The motor effect - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

The motor effect

Magnetism and electromagnetism

Lesson 3 of 8

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up

Magnetism and electromagnetism

Lesson 3 of 8

Answer each one, then check.

1

What is a force?

2

What is the unit of force?

3

What is a magnetic field?

4

What is current?

5

What is a conductor?

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up - Answers

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Learning Objectives

Magnetism and electromagnetism

Lesson 3 of 8

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Magnetism and electromagnetism

Lesson 3 of 8

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Fleming's Left-Hand Rule

Magnetism and electromagnetism

Lesson 3 of 8

Thumb: force. First finger: field. Second finger: current.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Calculating the Force

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Finding the Current

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Reversing and Changing the Force

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Why Does It Happen?

Magnetism and electromagnetism

Lesson 3 of 8

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Key Terms

Magnetism and electromagnetism

Lesson 3 of 8

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Magnetism and electromagnetism

Lesson 3 of 8

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.

WHAT 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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Can I...?

Magnetism and electromagnetism

Lesson 3 of 8

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Summary & Exam Focus

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Exam Practice: The motor effect

Magnetism and electromagnetism

Lesson 3 of 8

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…

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Exam Practice: The motor effect (cont.)

Magnetism and electromagnetism

Lesson 3 of 8

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…

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 1 · 2 marks · Calculate

Magnetism and electromagnetism

Lesson 3 of 8

“

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 1 · mark scheme

Magnetism and electromagnetism

Lesson 3 of 8

2 marks available. Award a mark for each point made.

Correct substitution

1 mark

0.24 N

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 2 · 2 marks · State

Magnetism and electromagnetism

Lesson 3 of 8

“

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 2 · mark scheme

Magnetism and electromagnetism

Lesson 3 of 8

2 marks available. Award a mark for each point made.

Reverse the current

1 mark

Reverse the field

1 mark

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 3 · 3 marks · Calculate

Magnetism and electromagnetism

Lesson 3 of 8

“

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 3 · mark scheme

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 4 · 3 marks · Explain

Magnetism and electromagnetism

Lesson 3 of 8

“

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 4 · mark scheme

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 5 · 3 marks · Explain

Magnetism and electromagnetism

Lesson 3 of 8

“

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 5 · mark scheme

Magnetism and electromagnetism

Lesson 3 of 8

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

openrevise.com