rior to about 1960, magnetic field strength was measured by means of a rotating

Question

rior to about 1960, magnetic field strength was measured by means of a rotating coil gaussmeter. This device used a small loop of many turns rotating on an axis perpendicular to the magnetic field at fairly high speed and connected to an ac voltmeter by means of slip rings like those shown in the figure below. The sensing coil for a rotating coil gaussmeter has 410 turns and an area of 1.4 cm2. The coil rotates at 170 rpm. If the magnetic field strength is 0.45 T, find the maximum induced emf in the coil and the orientation of the coil relative to the field for which this maximum induced emf occurs.

.............V

and

...................° to the magnetic field

Explanation / Answer

Induced EMF is given by:

EMF = -N*d(phi)/dt

phi = B.A = B*A*cos wt

EMF = -N*A*B*d(cos wt)/dt

EMF = N*A*B*w*sin wt

max EMF will be, when sin wt = 1

wt = 90 deg

EMF)max = N*A*B*w

Using given values:

w = 170 rpm = 170*2*pi/60 rad/sec = 17.80 rad/sec

EMF)max = 410*1.4*10^-4*0.45*17.80

EMF)max = 0.4597 = 0.46 V

Part B.

Max EMF occurs when wt = 90 deg

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