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Physics · Energy

Energy transfers, conservation and dissipation

Describe how energy is conserved but dissipated, explain ways of reducing unwanted energy transfers, and investigate thermal insulators (Required Practical 2, Physics only).

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    Can energy be destroyed?

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    No

  2. 2

    What is friction?

    Show answerHide answer

    A force opposing motion between surfaces

  3. 3

    Name a good thermal insulator.

    Show answerHide answer

    Wool, foam or fibreglass

  4. 4

    What is wasted energy?

    Show answerHide answer

    Energy transferred to stores that are not useful

  5. 5

    Which store increases when friction acts?

    Show answerHide answer

    Thermal

Learning Objectives

  1. 1Describe energy transfers in a closed system.
  2. 2Explain how energy is dissipated.
  3. 3Explain ways of reducing unwanted energy transfers: lubrication and thermal insulation.
  4. 4Describe how thickness and thermal conductivity affect the rate of cooling of a building.
  5. 5Describe an investigation into insulating materials.

DISSIPATION

Energy is never lost: it is transferred usefully, stored or dissipated. In every change some energy is dissipated into less useful stores.

The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction through it.

Reducing Unwanted Energy Transfers

  • Lubrication

    Oil between moving parts reduces friction, so less energy is dissipated to the thermal store.

  • Thermal insulation

    Materials with low thermal conductivity, like foam and wool, reduce the rate of heat transfer.

  • Thicker walls

    A thicker layer means a lower rate of energy transfer through it.

  • Lower conductivity

    Walls made of material with lower thermal conductivity cool more slowly.

A House Cooling

Explain how the thickness and thermal conductivity of a house's walls affect its rate of cooling.

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  1. 1 Thickness Thicker walls slow the transfer of energy from inside to outside, so the house cools more slowly
  2. 2 Conductivity Walls of lower thermal conductivity give a lower rate of energy transfer
  3. 3 Together Thick walls made of a low-conductivity material give the lowest rate of cooling

AnswerThe rate of cooling decreases with thicker walls and with lower thermal conductivity.

Required Practical 2: Method

Compare insulating materials.

  1. 1 Set up

    Put a beaker of hot water inside a larger beaker with the insulating material packed around it.
  2. 2 Same start

    Use the same volume of water at the same starting temperature (about 80 °C) for each material.
  3. 3 Measure

    Record the temperature every minute for 15 to 20 minutes.
  4. 4 Repeat

    Repeat with other materials and a control with no insulation.
  5. 5 Compare

    Plot cooling curves. The smallest temperature fall shows the best insulator.

Control Variables

Make it a fair test.

  • Independent variable

    The type of insulating material (or its thickness).

  • Dependent variable

    The temperature drop, or the time to cool by a set amount.

  • Control

    Volume of water, starting temperature, beaker size, lid, room temperature.

Insulate the Cup

You have four cups, cotton wool, foam, newspaper and bubble wrap. Plan a fair test to find the best insulator, listing what you keep the same.

1. State what you change.

2. State what you measure and keep the same.

A good answer shows: Independent: material. Dependent: temperature fall in 10 minutes. Control: volume and starting temperature of water, cup and lid, thickness of material, room conditions. Repeat and find a mean.

Can I...?

  1. 1Explain energy conservation.
  2. 2Explain dissipation.
  3. 3Give ways to reduce unwanted energy transfers.
  4. 4Explain lubrication.
  5. 5Link thermal conductivity to rate of transfer.
  6. 6Describe Required Practical 2.
  7. 7Name control variables.
  8. 8Read cooling curves.

Summary & Exam Focus

  • Total energy is conserved.
  • Energy is always dissipated to the surroundings.
  • Lubrication reduces friction; insulation reduces heat loss.
  • Low thermal conductivity means slower heat transfer.

Exam focus

A student compares three insulating materials by measuring how quickly hot water cools. Describe how to make the test fair. (3 marks) (3 marks)

Name what you change, measure and keep the same.

Key terms

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

Dissipated
Transferred to the surroundings and stored in less useful ways.
Thermal conductivity
How well a material conducts thermal energy.
Insulator
A material with low thermal conductivity.
Lubrication
Using oil or grease to reduce friction.
Closed system
A system where no energy enters or leaves.
Control variable
A variable kept the same in a fair test.

Questions and answers

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

1. Exam question Explain 2 marks Easier

A bicycle chain is lubricated with oil. Explain how this reduces the amount of energy wasted.

Mark scheme — 2 marks available

  • Reduces friction — 1 mark
  • Less energy transferred to thermal stores — 1 mark

Model answer

The oil reduces friction between the moving parts, so less energy is transferred to the thermal energy stores of the chain and surroundings.

2. Exam question Explain 2 marks Easier

A ball bounces, and each bounce is lower than the one before. Explain why, using the idea of conservation of energy.

Mark scheme — 2 marks available

  • Total energy is conserved or not destroyed — 1 mark
  • Some energy dissipated to thermal stores or surroundings — 1 mark

Model answer

Energy is not destroyed. Some energy is dissipated to the thermal energy store of the ball, floor and surroundings each time, so less is available as gravitational potential energy.

3. Exam question Use the graph 4 marks Easier

A student investigates how quickly hot water cools in three beakers, A, B and C. Each beaker is surrounded by a different insulating material. The graph shows the results. (a) Which beaker was surrounded by the best insulator? Give a reason. (b) Give two variables that should be controlled.

Three cooling curves for beakers A, B and C, with C cooling the least.

Mark scheme — 4 marks available

  • C — 1 mark
  • Smallest fall in temperature or slowest cooling — 1 mark
  • Any two controls, 1 mark each — 2 marks

Model answer

(a) Beaker C, because the temperature fell the least. (b) Any two from: volume of water, starting temperature, size of beaker, thickness of insulation, lid.

4. Exam question Explain 3 marks Easier

A house has walls of thickness 20 cm. The walls are replaced with walls of the same material but thickness 30 cm. Explain what happens to the rate at which the house cools.

Mark scheme — 3 marks available

  • Rate of cooling decreases — 1 mark
  • Thicker walls — 1 mark
  • Lower rate of energy transfer by conduction — 1 mark

Model answer

The rate of cooling decreases. Thicker walls reduce the rate of energy transfer by conduction through the wall, so the house loses energy more slowly.

5. Exam question Describe 6 marks Core

Describe an investigation to compare the effectiveness of different materials as thermal insulators. Your answer should include the apparatus, how you would make it a fair test, and how you would use the results.

Mark scheme — 6 marks available

  • Level 3 (5 to 6 marks): a clear method using beakers, hot water, thermometer and timer with each material; several controls named; temperature change compared to identify the best insulator; repeats — 5 to 6 marks
  • Level 2 (3 to 4 marks): a method with some controls and a way to compare — 3 to 4 marks
  • Level 1 (1 to 2 marks): simple statements about insulating a beaker — 1 to 2 marks

Model answer

See levels-of-response scheme.

6. Multiple choice 1 mark Easier

Energy that is not useful is described as...

  1. A created
  2. B wasted (dissipated) Correct
  3. C destroyed
  4. D conducted only

Why: It is dissipated to the surroundings.

7. Multiple choice 1 mark Core

Oil in a machine reduces...

  1. A mass
  2. B power
  3. C friction Correct
  4. D temperature only

Why: Less friction means less wasted energy.

8. Multiple choice 1 mark Core

A material with low thermal conductivity is a good...

  1. A conductor
  2. B fuel
  3. C magnet
  4. D insulator Correct

Why: Low conductivity slows the transfer of energy.

9. Multiple choice 1 mark Core

Thicker walls make the rate of cooling of a house...

  1. A smaller Correct
  2. B greater
  3. C the same
  4. D impossible to say

Why: Less energy is transferred per second.

10. Multiple choice 1 mark Stretch

Which is a control variable in the insulation experiment?

  1. A Type of insulating material
  2. B Starting temperature of the water Correct
  3. C Temperature fall
  4. D Colour of the beaker

Why: It must be the same for every test.