AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Power
Energy · Lesson 4 of 7
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
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
Learning Objectives
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.
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.
Same Work, Different Power
|
Same work, half the time means twice the power. |
Two motors lifting the same weight the same height; motor B does it in half the time so has twice the power.
Power Equations
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 |
Energy Transferred by a Kettle
|
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)
Work and Power of a Climber
|
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.)
Rearranging
|
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)
Watch Out
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.
Key Terms
|
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. |
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. |
A good answer shows: Work done by each = 600 × 3.0 = 1800 J. Power A = 225 W. Power B = 360 W.
Note: Ask why both do the same work but B has a greater power.
Can I...?
☐ 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.
Summary
✓ 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. |
Exam Practice: Power
Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes
▸ Question 1 · 2 marks · Calculate. A motor transfers 6000 J of energy in 30 s. Calculate the power of the motor. Use the equation: power = energy transferred ÷ time
▸ Question 2 · 2 marks · Calculate. A machine does 900 J of work in 12 s. Calculate the power.
▸ Question 3 · 3 marks · Compare. Motor A lifts a 20 N weight through 3.0 m in 6.0 s. Motor B lifts the same weight through the same height in 3.0 s. Compare the power of…
▸ Question 4 · 1 mark · State. State what is meant by a power of 1 watt.
▸ Question 5 · 4 marks · Calculate. A person of mass 60 kg climbs stairs of height 3.0 m in 5.0 s. Calculate the power developed by the person. Gravitational field strength =…
▸ Question 6 · 2 marks · Calculate. A 1500 W heater is switched on until it has transferred 450 000 J. Calculate the time it is switched on for.
Question 1 · 2 marks · Calculate
|
“A motor transfers 6000 J of energy in 30 s. Calculate the power of the motor. Use the equation: power = energy transferred ÷ time” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 200 W. 1 mark
▸ Model answer. 6000 ÷ 30 = 200 W
Question 2 · 2 marks · Calculate
|
“A machine does 900 J of work in 12 s. Calculate the power.” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 75 W. 1 mark
▸ Model answer. 900 ÷ 12 = 75 W
Question 3 · 3 marks · Compare
|
“Motor A lifts a 20 N weight through 3.0 m in 6.0 s. Motor B lifts the same weight through the same height in 3.0 s. Compare the power of the two motors. Show your calculations.” |
HOW TO ANSWER IT Command word: Compare. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Work done = 60 J. 1 mark
▸ A = 10 W and B = 20 W. 1 mark
▸ B has twice the power. 1 mark
▸ Model answer. Work done = 20 × 3.0 = 60 J. Power A = 60 ÷ 6.0 = 10 W. Power B = 60 ÷ 3.0 = 20 W. Motor B has twice the power of motor A.
Question 4 · 1 mark · State
|
“State what is meant by a power of 1 watt.” |
HOW TO ANSWER IT Command word: State. Worth 1 mark, so plan before writing.
Question 4 · mark scheme
1 mark available. Award a mark for each point made.
▸ 1 joule per second. 1 mark
▸ Model answer. An energy transfer of 1 joule each second.
Question 5 · 4 marks · Calculate
|
“A person of mass 60 kg climbs stairs of height 3.0 m in 5.0 s. Calculate the power developed by the person. Gravitational field strength = 9.8 N/kg. Give your answer to 2 significant figures.” |
HOW TO ANSWER IT Command word: Calculate. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Work done = mgh. 1 mark
▸ 1764 J. 1 mark
▸ 352.8 W. 1 mark
▸ 350 W. 1 mark
▸ Model answer. W = 60 × 9.8 × 3.0 = 1764 J; P = 1764 ÷ 5.0 = 352.8 W = 350 W
Question 6 · 2 marks · Calculate
|
“A 1500 W heater is switched on until it has transferred 450 000 J. Calculate the time it is switched on for.” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 6 · mark scheme
2 marks available. Award a mark for each point made.
▸ Rearranges to t = E ÷ P. 1 mark
▸ 300 s. 1 mark
▸ Model answer. t = 450 000 ÷ 1500 = 300 s