If a sine wave is produced by a rotating loop in a magnetic field, what waveform is observed?

• A. DC
• B. Sine wave
• C. Triangle wave
• D. Square wave

Answer: (B)

When a loop rotates in a steady magnetic field, the magnetic flux through the loop changes smoothly as the loop turns. Faraday’s law says the induced emf is proportional to the rate of change of this flux. If the rotation is uniform, the flux varies like cos(ωt), so its rate of change is proportional to sin(ωt). That makes the output voltage a sinusoidal (sine) waveform. The amplitude depends on the field strength, loop area, number of turns, and angular speed. DC would require constant flux (no rotation), and triangle or square waves would require nonuniform motion or a nonuniform field, which isn’t the case here.