AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
The motor effect
Magnetism and electromagnetism · Lesson 3 of 8
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is a force?
A push or a pull
2. What is the unit of force?
Newton (N)
3. What is a magnetic field?
A region where a force acts on a magnet
4. What is current?
A flow of charge
5. What is a conductor?
A material that lets current flow
Learning Objectives
1. Explain why a current-carrying wire in a magnetic field experiences a force.
2. Use Fleming's left-hand rule to find the direction of the force.
3. Recall and use F = BIl.
4. State when the force is greatest and when it is zero.
5. Explain how the force can be reversed.
The Motor Effect
force = magnetic flux density × current × length F = BIl
F in newtons (N), B in tesla (T), I in amperes (A) and l in metres (m). The conductor must be at right angles to the field. F = BIl is on the equation sheet.
Fleming's Left-Hand Rule
|
Thumb: force. First finger: field. Second finger: current. |
Fleming's left-hand rule showing the force, field and current directions.
Calculating the Force
|
A wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force. |
1. Write the equation
F = BIl
2. Substitute
F = 0.40 × 3.0 × 0.20
3. Answer
F = 0.24 N
Answer: 0.24 N
Finding the Current
|
A force of 0.60 N acts on a 0.30 m wire in a field of 0.50 T. Calculate the current. |
1. Rearrange
I = F ÷ (Bl)
2. Substitute
I = 0.60 ÷ (0.50 × 0.30)
3. Answer
I = 4.0 A
Answer: 4.0 A
Reversing and Changing the Force
Learn what happens.
|
Change |
Effect on the force |
|---|---|
|
Reverse the current |
Force reverses |
|
Reverse the magnetic field |
Force reverses |
|
Increase the current |
Force increases |
|
Stronger magnets (larger B) |
Force increases |
|
Wire parallel to the field |
No force |
Why Does It Happen?
Two magnetic fields.
▸ Two fields. The magnet's field and the field round the wire interact.
▸ Result. The two fields push on each other and the wire feels a force.
▸ Right angles. The force is greatest when the wire is at 90° to the field.
▸ Parallel. No force when the wire is parallel to the field.
Key Terms
|
Motor effect The force on a current-carrying conductor in a magnetic field. |
Magnetic flux density A measure of the strength of a magnetic field, in tesla (T). |
|
Tesla The unit of magnetic flux density. |
Fleming's left-hand rule A rule for finding the direction of the force on a current-carrying wire in a magnetic field. |
|
Conductor A material that allows a current to flow. |
Thumb Gives the direction of the force in Fleming's left-hand rule. |
Your Task: Hand Rule
10 minutes
|
A current flows left to right through a wire between a north pole (above) and a south pole (below). Use Fleming's left-hand rule. Which way is the force? Then calculate F for B = 0.30 T, I = 5.0 A, l = 0.10 m. 1. Thumb = force. 2. First finger = field. 3. Second finger = current. |
A good answer shows: The field points downwards (north to south), the current points to the right, so the force is into the page. F = 0.30 × 5.0 × 0.10 = 0.15 N.
Note: Ask learners to rehearse the rule with their left hand.
Can I...?
☐ State the motor effect.
☐ Use the left-hand rule.
☐ Write F = BIl.
☐ Give units.
☐ Use right angles.
☐ Reverse the force.
☐ Change the force.
☐ Calculate F, I, B or l.
Summary
✓ F = BIl (tesla, amperes, metres).
✓ Left-hand rule: thumb force, first finger field, second finger current.
✓ Reversing current or field reverses the force.
✓ No force if parallel to the field.
|
EXAM FOCUS A 0.20 m wire carries 3.0 A at right angles to a 0.40 T field. Calculate the force. (2 marks) F = BIl. |
Exam Practice: The motor effect
Answer all questions. Use the mark allocation as a guide to how much to write. · 14 minutes
▸ Question 1 · 2 marks · Calculate. A wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force on the…
▸ Question 2 · 2 marks · State. State two ways of reversing the direction of the force on a current-carrying wire in a magnetic field.
▸ Question 3 · 3 marks · Calculate. A force of 0.60 N acts on a wire of length 0.30 m in a magnetic field of 0.50 T. The wire is at right angles to the field. Calculate the…
▸ Question 4 · 3 marks · Explain. Explain why a wire carrying a current experiences a force when it is placed between the poles of a magnet.
▸ Question 5 · 3 marks · Explain. Explain what happens to the force on the wire in a magnetic field when the wire is turned so that it is parallel to the field lines. Give a…
Question 1 · 2 marks · Calculate
|
“A wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force on the wire. Use the equation: force = magnetic flux density × current × length” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 0.24 N. 1 mark
▸ Model answer. F = 0.40 × 3.0 × 0.20 = 0.24 N
Question 2 · 2 marks · State
|
“State two ways of reversing the direction of the force on a current-carrying wire in a magnetic field.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ Reverse the current. 1 mark
▸ Reverse the field. 1 mark
▸ Model answer. Reverse the direction of the current. Reverse the direction of the magnetic field (swap the magnets).
Question 3 · 3 marks · Calculate
|
“A force of 0.60 N acts on a wire of length 0.30 m in a magnetic field of 0.50 T. The wire is at right angles to the field. Calculate the current in the wire.” |
HOW TO ANSWER IT Command word: Calculate. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Rearranges to I = F ÷ (B l). 1 mark
▸ Substitution. 1 mark
▸ 4.0 A. 1 mark
▸ Model answer. I = F ÷ (Bl) = 0.60 ÷ (0.50 × 0.30) = 4.0 A
Question 4 · 3 marks · Explain
|
“Explain why a wire carrying a current experiences a force when it is placed between the poles of a magnet.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Current produces a magnetic field. 1 mark
▸ Interacts with the magnet's field. 1 mark
▸ Resulting force on the wire. 1 mark
▸ Model answer. The current produces its own magnetic field around the wire. This interacts with the field of the magnet, and the two fields exert a force on each other so the wire moves.
Question 5 · 3 marks · Explain
|
“Explain what happens to the force on the wire in a magnetic field when the wire is turned so that it is parallel to the field lines. Give a reason.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 5 · mark scheme
3 marks available. Award a mark for each point made.
▸ Force decreases to zero. 1 mark
▸ Force is greatest at right angles. 1 mark
▸ Fields no longer interact / no component at right angles. 1 mark
▸ Model answer. The force decreases to zero because the wire's field no longer interacts with the magnet's field; the force is largest when the wire is at right angles to the field.