4.I.7G

Electromagnetism
Part IB, 2006

Starting from Maxwell's equations, deduce Faraday's law of induction

dΦdt=ε\frac{d \Phi}{d t}=-\varepsilon

for a moving circuit CC, where Φ\Phi is the flux of B\mathbf{B} through the circuit and where the EMF ε\varepsilon is defined to be

ε=C(E+v×B)dr\varepsilon=\oint_{C}(\mathbf{E}+\mathbf{v} \times \mathbf{B}) \cdot d \mathbf{r}

with v(r)\mathbf{v}(\mathbf{r}) denoting the velocity of a point r\mathbf{r} of CC.

[Hint: consider the closed surface consisting of the surface S(t)S(t) bounded by C(t)C(t), the surface S(t+δt)S(t+\delta t) bounded by C(t+δt)C(t+\delta t) and the surface SS^{\prime} stretching from C(t)C(t) to C(t+δt)C(t+\delta t). Show that the flux of B\mathbf{B} through SS^{\prime} is CB(v×dr)δt-\oint_{C} \mathbf{B} \cdot(\mathbf{v} \times d \mathbf{r}) \delta t.]