Power.pptx

AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Power

Energy

Lesson 4 of 7

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up

Energy

Lesson 4 of 7

Answer each one, then check.

1

What is the unit of energy?

2

What is the unit of time used in power calculations?

3

Convert 2 minutes to seconds.

4

What is work done?

5

Write 1.5 kW in watts.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up - Answers

Energy

Lesson 4 of 7

1

What is the unit of energy?

Joule

2

What is the unit of time used in power calculations?

Seconds

3

Convert 2 minutes to seconds.

120 s

4

What is work done?

Energy transferred by a force

5

Write 1.5 kW in watts.

1500 W

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Learning Objectives

Energy

Lesson 4 of 7

1

Define power.

2

Recall and apply both power equations.

3

Explain that 1 W is 1 J/s.

4

Compare two devices doing the same work at different rates.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Energy

Lesson 4 of 7

POWER

Power is the rate at which energy is transferred or the rate at which work is done.

P = E/t and P = W/t. Power is in watts (W), energy and work in joules (J), time in seconds (s). 1 W = 1 J/s.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Same Work, Different Power

Energy

Lesson 4 of 7

Same work, half the time means twice the power.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Power Equations

Energy

Lesson 4 of 7

Learn both and the units.

Equation

Meaning

Units

P = E/t

Power is energy transferred divided by time

W, J, s

P = W/t

Power is work done divided by time

W, J, s

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Energy Transferred by a Kettle

Energy

Lesson 4 of 7

A kettle transfers 360 000 J of energy in 2.0 minutes. Calculate its power.

1

Convert time

2.0 min = 120 s

2

Substitute

P = 360 000 ÷ 120

3

Answer

P = 3000 W

ANSWER

3000 W (3 kW)

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Work and Power of a Climber

Energy

Lesson 4 of 7

A person of mass 60 kg climbs 3.0 m in 5.0 s. g = 9.8 N/kg. Calculate the power.

1

Work done = energy gained

W = mgh = 60 × 9.8 × 3.0 = 1764 J

2

Power

P = W ÷ t = 1764 ÷ 5.0

3

Answer

P = 352.8 W

ANSWER

About 350 W (2 s.f.)

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Rearranging

Energy

Lesson 4 of 7

A 1500 W heater is switched on. How long does it take to transfer 450 000 J?

1

Rearrange

t = E ÷ P

2

Substitute

t = 450 000 ÷ 1500

3

Answer

t = 300 s

ANSWER

300 s (5 minutes)

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Watch Out

Energy

Lesson 4 of 7

Common ways to lose marks.

Time in seconds

Always convert minutes and hours to seconds.

Power, not energy

Power is a rate: it tells you how quickly, not how much.

Unit

1 W is 1 joule per second, not 1 joule.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Key Terms

Energy

Lesson 4 of 7

Power

The rate at which energy is transferred or work is done.

Watt

The unit of power; one joule per second.

Work done

Energy transferred by a force moving an object.

Rate

How much something changes in each unit of time.

Kilowatt

1000 watts.

Energy transferred

The amount of energy moved from one store to another.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Energy

Lesson 4 of 7

YOUR TASK

Fair Race

10 minutes

Two students run up the same flight of stairs. Student A takes 8.0 s and student B takes 5.0 s. Both have a weight of 600 N and the stairs are 3.0 m high. Calculate the work done and the power of each student.

1

Work done first.

2

Then divide by each time.

WHAT A GOOD ANSWER SHOWS

Work done by each = 600 × 3.0 = 1800 J. Power A = 225 W. Power B = 360 W.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Can I...?

Energy

Lesson 4 of 7

Define power.

Use P = E ÷ t.

Use P = W ÷ t.

Convert minutes to seconds.

Explain what a watt is.

Rearrange for time.

Compare two devices with the same work.

Give the unit.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Summary & Exam Focus

P = E ÷ t or P = W ÷ t.

1 W = 1 J/s.

Higher power means energy transferred faster.

Time must be in seconds.

EXAM FOCUS

A motor transfers 6000 J of energy in 30 s. Calculate its power. (2 marks)

Write the equation, substitute, and give the unit W.

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