The National Grid - Teacher Slides.pptx

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The National Grid

Electricity

Lesson 8 of 10

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Warm-up

Electricity

Lesson 8 of 10

Answer each one, then check.

1

What does a power station generate?

2

What happens to a wire when a current flows?

3

Write the equation for power using current and resistance.

4

What does step-up mean?

5

What is a cable?

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Warm-up - Answers

Electricity

Lesson 8 of 10

1

What does a power station generate?

Electricity

2

What happens to a wire when a current flows?

It heats up

3

Write the equation for power using current and resistance.

P = I²R

4

What does step-up mean?

Increases the potential difference

5

What is a cable?

A wire used to carry current

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Learning Objectives

Electricity

Lesson 8 of 10

1

Describe the National Grid as a system of cables and transformers.

2

State the roles of step-up and step-down transformers.

3

Explain why the National Grid is an efficient way to transfer energy.

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Electricity

Lesson 8 of 10

THE NATIONAL GRID

Step-up transformers increase the potential difference for transmission, which reduces the current and so the energy wasted as heat in the cables.

The National Grid is a system of cables and transformers linking power stations to consumers.

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From Power Station to Home

Electricity

Lesson 8 of 10

High pd means low current.

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Why High pd Means Efficient

Electricity

Lesson 8 of 10

Follow the logic.

1

Power = pd × current

For the same power, a higher pd means a smaller current.

2

Less current

A smaller current means less heating in the cables.

3

Energy wasted

Power loss in the cable is P = I²R, so halving the current cuts the loss to a quarter.

4

Step-up transformers

Increase the pd before the cables.

5

Step-down transformers

Decrease the pd to a safe value for homes (230 V).

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Comparing Losses

Electricity

Lesson 8 of 10

100 kW is transmitted through cables of total resistance 2.0 Ω. Compare the power lost at 1000 V with the power lost at 10 000 V.

1

Current at 1000 V

I = P ÷ V = 100 000 ÷ 1000 = 100 A

2

Power lost

P = I²R = 100² × 2.0 = 20 000 W

3

Current at 10 000 V

I = 100 000 ÷ 10 000 = 10 A

4

Power lost

10² × 2.0 = 200 W

ANSWER

20 kW lost at 1000 V but only 200 W lost at 10 000 V: high pd wastes far less energy.

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Key Points

Electricity

Lesson 8 of 10

For an exam answer.

Efficient

High pd, low current means little energy wasted as heat in the cables.

Step-up

Used at the power station end.

Step-down

Used near homes and factories.

Safety

The high pd is reduced to 230 V before homes.

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Key Terms

Electricity

Lesson 8 of 10

National Grid

A system of cables and transformers linking power stations to consumers.

Transformer

A device that changes the size of an alternating pd.

Step-up transformer

Increases the pd.

Step-down transformer

Decreases the pd.

Transmission cables

Cables that carry electricity over long distances.

Efficient

Wasting little energy.

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Electricity

Lesson 8 of 10

YOUR TASK

Follow the Grid

10 minutes

Put these in the correct order: homes, step-up transformer, power station, step-down transformer, transmission cables.

1

Start at the power station.

2

End at homes.

WHAT A GOOD ANSWER SHOWS

Power station, step-up transformer, transmission cables, step-down transformer, homes.

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Can I...?

Electricity

Lesson 8 of 10

Describe the National Grid.

State what a step-up transformer does.

State what a step-down transformer does.

Explain why high pd is used.

Link current to energy wasted.

Compare power losses.

Draw or complete a grid diagram.

Explain why 230 V is needed in homes.

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Summary & Exam Focus

Step-up transformers increase pd, step-down decrease it.

High pd means low current for the same power.

Low current means less energy wasted as heat in the cables.

The National Grid is efficient.

EXAM FOCUS

Explain why the National Grid transmits electricity at a high potential difference. (3 marks)

Say higher pd means lower current, so less heating.

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Exam Practice: The National Grid

Electricity

Lesson 8 of 10

Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes

Question 1 · 2 marks · Complete

Name the two types of transformer used in the National Grid and state what each does.

Question 2 · 3 marks · Use the diagram

The diagram shows part of the National Grid. (a) Name the parts labelled A and B. (b) State why B is needed near homes.

Question 3 · 3 marks · Explain

Explain why using a high potential difference to transmit electricity is efficient.

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Exam Practice: The National Grid (cont.)

Electricity

Lesson 8 of 10

Question 4 · 4 marks · Calculate

A power of 100 kW is transmitted at 10 000 V through cables of total resistance 2.0 Ω. Calculate the power wasted as heat in the cables.

Question 5 · 2 marks · Explain

The potential difference in the transmission cables is much higher than in homes. Explain why the potential difference in homes is reduced.

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Question 1 · 2 marks · Complete

Electricity

Lesson 8 of 10

“

Name the two types of transformer used in the National Grid and state what each does.

HOW TO ANSWER IT

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

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Question 1 · mark scheme

Electricity

Lesson 8 of 10

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

Step-up increases pd

1 mark

Step-down decreases pd

1 mark

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Question 2 · 3 marks · Use the diagram

Electricity

Lesson 8 of 10

The diagram shows part of the National Grid. (a) Name the parts labelled A and B. (b) State why B is needed near homes. (3 marks)

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Question 2 · mark scheme

Electricity

Lesson 8 of 10

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

A: step-up transformer

1 mark

B: step-down transformer

1 mark

Reduces the pd to a safe value for homes

1 mark

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Question 3 · 3 marks · Explain

Electricity

Lesson 8 of 10

“

Explain why using a high potential difference to transmit electricity is efficient.

HOW TO ANSWER IT

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

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Question 3 · mark scheme

Electricity

Lesson 8 of 10

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

High pd means lower current for the same power

1 mark

Lower current means less heating of the cables

1 mark

So less energy wasted

1 mark

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Question 4 · 4 marks · Calculate

Electricity

Lesson 8 of 10

“

A power of 100 kW is transmitted at 10 000 V through cables of total resistance 2.0 Ω. Calculate the power wasted as heat in the cables.

HOW TO ANSWER IT

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

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Question 4 · mark scheme

Electricity

Lesson 8 of 10

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

Rearranges to I = P ÷ V

1 mark

10 A

1 mark

Correct substitution into I²R

1 mark

200 W

1 mark

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Question 5 · 2 marks · Explain

Electricity

Lesson 8 of 10

“

The potential difference in the transmission cables is much higher than in homes. Explain why the potential difference in homes is reduced.

HOW TO ANSWER IT

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

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Question 5 · mark scheme

Electricity

Lesson 8 of 10

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

Safe for homes or appliances designed for 230 V

1 mark

Step-down transformer decreases the pd

1 mark

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