Transformers - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Transformers

Magnetism and electromagnetism

Lesson 8 of 8

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Warm-up

Magnetism and electromagnetism

Lesson 8 of 8

Answer each one, then check.

1

What is the National Grid?

2

Why is high voltage used in the grid?

3

What is power?

4

What is a.c.?

5

What is a coil?

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Warm-up - Answers

Magnetism and electromagnetism

Lesson 8 of 8

1

What is the National Grid?

The network that transfers electrical power

2

Why is high voltage used in the grid?

To reduce energy lost as heat

3

What is power?

The rate of transfer of energy

4

What is a.c.?

Alternating current

5

What is a coil?

Loops of wire

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Learning Objectives

Magnetism and electromagnetism

Lesson 8 of 8

1

Describe the structure and function of a transformer.

2

Explain how a step-up and a step-down transformer work.

3

Use V_p / V_s = n_p / n_s.

4

Use V_s I_s = V_p I_p for a 100% efficient transformer.

5

Explain why the National Grid uses transformers.

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Magnetism and electromagnetism

Lesson 8 of 8

THE TRANSFORMER EQUATIONS

V_p/(V_s) = n_p/(n_s) and V_s × I_s = V_p × I_p

V_p, I_p, n_p are for the primary coil; V_s, I_s, n_s for the secondary. The second equation assumes 100% efficiency (power in = power out).

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A Transformer

Magnetism and electromagnetism

Lesson 8 of 8

The iron core links the magnetic field between the coils.

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How a Transformer Works

Magnetism and electromagnetism

Lesson 8 of 8

Follow the steps.

1

a.c. in the primary

An alternating current flows in the primary coil.

2

Changing field

It produces a changing magnetic field in the iron core.

3

Field through the secondary

The changing field passes through the secondary coil.

4

Generator effect

An alternating p.d. is induced in the secondary coil.

5

Turns ratio

The ratio of turns sets the ratio of potential differences.

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Step-up or Step-down?

Magnetism and electromagnetism

Lesson 8 of 8

STEP-UP TRANSFORMER

• More turns on the secondary coil.

• Increases the potential difference.

• Used at power stations before the grid.

STEP-DOWN TRANSFORMER

• Fewer turns on the secondary coil.

• Decreases the potential difference.

• Used near homes to bring the p.d. down to 230 V.

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Using the Turns Ratio

Magnetism and electromagnetism

Lesson 8 of 8

A transformer has 200 turns on its primary coil and 1000 on its secondary. The input is 12 V. Calculate the output potential difference.

1

Write the equation

V_p / V_s = n_p / n_s

2

Rearrange

V_s = V_p × n_s ÷ n_p

3

Substitute

V_s = 12 × 1000 ÷ 200

4

Answer

V_s = 60 V

ANSWER

60 V

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Power In Equals Power Out

Magnetism and electromagnetism

Lesson 8 of 8

A 100% efficient transformer has a primary p.d. of 230 V and current of 2.0 A. The secondary p.d. is 11.5 V. Calculate the secondary current.

1

Write the equation

V_s I_s = V_p I_p

2

Rearrange

I_s = V_p I_p ÷ V_s

3

Substitute

I_s = 230 × 2.0 ÷ 11.5

4

Answer

I_s = 40 A

ANSWER

40 A

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Why the National Grid Uses Them

Magnetism and electromagnetism

Lesson 8 of 8

High voltage, low current.

Step-up at the power station

Raises the p.d. (up to 400 000 V) and lowers the current.

Lower current

Less energy is wasted as heat in the cables.

Step-down near homes

Reduces the p.d. to 230 V for safe use.

Efficiency

The grid is efficient because transformers waste little energy.

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Key Terms

Magnetism and electromagnetism

Lesson 8 of 8

Transformer

A device that changes the potential difference of an alternating supply.

Primary coil

The input coil of a transformer.

Secondary coil

The output coil of a transformer.

Step-up transformer

A transformer that increases the potential difference.

Step-down transformer

A transformer that decreases the potential difference.

Soft iron core

A core that easily becomes magnetised and demagnetised, linking the coils.

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Magnetism and electromagnetism

Lesson 8 of 8

YOUR TASK

Transformer Sums

15 minutes

A transformer has n_p = 50 and n_s = 500. Input 10 V a.c. Find V_s. If the input current is 2.0 A, find I_s assuming 100% efficiency.

1

Step-up: p.d. up, current down.

WHAT A GOOD ANSWER SHOWS

Vs = 10 × 500 ÷ 50 = 100 V. Power in = 20 W, so Is = 20 ÷ 100 = 0.20 A.

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Can I...?

Magnetism and electromagnetism

Lesson 8 of 8

Name the parts.

Explain the working.

State step-up and step-down.

Use the turns ratio.

Use power in = power out.

Explain why a.c. is needed.

Explain the National Grid.

Give typical voltages.

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Summary & Exam Focus

V_p / V_s = n_p / n_s.

V_s I_s = V_p I_p for 100% efficiency.

Step-up increases the p.d. and decreases the current.

a.c. is needed for a changing field.

EXAM FOCUS

A transformer has 200 primary turns and 1000 secondary turns. The input is 12 V. Calculate the output. (2 marks)

Vs = Vp × ns ÷ np.

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Exam Practice: Transformers

Magnetism and electromagnetism

Lesson 8 of 8

Answer all questions. Use the mark allocation as a guide to how much to write. · 17 minutes

Question 1 · 2 marks · Calculate

A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. The input potential difference is 12 V. Calculate the…

Question 2 · 4 marks · Explain

Explain how a transformer changes the potential difference of an alternating supply.

Question 3 · 3 marks · Calculate

A transformer is 100% efficient. The primary p.d. is 230 V and the primary current is 2.0 A. The secondary p.d. is 11.5 V. Calculate the…

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Exam Practice: Transformers (cont.)

Magnetism and electromagnetism

Lesson 8 of 8

Question 4 · 3 marks · Explain

Explain why a transformer does not work with a direct current.

Question 5 · 4 marks · Explain

Explain why electricity is transmitted in the National Grid at a very high potential difference.

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Question 1 · 2 marks · Calculate

Magnetism and electromagnetism

Lesson 8 of 8

“

A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. The input potential difference is 12 V. Calculate the output potential difference. Use the equation: Vp ÷ Vs = np ÷ ns

HOW TO ANSWER IT

Command word: Calculate. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Magnetism and electromagnetism

Lesson 8 of 8

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

Correct rearrangement and substitution

1 mark

60 V

1 mark

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Question 2 · 4 marks · Explain

Magnetism and electromagnetism

Lesson 8 of 8

“

Explain how a transformer changes the potential difference of an alternating supply.

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

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Question 2 · mark scheme

Magnetism and electromagnetism

Lesson 8 of 8

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

a.c. in the primary makes a changing magnetic field

1 mark

Field carried by the iron core to the secondary

1 mark

Induced p.d. in the secondary (generator effect)

1 mark

Turns ratio sets the p.d. ratio

1 mark

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Question 3 · 3 marks · Calculate

Magnetism and electromagnetism

Lesson 8 of 8

“

A transformer is 100% efficient. The primary p.d. is 230 V and the primary current is 2.0 A. The secondary p.d. is 11.5 V. Calculate the secondary current.

HOW TO ANSWER IT

Command word: Calculate. Worth 3 marks, so plan before writing.

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Question 3 · mark scheme

Magnetism and electromagnetism

Lesson 8 of 8

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

Vs Is = Vp Ip

1 mark

Correct substitution

1 mark

40 A

1 mark

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Question 4 · 3 marks · Explain

Magnetism and electromagnetism

Lesson 8 of 8

“

Explain why a transformer does not work with a direct current.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 4 · mark scheme

Magnetism and electromagnetism

Lesson 8 of 8

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

d.c. produces a constant field

1 mark

No change in field in the secondary

1 mark

No p.d. induced

1 mark

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Question 5 · 4 marks · Explain

Magnetism and electromagnetism

Lesson 8 of 8

“

Explain why electricity is transmitted in the National Grid at a very high potential difference.

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

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Question 5 · mark scheme

Magnetism and electromagnetism

Lesson 8 of 8

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

Step-up transformer raises p.d.

1 mark

Current is lower for the same power

1 mark

Less energy wasted as heat

1 mark

Step-down near homes for safety

1 mark

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