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Oak National Academy
Oak National Academy
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Pupils
Year 10
AQA
Foundation

Efficiency (in terms of energy and power)

I can calculate the efficiency of an energy transfer and describe ways in which energy is dissipated.

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Year 10
AQA
Foundation

Efficiency (in terms of energy and power)

I can calculate the efficiency of an energy transfer and describe ways in which energy is dissipated.

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Lesson details

Key learning points

  1. Energy is dissipated due to friction which causes particles to vibrate more quickly and heats up the surroundings.
  2. Energy is dissipated due to drag which causes particles in the surroundings to move more quickly.
  3. Efficiency = useful output energy transfer / total input energy transfer
  4. Efficiency = useful power output / total power input

Keywords

  • Dissipate - Friction can cause energy to dissipate (spread out and becomes unusable) into the surroundings, causing them to heat up.

  • Efficiency - Efficiency is the fraction of energy supplied to an object or system that is usefully transferred by it.

  • Useful output energy transfer - The amount of energy usefully transferred by an object or system is the useful output energy transfer.

  • Total input energy transfer - The total amount of energy transferred to an object or system is the total input energy transfer.

Common misconception

Pupils may not realise that energy transfers usually result in the heating of the surroundings and therefore some energy ends up in a thermal store.

Pupils should have opportunity to identify and describe places in a system where energy is dissipated and transferred into a thermal store.

The practical activity in this lesson allows pupils to investigate and describe an example in which energy is dissipated in order to help develop understanding of ideas about efficiency. The activity is also useful for providing practice of measurement techniques and control variables.
Teacher tip

Equipment

A selection of balls that bounce, a metre ruler, masking tape.

Content guidance

  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which of the following forces slow down a sledge that is sliding down a snow-covered hill?
Correct answer: air resistance
Correct answer: friction
gravitational
normal reaction force
Q2.
Which of the following energy stores does friction transfer energy into?
chemical store
gravitational store
kinetic store
Correct answer: thermal store
Q3.
Match the energy stores on the left to their descriptions on the right.
Correct Answer:chemical store,energy due to chemical reactions

energy due to chemical reactions

Correct Answer:gravitational store,energy due to an object's mass and height

energy due to an object's mass and height

Correct Answer:kinetic store,energy due to an object's mass and speed

energy due to an object's mass and speed

Correct Answer:thermal store,energy due to particle movement

energy due to particle movement

Q4.
Why does a hot cup of tea cool down?
All of its energy is used up.
All of its energy is transferred to the surroundings.
Some of its energy is used up.
Correct answer: Some of its energy is transferred to the surroundings.
Q5.
Why is the rebound height of a ball not the same as the height that it was dropped from?
Earth's gravitational force is pulling the ball downwards.
The normal reaction force from the ground is less than the gravitational force.
Pushing air out of the way uses up some of the ball's energy.
Correct answer: Pushing air out of the way transfers some energy to the air particles.
Q6.
The law of conservation of energy states that energy cannot be created or ...
Correct Answer: destroyed, used up

Exit quiz

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6 Questions

Q1.
A ball has 50 J of energy in the gravitational store and 20 J in the thermal store. Which of the following could be the amount of energy in each store after the ball has been dropped and rebounded?
40 J in the gravitational store and 10 J in the thermal store.
40 J in the gravitational store and 20 J in the thermal store.
Correct answer: 40 J in the gravitational store and 30 J in the thermal store.
Q2.
Energy is when it is transferred to the thermal store of the surroundings.
Correct Answer: dissipated, dissipate, dissipates
Q3.
Which of the following is a correct definition for efficiency?
The amount of energy that makes something useful happen.
The amount of energy that dissipates.
Correct answer: The fraction of the energy supplied that makes something useful happen.
The fraction of the energy supplied that dissipates.
Q4.
Aisha is investigating how the temperature of a tennis ball affects its rebound height. Match up the variable on the left to the type of variable for this experiment on the right.
Correct Answer:temperature of the ball,independent variable

independent variable

Correct Answer:rebound height of the ball,dependent variable

dependent variable

Correct Answer:height that the ball is dropped from,control variable

control variable

Q5.
Which of the following is the correct equation for efficiency?
efficiency = useful output energy transfer + total input energy transfer
efficiency = useful output energy transfer – total input energy transfer
efficiency = useful output energy transfer × total input energy transfer
Correct answer: efficiency = useful output energy transfer ÷ total input energy transfer
Q6.
A forklift truck transfers 24 000 J of energy when lifting a box onto a high shelf. Calculate the efficiency of the truck if 18 000 J of energy is transferred to the gravitational store.
Correct answer: 0.75
1.33
6000 J
18 000 J
0.75 J

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