A rectangular coil of N turns and of length a and width b is rotated at frequency f in a uniform magnetic field of magnitude B, as indicated in the figure. The coil is connected to co-rotating cylinders, against which metal brushes slide to make contact. The emf induced in the coil is given (as a function of time t) by 8 = 2xifNabBsin(2aft) = zosin(2aft). This is the principle of the commercial alternating-current generator. What value of Nab gives an emf with to = 256 V when the loop is rotated at 46.8 rev/s in a uniform magnetic field of 0.574 T? Sliding contacts BXXXXXXXXXX XXXX X X X b - R X X XXXXXXX X XXXXXXXXX XX Number Units