Motor Effect

A wire is placed between the poles of a horseshoe magnet. There is a current in the wire in the direction shown, and this causes a force to act on the wire. Three other arrangements, P, Q, and R, of the wire and magnet are set up as shown. Which arrangement or arrangements will cause a force in the same direction as the original arrangement?

• P, Q, and R

• P and Q only

• P only

• R only

A toy railway engine is driven around a track by a d.c. electric motor. How can the speed of the motor be increased?

• Use a motor made with fewer turns of wire.

• Use a smaller d.c. voltage.

• Use a stronger magnet in the motor.

• Use the supply with its connections reversed.

A coil carries a current in a magnetic field. The coil experiences a turning effect. Which device uses this effect?

• a d.c. motor

• an electromagnet

• a relay

• a transformer

The diagram shows a flat, rectangular coil placed between the poles of a magnet. There is a current in the coil that makes it turn in the direction shown in the diagram. Which change would make the coil turn in the opposite direction?

• Decreasing the current in the coil

• Increasing the number of turns on the coil

• Reversing both the direction of the current in the coil and the poles of the magnet

• Reversing only the direction of the current in the coil

A current-carrying wire XY lies in the magnetic field between the two poles of a U-shaped electromagnet. A force acts on the wire XY because of the magnetic field. Each of the following actions is carried out separately:

A. The current in the wire XY is reversed
B. The magnetic field is reversed
C. both the current in the wire XY and the magnetic field are reversed at the same time.

How many of these actions cause the direction of the force on the wire XY to be reversed?

• 0

• 1

• 2

• 3

The diagram shows a simple d.c. electric motor which is rotating. What change would increase the speed of rotation?

• Increasing the number of turns on the coil

• Removing the magnets

• Reversing the battery

• Reversing the polarity of the magnets

A metal rod PQ rests on two horizontal metal wires that are attached to a battery. The rod lies between the poles of a magnet. When the switch is closed the metal rod moves right, what change would make the rod move left?

• Open the switch.

• Reverse the battery terminals and exchange the poles of the magnet.

• Reverse the battery terminals but without exchanging the poles of the magnet.

• Turn the metal rod around (P and Q exchanged).

The diagram shows a wire placed between two magnetic poles of equal strength. A current passes through the wire in the direction shown. The current causes a downward force on the wire. What is the arrangement of the magnetic poles?

• S - N

• N - S

• N - N

• S - S

In which direction does a force act on the electron when it enters the magnetic field?

• into the page

• out of the page

• towards the bottom of the page

• towards the top of the page

Why might a current-carrying wire be placed between a magnetic field?

• To increase the current from the interaction of magnetic fields

• To increase the voltage from the interaction of magnetic fields

• To produce a force from the interaction of magnetic fields