Motor & Generator

A student pushes the N-pole of a bar magnet into the end Q of a long solenoid and observes a deflection to the right on the sensitive ammeter. What is produce a deflection in the same direction?

• pulling the N-pole out of end Q

• pulling the S-pole out of end P

• pushing the N-pole into the end P

• pushing the S-pole into end P

A simple a.c. generator produces an alternating e.m.f as shown.

The speed of generator is doubled. Which graph best represents the new output?

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A step-down transformer changes 240V a.c. to 12V a.c. There are 600 turns on the primary coil. How many turns are on the secondary coil?

• 20

• 30

• 600

• 12 000

Whihc statement about the action of a transformer is correct?

• an e.m.f is induced in the secondary coil when an alternating voltage is applied to the primary coil.

• an e.m.f is induced in the secondary coil when there is a steady direct current in the primary coil.

• the current in the secondary coil is always larger than the current in the primary coil.

• the voltage in the secondary coil is always larger than voltage in the primary coil.

The diagram shows how a magnet and a coil may be used to induce an electric current.

How could the ammeter reading be increased?

• Move the coil more slowly

• Put a resistor in series with the ammeter

• Turn the magnet round, then move the coil

• Use a coil with more turns

The diagram shows part of an a.c. generator when its coil is in a horizontal position.

The graph shows the voltage output plotted against time. Which point on the graph shows when the coil is in a vertical position?

• A

• B

• C

• D

The electromotive force (e.m.f) induced in a conductor moving at right-angles to a magnetic field does not depend upon

• the length of the conductor

• the resistance of the conductor

• the speed of the conductor

• the strength of magnetic field

The coil in an electric motor is wound onto a cylinder. Why is the cylinder made of soft iron?

• to deflect the magnetic field away from the coil

• to increase the current through the coil

• to increase the strength of the magnetic field through the coil.

• to support the coil and prevent it from collapsing

The diagram shows a d.c. motor. Why is split-ring commutator used?

• to change the current direction in the coil as the coil passes the horizontal position

• to change the current direction in the coil as the passes the vertical position

• to change the current direction in the d.c. supply supply as the coil passes the horizontal position

• to change the current direction in the d.c. supply as the coil passes the vertical position

Which is correct about a working transformer?

• The input voltage is d.c.

• The input voltage is greater than the output voltage

• The input voltage is less than the output voltage

• The input voltage is the same as the output voltage