AQA GCSE Combined Science (8464), Higher tier · Physics › Magnetism and electromagnetism › The motor effect
Practise Fleming's left-hand rule (HT only). 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.
When a wire carrying a current is in a magnetic field, the wire and the magnet exert a force on each other: the motor effect. You need to use Fleming's left-hand rule to find the direction of the force, recall what affects its size, and calculate it with F = B I l, which is given on the equation sheet. Higher tier only.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
4
State what the motor effect isA wire carrying a current in a magnetic field has a force on it, and so does the magnet.
5
Recall the factors that affect the forceThe force is bigger for a stronger field (greater magnetic flux density), a bigger current and a longer length of wire in the field.
5
Use Fleming's left-hand rule to find directionFirst finger = field (N to S), second finger = current (+ to −), thumb = force (motion).
5
Explain why a current-carrying wire movesThe magnetic field of the current interacts with the field of the magnet, producing a force on the wire.
6
Calculate the force with F = BIlSubstitute B in tesla, I in amperes and l in metres to get F in newtons.
7
Rearrange F = BIl after converting unitse.g. find B in tesla when the length is given in mm or cm.
8
Predict how changes affect the forcee.g. reversing both the current and the field leaves the direction unchanged; doubling I and halving l leaves F unchanged.
Notes
The motor effect
When a wire carrying a current is placed in a magnetic field, the magnet and the wire exert a force on each other. This is the motor effect.
It happens because the magnetic field of the current interacts with the magnetic field of the magnet.
The force is biggest when the wire is at right angles to the field. If the wire is parallel to the field, there is no force.
Reversing the current or reversing the field reverses the direction of the force. Reversing both leaves the direction unchanged.
Fleming's left-hand rule
Hold the thumb, first finger and second finger of your left hand all at right angles to each other.
First finger → Field (from north to south).
Second finger → Current (conventional current, from + to −).
Thumb → Motion: the direction of the force on the wire.
The rule shows that the force is at right angles to both the current and the field.
The size of the force
The force is bigger if you increase the magnetic flux density (the strength of the field), the current, or the length of wire in the field.
For a wire at right angles to the field: \(F = BIl\)
F = force (N), B = magnetic flux density (tesla, T), I = current (A), l = length of wire in the field (m).
Convert first: cm → m (÷ 100), mm → m (÷ 1000), mT → T (÷ 1000), mA → A (÷ 1000).
Cheatsheet
Motor effect: a wire carrying a current in a magnetic field has a force on it
\(F = BIl\) for a wire at right angles to the field (given on the equation sheet)
F in N, B in T (tesla), I in A, l in m
Force bigger for bigger B, bigger I or longer l
Left hand: First finger = Field, seCond finger = Current, thuMb = Motion (force)
Field goes N → S; conventional current goes + → −
Wire parallel to the field: no force
Reverse the current or the field: force reverses. Reverse both: no change
How to answer each type of question
Calculate the force using F = B I l
2 to 3 marks6
Write down F = B I l from the equation sheet.
Convert the length to metres (and mT to T, mA to A).
Substitute and work out the answer.
Give the unit: newtons (N).
Example. A 6.0 cm length of wire is at right angles to a uniform magnetic field of flux density 0.25 T. The current in the wire is 3.2 A. Calculate the force on the wire. [3 marks]
Show the model answer
l = 6.0 cm = 0.060 m (1) F = 0.25 × 3.2 × 0.060 (1) F = 0.048 N (1)
Rearrange F = B I l to find B, I or l
3 to 4 marks7
Convert all values to N, A, m and T first.
Substitute into F = B I l before rearranging: this can earn a mark even if you slip later.
Divide to find the unknown.
Give the unit if asked (tesla, T, for magnetic flux density).
Example. A wire carries a current of 1.5 A. A 40 mm length of the wire is at right angles to a magnetic field. The force on the wire is 4.8 × 10−3 N. Calculate the magnetic flux density of the field. Give the unit. [4 marks]
Show the model answer
l = 40 mm = 0.040 m (1) 4.8 × 10−3 = B × 1.5 × 0.040 (1) B = 4.8 × 10−3 ÷ 0.060 = 0.080 (1) tesla / T (1)
Use Fleming's left-hand rule to find a direction
1 to 3 marks5
Point your first finger from N to S (the field).
Turn your hand until your second finger points along the current (+ to −).
Your thumb now shows the direction of the force.
To reverse the force, reverse the current or the field (not both).
Example. A horizontal wire runs across the page from left to right, between the poles of a magnet. The north pole is above the wire and the south pole is below it. The current in the wire is from left to right. (a) Use Fleming's left-hand rule to find the direction of the force on the wire. [1 mark] (b) State two different changes, each of which would reverse the direction of the force. [2 marks]
Show the model answer
(a) Into the page (1) (b) Reverse the direction of the current (1). Reverse the magnetic field / swap the poles of the magnet (1).
Explain why a wire in a magnetic field moves
2 to 4 marks5
Say the current produces a magnetic field around the wire.
Say this field interacts with the field of the magnet, giving a force (the motor effect).
For 'give ways to increase the force', choose from: bigger current, stronger magnet, longer length of wire in the field.
Example. A short copper rod rests across two horizontal metal rails between the poles of a magnet. When there is a current in the rod, the rod rolls along the rails. (a) Explain why the rod moves. [2 marks] (b) Give two ways of making the force on the rod bigger. [2 marks]
Show the model answer
(a) The current in the rod produces a magnetic field around it (1). This interacts with the field of the magnet, so there is a force on the rod (the motor effect) (1). (b) Any two, 1 mark each: increase the current; use a stronger magnet (greater magnetic flux density); make a longer length of the rod lie in the field.
Shortcuts and memory tricks
Fleming's LEFT hand for Motors: in the UK, motor cars drive on the left.
The left-hand rule in order: thuMb = Motion, First finger = Field, seCond finger = Current.
Quick check: the force is always at right angles to both the wire and the field. If your answer is along the wire or along the field lines, start again.
Read \(F = BIl\) as 'F equals Bil' to remember the order.
Where marks are lost
Using the length in cm or mm instead of metres.
Using the right hand, or swapping the field and current fingers.
Taking the current from − to +: use conventional current, + to −.
Pointing the field finger from S to N: the field goes from N to S.
Using the whole length of the wire instead of only the length inside the field.
Forgetting the unit tesla (T) when asked for a magnetic flux density.
Exam technique
Write the equation, then the substitution with converted units, then the answer with its unit: each step can earn a mark.
Use your left hand in the exam: turn your hand, not the paper, until the first two fingers match the diagram.
For 'factors that affect the size of the force', name all three: magnetic flux density (or 'a stronger magnet'), current and length. 'Bigger magnet' is not the same as 'stronger magnet'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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