Chapter 10 Practice Paper: Electromagnetism — question paper (print)

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Secondary 4 · Physics (G2 / G3)

Chapter 10 Practice Paper

Electromagnetism

Duration: 40 minutes

Total: 30 marks

Name:

Class:

Date:

READ THESE INSTRUCTIONS FIRST

Write your name, class and register number in the spaces at the top of this page.
Answer all questions.
Give numerical answers correct to 3 significant figures unless stated otherwise, with units where a quantity is calculated.
The number of marks is given in brackets [ ] at the end of each question or part question.
The total number of marks for this paper is 30.

Original practice material written for the MOE Physics syllabus. It is not an official MOE or school examination paper. Magnetic field of a current-carrying wire and solenoid, the right-hand grip rule, electromagnets and their uses (relay, electric bell), factors affecting solenoid strength, the motor effect F = BIL with Fleming’s left-hand rule, and the simple d.c. motor with its split-ring commutator.

For Examiner’s Use
Section A	/ 10
Section B	/ 20
Total	/ 30

Section A: Multiple-choice questions[10 marks]

Choose the one correct answer for each question.

1
A straight wire carries current vertically upwards. Looking from above, the magnetic field lines around the wire are
1. Aradial spikes pointing away from the wire
2. Bconcentric circles around the wire
3. Cstraight lines parallel to the wire
4. Dclosed loops that shrink into the wire
( )
2
The magnetic field inside a current-carrying solenoid is
1. Astrong and nearly uniform along the axis
2. Bzero at the centre
3. Cstrongest at the middle of the outside surface
4. Dcircular around the coil axis
( )
3
A wire carrying 4.0 A lies at right angles to a magnetic field of flux density 0.50 T. The length of wire in the field is 0.030 m. The force on the wire is
1. A0.0060 N
2. B0.60 N
3. C0.060 N
4. D6.0 N
( )
4
In a simple d.c. motor, the split-ring commutator reverses the
1. Amagnetic field every quarter turn
2. Bcurrent in the coil every half turn
3. Cdirection of rotation every half turn
4. Dcurrent in the coil every quarter turn
( )
5
The soft-iron armature of an electric bell makes and breaks the circuit because when the armature moves towards the gong, the
1. Aspring strengthens the field
2. Bgong completes the circuit
3. Ccurrent increases until the coil burns out
4. Dcontact points separate and cut the current
( )

Section B: Structured questions[20 marks]

Answer all questions in the spaces provided.

1
A wire carries a current of 3.0 A at right angles to a magnetic field of flux density 0.60 T. The length of wire inside the field is 5.0 cm.
1. (a)
  Determine the force on the wire.
  force =N[2]
2. (b)
  State and explain the effect of doubling the current only.
  [2]
3. (c)
  State the effect of turning the wire until it lies parallel to the magnetic field.
  [2]
2
A relay is used so that a low-voltage control circuit can switch a separate 230 V heater circuit safely.
1. (a)
  Describe the structure of the relay, naming the material of its moving armature.
  [3]
2. (b)
  Explain how a small current in the control circuit closes the heater circuit.
  [3]
3. (c)
  State one safety advantage of operating the heater circuit through a relay.
  [1]
3
The coil of a simple d.c. motor rotates in a magnetic field between the north and south poles of a magnet. The split-ring commutator and carbon brushes connect the coil to a battery.
1. (a)
  Explain why the coil begins to rotate when the current is switched on.
  [3]
2. (b)
  Explain the purpose of the split-ring commutator.
  [2]
3. (c)
  State two changes, each made separately, that would increase the speed of the motor.
  [2]

— End of paper —