Magnetic fields and electromagnetic inductionAQA A-Level Physics: Topic test

20 questions, 54 marks

AQA A-Level Physics

Magnetic fields and electromagnetic induction topic test

Total 54 marks

Name

Class

Date

1
A straight wire of length 0.12 m carries a current of 6.0 A in a uniform magnetic field of flux density 80 mT. The wire is perpendicular to the field.
(a)
What is the magnitude of the force on the wire?
[1 mark]
A5.8×10⁻² N
B0.72 N
C1.6×10⁻³ N
D58 N
(b)
The field is directed vertically downwards and the conventional current flows horizontally towards the north. What is the direction of the force on the wire?
[1 mark]
Atowards the east
Btowards the west
Cvertically upwards
Dvertically downwards
(c)
Define the tesla.
[2 marks]
Total for question 1: 4 marks
2
Singly charged sodium ions, each of mass 3.8×10⁻²⁶ kg, enter a region of uniform magnetic field of flux density 0.30 T at a speed of 2.0×10⁵ m s⁻¹. They enter at right angles to the field. Use e = 1.60×10⁻¹⁹ C.
(a)
Which statement explains why the ions follow a circular path in the field?
[1 mark]
AThe force acts along the direction of motion, so it steadily increases the speed.
BThe force has a fixed direction, so it pulls the ions steadily to one side.
CThe force is always perpendicular to the velocity, so it changes the direction of the motion but not the speed.
DThe force does work on the ions, so their kinetic energy increases.
(b)
What is the radius of the circular path?
[1 mark]
A0.32 m
B6.3 m
C0.079 m
D0.16 m
(c)
Ions of a heavier isotope, with the same charge and speed, enter the same field. State and explain how the radius of their path differs.
[2 marks]
Total for question 2: 4 marks
3
A square coil of side 5.0 cm has 120 turns. It is placed in a uniform magnetic field of flux density 0.30 T, with the normal to the plane of the coil at 40° to the field direction.
(a)
Calculate the magnetic flux linkage of the coil.
[3 marks]
(b)
The coil is rotated through a further angle in 0.20 s, until its plane is parallel to the field. Calculate the average emf induced in the coil.
[4 marks]
Total for question 3: 7 marks
4
A circular coil of 350 turns and radius 2.4 cm is placed with its plane perpendicular to a uniform magnetic field between the poles of an electromagnet. The coil is connected across a sensitive voltmeter of very high resistance. The flux density is 0.50 T and is then reduced uniformly to zero in 0.80 s.
(a)
Calculate the average emf induced in the coil. Use Lenz's law to explain the direction of the induced current relative to the original field.
[6 marks]
(b)
A student claims that doubling the number of turns on the coil and halving the time taken to reduce the field to zero will double the emf. Evaluate this claim by calculation.
[6 marks]
Total for question 4: 12 marks
5
A sinusoidal alternating supply has a peak-to-peak voltage of 24 V and a frequency of 250 Hz. It is connected across a resistor of resistance 18 Ω.
(a)
What is the rms voltage of the supply?
[1 mark]
A17 V
B8.5 V
C12 V
D6.0 V
(b)
What is the period of the supply?
[1 mark]
A2.0 ms
B8.0 ms
C4.0 ms
D250 ms
(c)
Calculate the mean power dissipated in the resistor.
[2 marks]
Total for question 5: 4 marks
6
A laboratory transformer has 1200 turns on its primary coil and 150 turns on its secondary coil. The primary coil is connected to a 240 V rms alternating supply. When the secondary coil supplies a load, the output power is 60 W and the input power is 75 W.
(a)
What is the rms voltage across the secondary coil?
[1 mark]
A1920 V
B240 V
C3.8 V
D30 V
(b)
What is the efficiency of the transformer?
[1 mark]
A80%
B125%
C20%
D64%
(c)
Describe how eddy currents arise in the core of the transformer and state one way in which they are reduced.
[2 marks]
Total for question 6: 4 marks
7
A wind farm generates 4.0 MW of electrical power. This is transmitted to a town through cables of total resistance 15 Ω. Assume that the transmitted power is the same at any transmission voltage and that the load is purely resistive.
(a)
The power is transmitted at 33 kV. Calculate the current in the cables and the power lost in the cables.
[3 marks]
(b)
The same power is now transmitted at 11 kV. Calculate the percentage of the 4.0 MW lost in the cables and explain why transmission at a high voltage is preferred.
[4 marks]
Total for question 7: 7 marks
8
A small alternator has a rectangular coil of 200 turns and area 4.0×10⁻³ m², rotating at 30 revolutions per second in a uniform magnetic field of flux density 0.15 T. Its output is connected to the primary coil of a step-up transformer with 40 primary turns and 1000 secondary turns. The transformer is 92% efficient and delivers 180 W to a load.
(a)
Calculate the peak emf and the rms emf of the alternator. State the position of the coil relative to the field when the emf is zero, and explain why the emf is zero then.
[6 marks]
(b)
Calculate the rms voltage across the secondary coil, the current in the load, and the current drawn from the alternator by the primary coil. State two reasons why the transformer is not 100% efficient.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).