When a wire carries an AC current with a known frequency, you can use a Rogowski coil

Question:

When a wire carries an AC current with a known frequency, you can use a Rogowski coil to determine the amplitude I max of the current without disconnecting the wire to shunt the current in a meter. The Rogowski coil, shown in Figure P31.16, simply clips around the wire. It consists of a toroidal conductor wrapped around a circular return cord. The toroid has n turns per unit length and a cross-sectional area A. The current to be measured is given by I (t) = I max sin wt.
(a) Show that the amplitude of the emf induced in the Rogowski coil is Imax = /0nAw Imax.
(b) Explain why the wire carrying the unknown current need not be at the center of the Rogowski coil, and why the coil will not respond to nearby currents that it does not enclose.

(t)