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## Homework Statement

The magnetic field inside an air-filled solenoid 39.9cm long and 2.00cm in diameter is 0.800T. Approximately how much energy is stored in this field?

## Homework Equations

U=(1/2)LI^2

=> U = (1/2)(B^2)(area*length)/(u)

## The Attempt at a Solution

I discovered how to derive the bottom equation from the top equation from http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/indeng.html#c2

The derivation on the site seems to make sense, but when I try to use the bottom equation I get:

U = (1/2)(0.8^2)(Pi*0.01^2)*(0.399)/(8.85*10^-7) = 45.324 J

What am I missing here?

Thanks,

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