# Homework Help: Amount of energy stored in a magnetic field

1. Apr 6, 2009

### rinarez7

1. 1. An air-core solenoid with 57 turns is 4.96 cm
long and has a diameter of 1.46 cm.
The permitivity of free space is 4×10−7 T·
m/A.
How much energy is stored in its magnetic
field when it carries a current of 0.634 A ?
2.
B = mu-o (I) (N)/ L
Induction= mu-0 (N^2/l) A
U= (1/2) Induction (I ^2)

3. First I calculated Induction= 4pie-7 ( 57turns ^2/ .0496m) (pi(.0146^2))= 1.3785e-5

Then I used U = (1/2) induction (0.634 A ^2)= 2.7355μJ

I am on the wrong path? I thought of calculating the magentic field as well
using my first eqaution= 9.09795 e-4 T but I couldn't find the correct eqaution/ relationship to calculate the energy stored. Thanks in advance for any help!

2. Apr 6, 2009

### rl.bhat

0.0146 is the diameter, not the radius.

3. Apr 6, 2009

### rinarez7

My mistake, I did use the diameter/ 2 in my calculations ( jsut translated it incorrectly) so I still had the same calculation. Is there something else I am missing?

4. Apr 6, 2009

### rl.bhat

Check the calculation of inductance.