4.5.4+4.4.5: Forces on a current-carrying wire and d.c. motor

A simple d.c. electric motor is shown in the diagram.

Which of the following changes would make the coil turn more quickly?

• Reducing the strength of the magnetic field

• Reversing the direction of the current and swapping the magnetic poles

• Swapping the magnetic poles

• Increasing the current in the coil

A class is designing a d.c. motor. To achieve a greater turning effect, three suggestions are made.

1. Have larger current in the coil of the motor

2. 2. Have a stronger magnet in the motor

3. 3. Put a larger number of turns on the coil

4. Which suggestions will help to increase the turning effect?

• 1, 2 and 3

• 1 and 2 only

• 1 and 3 only

• 2 and 3 only

A student makes a small d.c. motor. The teacher supplies the following equipment

1. 1. battery

2. 2. ammeter

3. 3. voltmeter

4. 4. coil of wire

5. 5. magnets

6. 6. resistor

7. The student is also able to use other equipment if he wishes.

8. Which equipment must be used?

• ammeter, battery and resistor

• ammeter, coil of wire and voltmeter

• battery, coil of wire and magnets

• magnets, resistor and voltmeter

What is the role of the split ring commutator in a d.c. motor?

• The split ring commutator increases the speed of the motor.

• The split ring commutator regulates the voltage in the motor.

• The split ring commutator reverses the direction of current in the armature coil.

• The split ring commutator reduces the friction in the motor.

What happens if the direction of the current in the armature coil is reversed in a d.c. motor?

• The motor will rotate in the opposite direction for a short period and then stop.

• The direction of the motor's rotation will be reversed.

• The motor will rotate faster.

• The motor will stop working.

The diagram shows the magnetic field pattern due to a current in a solenoid. Which statement is correct?

• The strength of the field is greater at K than it is at L

• The strength of the field is greater at M than it is at L

• The strength of the field is the same at K as it is at M

• The strength of the field is the same at L as it is at M

A current-carrying wire is placed between the due to the current try to move the wire?

• A

• B

• C

• D

The diagram shows a current in a conductor in a magnetic field. What is the direction of the force on the conductor?

• into the page

• out of the page

• towards the N-pole

• towards the S-pole

The diagram shows a coil in a magnetic field. When the coil is part of a d.c. motor, what must be connected directly to X and Y?

• d.c. supply

• slip rings

• soft-iron core

• split-ring commutator

Which diagram shows the direction of the force F that is produced by a current I in a magnetic field?

• A

• B

• C

• D