7.4 Electric motors
9 exam questions worth 28 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the job of the commutator in a d.c. motor?
Choose one answer.
Correct Not quite: the answer is To reverse the current in the coil every half turn
Mark scheme
B: To reverse the current in the coil every half turn
Model answer
To reverse the current in the coil every half turn (B).
Name the parts of a simple d.c. motor.
Mark scheme
any four from: coil (1); permanent magnets (1); split-ring commutator (1); brushes (1); d.c. supply (1); axle (1)
Model answer
A coil, two magnets, a split-ring commutator, brushes and a d.c. supply.
Explain why the coil in a motor turns when a current flows through it.
Mark scheme
the coil is in a magnetic field and carries a current, so each side experiences a force (1); the current flows in opposite directions on opposite sides of the coil, so the forces are in opposite directions (1); this produces a turning effect / moment on the coil (1)
Model answer
Each side of the coil carries a current in a magnetic field, so it experiences a force. The currents are in opposite directions, so the forces are opposite, and this turns the coil.
Explain what would happen to the coil if the commutator was replaced with two continuous slip rings.
Mark scheme
the current would not reverse every half turn (1); the coil would turn until it was vertical (1); then the forces would pull it back, so it would rock back and forth instead of continuing to rotate (1)
Model answer
The current would not be reversed every half turn. The coil would turn to the vertical position, and then the forces would pull it back, so it would swing back and forth.
State three ways of increasing the speed of a motor.
Mark scheme
increase the current (1); use a stronger magnet (1); use more turns on the coil (1)
Model answer
Increase the current, use stronger magnets, or use more turns of wire on the coil.
State two ways to make a motor turn in the opposite direction.
Mark scheme
reverse the current (1); reverse the magnetic field / swap the magnet poles (1)
Model answer
Swap the connections to the supply, or turn the magnets round.
Each side of a coil has a length of 0.040 m in a field of 0.50 T, carrying 3.0 A. The coil has 50 turns. Calculate the total force on one side of the coil.
Mark scheme
F per turn = B I l = 0.50 × 3.0 × 0.040 (1); 0.060 N (1); × 50 turns (1); 3.0 N (1)
Model answer
Force per turn = 0.50 × 3.0 × 0.040 = 0.060 N. With 50 turns, the force is 50 × 0.060 = 3.0 N.
Describe the energy transfers in an electric motor, including the wasted energy.
Mark scheme
electrical energy is transferred from the supply (1); to the kinetic energy store of the motor / its load (1); some is wasted as thermal energy (and sound) due to resistance / friction (1)
Model answer
Electrical energy from the supply is transferred to kinetic energy. Some is wasted as thermal energy, because of the resistance of the coil and friction in the bearings.
Describe how the forces on the two sides of a motor's coil keep the coil rotating in the same direction, using the commutator.
Mark scheme
the current is in opposite directions on opposite sides (1); the forces on the two sides are opposite, giving a turning effect (1); when the coil reaches the vertical position the commutator switches the brush contacts (1); the current in each side reverses (1); so the forces continue to turn the coil in the same direction (1)
Model answer
The current flows in opposite directions along the two sides of the coil, so the forces are opposite and turn the coil. When the coil passes the vertical position, the commutator swaps the connections so the current in each side reverses. The forces therefore carry on turning the coil in the same direction.
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