AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
The motor effect
Magnetism and electromagnetism · Lesson 3 of 8
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.
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. |