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darksyesider
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Homework Statement
A toroid with N windings and radius R has cross sectional radius x (x<<R).
The current running through the wires is given by ##I = I_0\sin (\omega t)##.
There is a magnetic field in the center of the toroid.
A loop of wire of radius three times that of the toroid is placed around the cross section of the toroid
(like http://s3.amazonaws.com/answer-board-image/9fa3691b-46c6-4de7-b608-82705d9efa60.gif, but it's circular, not a rectangle).
Find the induced emf in the circular loop of wire.
Homework Equations
E = -dphi/dt
The Attempt at a Solution
##\int B\cdot ds = \mu_0 I\implies B(2\pi R) = N\mu_0 I \implies B = \dfrac{\mu_0 N (I_0\sin\omega t)}{2\pi R}## (*)
then ##\mathcal{E} = \dfrac{- d\phi}{dt} = - \dfrac{d(BA)}{dt} = -\pi x^2 \dfrac{dB}{dt}##, where dB/dt is just the derivative of (*).
My question is, that should it be ##-\pi x^2## (the cross section of the toroid) or should it be the area of the loop ( pi*(3x)^2) ?
My reasoning for pi*x^2 is that since there is no flux at parts of the circular loop, it's just the area of the cross section (no flux since no magnetic field!).