Electromagnetic Induction and Alternating Current

A bar magnet is held vertically above a loop of wire. It is dropped and falls freely through the loop as shown in the following diagram.

As the magnet approaches the loop from above and then falls through the loop, the direction of the induced current through the loop (when viewed from above) is:

• anticlockwise

• clockwise

• clockwise then anticlockwise

• anticlockwise then clockwise

Answer explanation

As the magnet approaches the loop, the magnetic flux through the loop in the upward direction is increasing. Lenz’s Law states that the induced current will flow in a direction to oppose this increase in flux. The induced magnetic field will be in a downward direction through the loop, and so the right-hand-grip rule indicates that the induced current must flow in a clockwise direction.

After the magnet has passed through the loop and moves away from it, the flux through the loop in the upward direction is decreasing, so the induced field acts in an upward direction to balance the decreasing flux. The induced current then flows in an anticlockwise direction to produce an induced field in the upward direction.

Which of the following would increase the EMF supplied by a generator?

• Decreasing the area of the coil

• Decreasing the time for one revolution

• Decreasing the number of turns in the coil

• Decreasing the strength of the magnetic field

Answer explanation

Turning the coil at a faster rate reduces delta t in the Faraday's Law equation.

When the coil of a generator is rotated at a rate of 5 Hz, the maximum EMF generated is 2.5 V. If the coil is now rotated at 10 Hz, the maximum EMF generated is:

• 2.5 V

• 1.25V

• 6.25V

• 5.0V

Answer explanation

If the coil is rotated twice as fast, the EMF generated will double (because delta t is halved in Faraday's Law)

x depicts....

• RMS Voltage

• Peak Voltage

• Peak to peak voltage

• Mean Voltage

x depicts....

• RMS Voltage

• Peak Voltage

• Peak to peak voltage

• Mean Voltage

What does X represent?

• Wavelength

• Period

• Both

• Neither

From the upper to the lower image, which parameter/s is/are doubled?

• Amplitude

• Period

• Frequency

• Peak Voltage

From the upper to the lower image which parameter/s is/are doubled?

• Frequency

• Period

• Amplitude

• RMS voltage

A rectangular loop takes 3.0 s to fully enter a perpendicular magnetic field of strength 0.82 T, as shown in the following diagram. The loop has a resistance of 0.60 Ω.

The magnitude of the EMF induced in the loop is:

• 0.098 V

• 0.27 V

• 0.033 V

• 0.12 V

The loop is moved fully into the field and then withdrawn fully from the field.

The induced current through the loop flows:

• clockwise

• anticlockwise

• clockwise then anticlockwise

• anticlockwise then clockwise