Problem 3. Calculating EMF: How great is the induced EMF? A uniform magnetic field is directed at an angle of 30 with the plane of a circular coil of radius 2 cm and 2000 turns. If the magnetic field changes at a rate of 4T per second, calculate the induced emf. Approach We are given the angle 30, but note that this is the angle of B with respect to the plane of the coil. Thus, the angle with respect to the area vector is 60. We also know the radius of the coil. Thus, we can calculate its area: A = Ir2 = (0.02m)2 = 1.26x10-3m2 The emf is induced because the flux is changing. In this case, the reason for the change in flux is the dB increasing magnetic field (we know this because the rate of change given is positive) at at S . We can write this into the law of induction. N 8 = -N d(BAcos60") dt dt The factor A cos 60 is not part of the change so we can take it out of the parentheses. dB E = -NAcos60 dt