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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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Warm-up

Answer each one, then check.

  1. 1

    Can energy be destroyed?

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    No

  2. 2

    What is friction?

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    A force opposing motion between surfaces

  3. 3

    Name a good thermal insulator.

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    Wool, foam or fibreglass

  4. 4

    What is wasted energy?

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    Energy transferred to stores that are not useful

  5. 5

    Which store increases when friction acts?

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    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.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Explain 2 marks

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

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    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.

    Mark scheme

    • Reduces friction — 1 mark
    • Less energy transferred to thermal stores — 1 mark
  2. Question 2 Explain 2 marks

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

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    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.

    Mark scheme

    • Total energy is conserved or not destroyed — 1 mark
    • Some energy dissipated to thermal stores or surroundings — 1 mark
  3. Question 3 Use the graph 4 marks

    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.
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    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.

    Mark scheme

    • C — 1 mark
    • Smallest fall in temperature or slowest cooling — 1 mark
    • Any two controls, 1 mark each — 2 marks
  4. Question 4 Explain 3 marks

    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.

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    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.

    Mark scheme

    • Rate of cooling decreases — 1 mark
    • Thicker walls — 1 mark
    • Lower rate of energy transfer by conduction — 1 mark
  5. Question 5 Describe 6 marks

    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.

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    Model answer

    See levels-of-response scheme.

    Mark scheme

    • 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

Quick check

  1. Energy that is not useful is described as...

    1. Acreated
    2. Bwasted (dissipated)
    3. Cdestroyed
    4. Dconducted only
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    B: wasted (dissipated)

    It is dissipated to the surroundings.

  2. Oil in a machine reduces...

    1. Amass
    2. Bpower
    3. Cfriction
    4. Dtemperature only
    Show answerHide answer

    C: friction

    Less friction means less wasted energy.

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

    1. Aconductor
    2. Bfuel
    3. Cmagnet
    4. Dinsulator
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    D: insulator

    Low conductivity slows the transfer of energy.

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

    1. Asmaller
    2. Bgreater
    3. Cthe same
    4. Dimpossible to say
    Show answerHide answer

    A: smaller

    Less energy is transferred per second.

  5. Which is a control variable in the insulation experiment?

    1. AType of insulating material
    2. BStarting temperature of the water
    3. CTemperature fall
    4. DColour of the beaker
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    B: Starting temperature of the water

    It must be the same for every test.

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