomfrank

1. The problem statement, all variables and given/known data
Considering the earth magnetic field, I need to find the magnetic energy stored in our classroom.

2. Relevant equations

U= (1/2*$$\mu$$) B2

3. The attempt at a solution

Any suggestions?

2. Nov 21, 2009

### kuruman

Your expression is the energy density (Joules/m3). How many cubic meters is the volume of your classroom?

3. Nov 22, 2009

### tomfrank

around 300 cubic meters

4. Nov 22, 2009

### ideasrule

Do you still need help? You can find the value of B online, so you can find the energy density using your formula.

5. Nov 22, 2009

### tomfrank

so once I have B i just put in the equation and solve for U

and then I multiply U by the volume of the classroom?

where do I found accurate value of the magnetic field of the Earth online?

6. Nov 22, 2009

### kuruman

7. Nov 22, 2009

### tomfrank

U= (1/8*pi*10^-7 ) *(54,191.9 nT )^2 = 0.001168500483 (J/m^3)

now

U*Volume= Energy classroom = 0.00117 (J/m^3)* 300 = .35 Joule

is this right?

8. Nov 22, 2009

### kuruman

I didn't put in the numbers, but it looks right.

9. Nov 23, 2009

thank you