#### transgalactic

##### Banned

**toroid question..**

http://i31.tinypic.com/2061tue.jpg

i tried to solve A like this:

[latex]B=\frac{\mu _0NI}{2\pi r}\\[/latex]

[latex]\phi=BA=LI\\[/latex]

[latex]A=(b-a)h\\[/latex]

[latex]\phi=\frac{\mu _0NI}{2\pi a}(b-a)h=LI[/latex]

I (current) cancels out

where is the mistake here

the solution is totaly different

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