(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

http://www.aijaa.com/img/b/00106/3735947.jpg [Broken]

2. Relevant equations

[tex] \epsilon =- \frac {d \Phi}{dt}=- \frac {d BA}{dt}= \oint \vec{E} \cdot d \vec{l} [/tex]

[tex] B= \mu _0nIA [/tex], where n is solenoid's turns per meter.

3. The attempt at a solution

I was able to solve this through trial and error, almost. What I got is [tex] \epsilon =- \frac { \mu _0 nAdI}{dt} \Longrightarrow \frac {dI}{dt}=- \frac {E2 \pi r_1}{ \mu _0n \pi r_{2}^2}=-9.21 \frac{A}{s}[/tex], where r_{1}=0.035m and r_{2}=0.011m. Our assistant teacher wasn't able to tell me why we to take the line integral over a circle in [tex] \epsilon = \oint \vec{E} \cdot d \vec{l} [/tex] and why my sign is wrong. Can somebody help?

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# The rate of current through a solenoid - clarification needed.

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