AQA GCSE Physics (8463), Higher tier · Magnetism and electromagnetism › The motor effect
Practise Fleming's left-hand rule. 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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A wire carrying a current is placed in a magnetic field. The wire experiences a force. What is the name of this effect?
The motor effect
Calculate the force on the wire. Use the Physics Equations Sheet.
0.040 N
A U-shaped magnet is placed on a top-pan balance. A straight horizontal wire is clamped so that it passes between the poles of the magnet, at right angles to the magnetic field, without touching the magnet. The length of wire between the poles is 5.0 cm. When there is no current, the balance reads 152.40 g. When there is a current of 3.5 A in the wire, the balance reads 154.90 g. The gravitational field strength is 9.8 N/kg. Calculate the size of the force on the wire.
0.0245 N
Name the rule used to work out the direction of the force on a current-carrying wire in a magnetic field.
Fleming's left-hand rule
State what happens to the direction of the force if the current in the wire is reversed.
It reverses direction.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
A wire carrying a current is placed in a magnetic field. The wire experiences a force.
(a) What is the name of this effect?[1]
(b) Fleming's left-hand rule uses the thumb, first finger and second finger of the left hand. State what each of these shows the direction of.[3]
(c) Give one way of increasing the size of the force on the wire.[1]
Show the answer and mark scheme
(a)Answer: The motor effect
the motor effect
(b)Answer: Thumb – force (motion); first finger – field; second finger – current.
thumb: force / motion (thrust)
first finger: (magnetic) field
second finger: current
(c)Answer: Increase the current.
increase the current / use a stronger magnet (greater magnetic flux density) / increase the length of wire in the field
Question 2Medium6 marks
A straight wire is placed at right angles to a magnetic field. The magnetic flux density is 0.25 T. The length of wire in the field is 0.050 m. The current in the wire is 3.2 A.
(a) Calculate the force on the wire. Use the Physics Equations Sheet.[2]
(b) Give the unit of magnetic flux density in words.[1]
(c) The wire is turned so that it lies parallel to the magnetic field. What is the force on the wire now?[1]
(d) The wire is turned back to its original position and the current is increased to 4.8 A. Calculate the new force on the wire.[2]
Show the answer and mark scheme
(a)Answer: 0.040 N
F = 0.25 × 3.2 × 0.050
0.04(0) (N)
(b)Answer: tesla
tesla
(c)Answer: 0 N
0 / zero
(d)Answer: 0.060 N
F = 0.25 × 4.8 × 0.050
0.06(0) (N)
Question 3Hard6 marks
In an experiment, a 4.0 cm length of wire lies at right angles to the magnetic field between two magnets. When the current in the wire is 450 mA, the force on the wire is 3.6 mN.
(a) Calculate the magnetic flux density between the magnets. Use the Physics Equations Sheet.[3]
(b) The current is doubled and the length of wire in the field is halved. What is the force on the wire now? Give a reason for your answer.[2]
(c) Suggest why this force would be difficult to measure with a school newtonmeter.[1]
Show the answer and mark scheme
(a)Answer: 0.20 T
F = 0.0036 N, I = 0.45 A and l = 0.040 m
0.0036 = B × 0.45 × 0.040
B = 0.20 (T)
(b)Answer: 3.6 mN (unchanged) mN
3.6 (mN) / unchanged
force is proportional to current and to length, so doubling one and halving the other has no overall effect
(c)Answer: 3.6 mN is too small for a newtonmeter to measure.
the force is very small / smaller than the resolution of the newtonmeter