AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
Exam Practice: Electric motors
Electric motors · Magnetism and electromagnetism · Lesson 4 of 8 · 14 marks · 15 minutes
|
Name |
|
Date |
|
Instructions
• Answer all the questions.
• Write your answers in the spaces provided.
• The marks for each question are shown in brackets - use this as a guide to how much to write.
• The answers are on separate pages at the back. Attempt every question before you look at them.
• Answer all questions. Use the mark allocation as a guide to how much to write.
Question 1 STATE (2 marks)
State two ways of increasing the speed of rotation of a d.c. motor.
|
|
|
|
|
|
(Total for Question 1 = 2 marks)
Question 2 EXPLAIN (3 marks)
Explain why the coil in a d.c. motor turns when a current flows through it.
|
|
|
|
|
|
|
|
(Total for Question 2 = 3 marks)
Question 3 EXPLAIN (3 marks)
Explain the purpose of the split-ring commutator in a d.c. motor.
|
|
|
|
|
|
|
|
(Total for Question 3 = 3 marks)
Question 4 EXPLAIN (2 marks)
The battery in a d.c. motor is connected the other way round. Describe and explain the effect.
|
|
|
|
|
|
(Total for Question 4 = 2 marks)
Question 5 EXPLAIN (4 marks)
A motor rotates slowly. Explain two changes that would make it rotate faster.
|
|
|
|
|
|
|
|
|
|
|
|
(Total for Question 5 = 4 marks)
TOTAL FOR PAPER = 14 MARKS
|
|
Answers and mark scheme
Check your answer only once you have written one.
Question 1 (2 marks)
Increase the current. Use stronger magnets. Use more turns. (any two)
• One way 1 mark
• Another way 1 mark
Question 2 (3 marks)
The current in the coil is at right angles to the magnetic field so each long side of the coil experiences a force. The current in the two sides is in opposite directions, so the forces are in opposite directions and turn the coil.
• Coil in a field carrying a current experiences forces 1 mark
• Current in opposite directions in the two sides 1 mark
• Opposite forces turn the coil 1 mark
Question 3 (3 marks)
It reverses the direction of the current in the coil every half turn so that the forces keep turning the coil in the same direction, giving continuous rotation.
• Reverses the current 1 mark
• Every half turn 1 mark
• Keeps the coil turning the same way 1 mark
Question 4 (2 marks)
The current in the coil is reversed, so the forces are reversed and the motor turns in the opposite direction.
• Turns in the opposite direction 1 mark
• Current (forces) reversed 1 mark
Question 5 (4 marks)
Increase the current, which increases the force on the coil. Use stronger magnets, which increases the magnetic flux density so the force is larger. More turns also increase the force.
• Increase the current 1 mark
• Larger force 1 mark
• Stronger magnets or more turns 1 mark
• Larger force or flux density 1 mark