Year 10
AQA
Higher

# Calculating efficiency (in terms of energy and power)

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

Year 10
AQA
Higher

# Calculating 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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## Slide deck

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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
5. Efficiency can be increased by lubrication, bearings or wheels.

### 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.

### 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.

### Equipment

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

### Content guidance

• Equipment requiring safe usage.

### Supervision

Adult supervision recommended.

### 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).

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## Worksheet

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## Starter quiz

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### 6 Questions

Q1.
Which of the following energy stores does friction transfer energy into?
chemical store
gravitational store
kinetic store
Correct answer: thermal store
Q2.
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.
Q3.
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.
Q4.
The law of conservation of energy states that energy cannot be created or .
Correct Answer: destroyed, used up
Q5.
Match the values on the left to the equivalent values on the right.
Correct Answer:1.5 kW,1500 W

1500 W

Correct Answer:3400 J,3.4 kJ

3.4 kJ

Correct Answer:0.34 kJ,340 J

340 J

Correct Answer:15 W,0.015 kW

0.015 kW

Q6.
Which of the following statements explains why an electric heater can be considered to be 100% efficient?
An electric heater transfers no energy to the surroundings.
Correct answer: An electric heater transfers all energy usefully into thermal stores.
An electric heater has a low power input.
An electric heater has a high power output.

## Exit quiz

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### 6 Questions

Q1.
Energy is when it is transferred to the thermal store of the surroundings.
Correct Answer: dissipated, dissipate, dissipates
Q2.
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.
Q3.
Which of the following equations show the correct relationship between efficiency, useful output energy transfer and total input energy transfer?
efficiency = useful output energy transfer × total input energy transfer
Correct answer: efficiency = useful output energy transfer ÷ total input energy transfer
Correct answer: useful output energy transfer = efficiency × total input energy transfer
Correct answer: total input energy transfer = useful output energy transfer ÷ efficiency
total input energy transfer = useful output energy transfer × efficiency
Q4.
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
Q5.
Aisha is investigating how the temperature of a tennis ball affects its rebound height. Which of the following are control variables for this experiment?
temperature of the ball
rebound height of the ball
Correct answer: height that the ball is dropped from
Correct answer: force that the ball is dropped with
Correct answer: surface that the ball is dropped on to
Q6.
An elevator has an efficiency of 0.8. When it lifts four people to the tenth floor, it uses 160 000 J of energy. How much energy is transferred to the people?
32 000 J
50 000 J
Correct answer: 128 000 J
200 000 J

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