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Physics · Energy

Energy stores and systems

Describe the changes in the way energy is stored when a system changes, using energy stores and transfer pathways.

  • 6 key terms
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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is the unit of energy?

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    The joule (J)

  2. 2

    What is a system?

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    An object or group of objects

  3. 3

    Name the energy store in a stretched spring.

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    Elastic potential

  4. 4

    Name the energy store in food and fuels.

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    Chemical

  5. 5

    Can energy be created or destroyed?

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    No, it can only be transferred, stored or dissipated

Learning Objectives

  1. 1Define a system and describe changes in the way energy is stored when a system changes.
  2. 2Name the main energy stores and the four transfer pathways.
  3. 3Describe the energy changes in common situations, such as an object projected upwards or a kettle boiling water.
  4. 4Write clear statements using the phrase 'energy is transferred from ... to ...'.

CONSERVATION OF ENERGY

Energy cannot be created or destroyed. It can be transferred usefully, stored or dissipated.

A system is an object or group of objects. When a system changes, there are changes in the way energy is stored.

The Energy Stores

Learn the main stores and an example of each.

  • Kinetic

    Examples: Any moving object

  • Gravitational potential

    Examples: An object above the ground

  • Elastic potential

    Examples: A stretched or compressed spring, an elastic band

  • Thermal

    Examples: A hot object; the particles have more energy

  • Chemical

    Examples: Fuels, food, batteries

  • Nuclear

    Examples: The nucleus of an atom

  • Magnetic

    Examples: Magnets and electromagnets

  • Electrostatic

    Examples: Charged objects, such as a charged balloon

Ways Energy Is Transferred

  • Mechanically

    A force moves an object through a distance (work done).

  • Electrically

    Charge flows in a circuit (work done by the current).

  • By heating

    Energy flows from a hotter object to a cooler one.

  • By radiation

    Energy is carried by waves, such as light and sound.

A Ball Thrown Upwards

Describe the changes in the way energy is stored when a ball is thrown upwards, until it reaches its highest point.

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  1. 1 Just after leaving the hand The ball has a large kinetic energy store
  2. 2 As it rises It slows down, so the kinetic energy store decreases
  3. 3 At the same time Its height increases, so the gravitational potential energy store increases
  4. 4 Small effect Some energy is transferred to the thermal store of the air by friction

AnswerEnergy is transferred mechanically from the kinetic store to the gravitational potential store, with a little dissipated to the thermal store of the air.

Boiling Water in a Kettle

Describe the changes in the way energy is stored when a kettle boils water.

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  1. 1 Electrical work Charge flowing through the heating element transfers energy from the mains
  2. 2 Heating element The thermal energy store of the element increases
  3. 3 By heating Energy is transferred to the thermal energy store of the water, which increases
  4. 4 Wasted Some energy heats the kettle and the surroundings

AnswerEnergy is transferred electrically and then by heating to the thermal store of the water; some is dissipated to the surroundings.

Writing Good Answers

Use the language of stores and transfers.

  • Name the stores

    Say 'the kinetic energy store decreases', not 'kinetic energy is lost'.

  • Say how

    Give the pathway: mechanically, electrically, by heating or by radiation.

  • Say where the rest goes

    Mention dissipation to the surroundings if the system is not perfectly efficient.

  • Total energy

    The total energy after the change is the same as before.

Store to Store

For each situation, name the store that decreases and the store that increases. (a) A car braking. (b) A book falling off a shelf. (c) A battery powering a motor lifting a weight. (d) A bow being drawn and released.

1. Name the store before.

2. Name the store after.

A good answer shows: (a) Kinetic decreases; thermal (brakes) increases. (b) Gravitational potential decreases; kinetic increases. (c) Chemical decreases; gravitational potential increases (plus some thermal). (d) Chemical (archer) to elastic potential, then elastic decreases; kinetic of the arrow increases.

Can I...?

  1. 1Name the main energy stores.
  2. 2Name the four transfer pathways.
  3. 3Define a system.
  4. 4Describe energy changes for an object projected upwards.
  5. 5Describe energy changes when a vehicle slows down.
  6. 6Describe how a kettle heats water.
  7. 7Use 'increases' and 'decreases' correctly.
  8. 8State that total energy is conserved.

Summary & Exam Focus

  • Energy can be transferred, stored or dissipated but not created or destroyed.
  • Describe changes with 'store decreases' and 'store increases'.
  • Pathways: mechanically, electrically, by heating, by radiation.
  • In a closed system the total energy does not change.

Exam focus

A car slows down to a stop using its brakes. Describe the changes in the way energy is stored. (2 marks) (2 marks)

Say which store decreases and which increases.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

System
An object or group of objects.
Energy store
A way energy is held, such as kinetic or thermal.
Transfer
Energy moving from one store to another.
Dissipated
Spread out to the surroundings in less useful stores.
Closed system
A system where no energy enters or leaves.
Work done
Energy transferred by a force or a current.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Which 1 mark Easier

A ball is thrown upwards. Which energy store increases as the ball rises? Tick one box. kinetic / gravitational potential / thermal / chemical

Mark scheme — 1 mark available

  • Gravitational potential — 1 mark

Model answer

Gravitational potential.

2. Exam question Describe 2 marks Easier

A car slows down to a stop using its brakes. Describe the changes in the way energy is stored.

Mark scheme — 2 marks available

  • Kinetic energy store decreases — 1 mark
  • Thermal energy store of the brakes or surroundings increases — 1 mark

Model answer

The kinetic energy store of the car decreases. The thermal energy store of the brakes (and surroundings) increases.

3. Exam question Describe 3 marks Easier

An electric kettle is used to bring water to the boil. Describe the changes in the way energy is stored.

Mark scheme — 3 marks available

  • Electrical transfer from the mains — 1 mark
  • Thermal store of the water increases — 1 mark
  • Some energy dissipated to the surroundings — 1 mark

Model answer

Energy is transferred electrically to the thermal energy store of the heating element and then by heating to the thermal energy store of the water, which increases. Some energy is transferred to the thermal energy store of the surroundings.

4. Exam question Describe 2 marks Easier

A box slides across a rough floor and comes to rest. Describe the changes in the way energy is stored.

Mark scheme — 2 marks available

  • Kinetic store decreases — 1 mark
  • Thermal stores of box, floor or surroundings increase — 1 mark

Model answer

The kinetic energy store of the box decreases. The thermal energy stores of the box and floor increase because of friction.

5. Exam question Describe 4 marks Easier

A bungee jumper steps off a platform, falls, and comes to rest momentarily at the lowest point when the cord is stretched. Describe the changes in the way energy is stored during the fall.

Mark scheme — 4 marks available

  • Gravitational potential store decreases — 1 mark
  • Kinetic store increases then decreases to zero — 1 mark
  • Elastic potential energy store of the cord increases — 1 mark
  • Some energy dissipated to the surroundings — 1 mark

Model answer

The gravitational potential energy store decreases as the jumper falls. The kinetic energy store increases. As the cord stretches, the elastic potential energy store of the cord increases while the kinetic store decreases to zero at the lowest point. A small amount of energy is dissipated to the thermal energy store of the air.

6. Multiple choice 1 mark Easier

Which of these is an energy store?

  1. A Force
  2. B Kinetic Correct
  3. C Speed
  4. D Weight

Why: Kinetic is a store; force, speed and weight are not.

7. Multiple choice 1 mark Core

A bow is drawn back. Which store increases?

  1. A Chemical
  2. B Nuclear
  3. C Elastic potential Correct
  4. D Magnetic

Why: The bow is stretched, so its elastic potential energy store increases.

8. Multiple choice 1 mark Core

Which is NOT a way of transferring energy?

  1. A By heating
  2. B Electrically
  3. C By radiation
  4. D By insulation Correct

Why: The pathways are mechanically, electrically, by heating and by radiation.

9. Multiple choice 1 mark Core

In a closed system the total energy...

  1. A stays the same Correct
  2. B increases
  3. C decreases
  4. D is destroyed

Why: Energy is conserved.

10. Multiple choice 1 mark Stretch

A falling apple: which store decreases?

  1. A Kinetic
  2. B Gravitational potential Correct
  3. C Thermal
  4. D Elastic potential

Why: As it falls, gravitational potential energy is transferred to kinetic energy.