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Energy transfer by conduction

Key idea: Learn how conduction transfers energy through solids, why metals conduct well through mobile electrons, and how to explain everyday examples.

  • SEC G3 Physics 2027
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Learning objectives

  • Compare physical properties of solids, liquids and gases
  • Explain state properties using particle arrangement, motion, forces and separation
  • Infer random molecular motion from a Brownian-motion experiment
  • Relate temperature rise to increased average kinetic energy of particles
  • Explain gas pressure using particle collisions with container walls
  • Explain heating from higher to lower temperature until thermal equilibrium
  • Describe conduction in solids through particle vibration and mobile electrons
  • Describe convection in fluids through density changes and bulk motion
  • Explain that energy transfer by electromagnetic radiation needs no material medium
  • Explain how surface colour, texture, temperature and area affect radiation transfer rate
  • Apply conduction, convection and radiation in everyday systems
  • describe internal energy as an energy store that is made up of the total kinetic energy associated with the random motion of the particles and the total potential energy between the particles in the system
  • Define heat capacity and specific heat capacity
  • Apply energy transferred = mass × specific heat capacity × temperature change
  • describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature
  • Explain the difference between boiling and evaporation
  • Define latent heat and specific latent heat
  • Apply energy transferred for a change of state = mass × specific latent heat
  • Explain latent heat using particle behaviour
  • Sketch and interpret a cooling curve

1. Definition

A. Conduction

Conduction is energy transfer through a material from a higher-temperature region to a lower-temperature region, without bulk movement of the material.

2. Key Ideas

  • Conduction is fastest in solids (especially metals) and slow in liquids and gases.
  • In solids, conduction happens by:
    • vibrations and collisions of particles, and
    • in metals, movement of free electrons.
  • Metals are good thermal conductors; many non-metals (wood, plastic, air) are thermal insulators.
  • Good conductors can feel colder even at the same temperature because they transfer energy away from your hand faster.
Why metal and plastic feel different at the same temperatureTwo hands touch blocks at the same temperature. Several solid arrows show faster energy transfer into metal, while one dashed arrow shows slower transfer into plastic.MetalPlasticsame starting temperaturesame starting temperatureskinskinfaster energy transferslower energy transfer
At the same starting temperature, metal cools the contact region of the skin faster than plastic. The arrows compare transfer rate, not object temperature.
MaterialThermal conductionExamples / uses
Good conductorfastmetals (pans, kettles)
Poor conductor (insulator)slowplastic/wood (handles), air/fabric (clothing)

3. Detailed Explanations

A. Conduction in solids (particle vibrations)

In a solid, particles are close together. When one end is heated:

  • particles there vibrate more vigorously (greater average kinetic energy)
  • interactions with neighbouring particles transfer energy through the lattice
  • energy spreads from hot region to cold region through the solid

B. Conduction in metals (free electrons)

Metals conduct very well because they have many free electrons.

  • mobile electrons near the hot end gain energy
  • they move through the metal and transfer energy in collisions with ions and other electrons
  • energy is transferred quickly from hot to cold regions

This is why a metal spoon becomes hot along its length when one end is placed in hot water.

C. Conduction in liquids and gases

Liquids and gases are generally poor conductors. They have no rigid lattice to pass vibrations along and no sea of mobile electrons as metals do. Gas particles are also far apart, so interactions are infrequent.

Comparing conduction and convection in waterA top-heated test tube with ice held at the bottom by gauze is compared with a bottom-heated beaker containing a circulating convection current.Heat at the top: conduction testheatericegauzeWarm, less-dense wateris already above cooler water.No convection loop carriesenergy down to the ice.Energy moves downwardslowly by conduction.Heat from below: convectionHeated fluid expands andbecomes less dense.Buoyancy makes it rise;cooler, denser fluid sinks.Bulk fluid motion carries energy.
With ice held below by gauze, heating water at the top suppresses convection and exposes slow conduction. Heating from below produces a density-driven convection current.

In this setup:

  • gauze holds the ice at the bottom instead of allowing it to float
  • heating is at the top, so the warmer, less-dense water is already above the cooler, denser water
  • convection does not carry energy down to the ice
  • only slow conduction would transfer energy down, so the ice can remain unmelted for a long time

D. Conduction vs convection vs radiation (quick contrast)

Don’t mix these up
  • Conduction: energy transfer without bulk movement (mostly solids).

  • Convection: energy transfer by bulk movement of a fluid (liquids/gases).

  • Radiation: energy transfer by infrared waves (no medium needed).


See: Convection and Radiation.

4. Common Mistakes

  • Saying “heat rises because of conduction” (rising is convection, not conduction).
  • Saying “cold flows into the object”. Energy is transferred by heating from the higher-temperature region to the lower-temperature region.
  • Forgetting that metals and plastics in the same room are usually at the same temperature; they just transfer energy at different rates.
  • Saying particles in a solid “move around freely”. They mainly vibrate about fixed positions.
  • Explaining the metal–plastic comparison by saying that metal starts colder. The question may state that both objects have the same temperature; the difference is the rate of energy transfer.

5. Exam Tips

  • Use the mark-scheme phrases:
    • “without bulk movement of the material”
    • “vibration/collisions of particles”
    • “free electrons transfer energy in metals”
  • When asked why a material is a good conductor, mention free electrons (for metals).
  • When asked why a material is an insulator, mention lack of free electrons and/or trapped air.

6. Worked Examples

Modelled example 1

Conduction in a metal rod (microscopic explanation)

Core

Problem

One end of a copper rod is heated. Explain how energy reaches the cooler end without bulk movement of the solid.
Study the worked solution

  1. Transfer through the lattice

    Method

    Increase vibrations at the hot end and pass energy through interactions with neighbouring ions.

    Reason

    Solid particles remain near fixed positions but can transfer energy through the lattice.

    Working

    More vigorous vibration at hot end → energy passed toward cooler regions.

  2. Add the metallic mechanism

    Method

    Use mobile electrons to carry energy rapidly through the copper.

    Reason

    Free electrons gain energy near the hot end and transfer it in collisions.

    Working

    Lattice interactions + mobile electrons → rapid metal conduction.

Guided practice 2

Saucepan handle material

About 4 min

Problem

Explain why saucepan handles are often plastic or wood rather than metal.

Match material property to function

Useful property

Hints

Hint 1: follow energy from the pan
The hot pan can transfer energy toward the hand.
Hint 2: slow that pathway
Plastic and wood are thermal insulators.
View solution step by step

  1. Identify the material property

    Method

    Describe plastic and wood as poor conductors.

    Reason

    They lack the mobile-electron mechanism that makes metals conduct rapidly.

    Working

    Low conduction rate through the handle.

  2. Connect property to safety

    Method

    Keep the hand end cooler for longer.

    Reason

    Energy from the hot pan reaches the hand more slowly.

    Working

    Poor conductor → safer handle.

Common misconception 3

Metal feels colder than plastic

Find and correct the mistake

Learner response

A metal handle and plastic handle have been in the same room for hours. A student says the metal feels colder because its temperature is lower. Diagnose the claim.

Separate temperature from transfer rate

Starting temperatures

View solution step by step

  1. Establish the initial condition

    Method

    State that both handles are at approximately room temperature.

    Reason

    They have had time to reach thermal equilibrium with the same surroundings.

    Working

    Tₘₑₜₐₗ ≈ T_plastic before touch.

  2. Explain the sensation

    Method

    Transfer energy from the warmer hand faster into metal.

    Reason

    Metal’s greater conductivity cools the skin at the contact region more quickly.

    Working

    Faster hand-energy loss → colder sensation.

Examiner practice 4

Water heated at the top

3 marks

Examination question

Gauze holds ice at the bottom of a water-filled test tube while the water is heated at the top. Explain why the ice melts slowly. [3 marks]

Address convection and conduction separately

View solution step by step

  1. Suppress convection

    2 marks

    Method

    Keep warmer, less-dense water above cooler, denser water.

    Reason

    This stable arrangement prevents a convection current carrying energy downward.

    Working

    Heating at top → no downward convection.

  2. Use slow liquid conduction

    1 mark

    Method

    Transfer energy downward only slowly by conduction.

    Reason

    Water is a poor thermal conductor.

    Working

    Slow conduction → slow melting.

Challenge 5

Insulation and trapped air

Minimal support

Building-design transfer

Double glazing traps air between two panes. Explain how the air layer reduces energy transfer by conduction.

Connect gas particle spacing to conduction rate

Hints

Hint 1: compare materials
Air conducts much less effectively than glass or metal.
Hint 2: use particle spacing
Gas particles are far apart, so energy-transfer interactions are less frequent.
View solution step by step

  1. Identify air as an insulator

    Method

    State that air is a poor conductor.

    Reason

    Its widely separated particles interact less frequently than particles in a solid.

    Working

    Low collision/interaction rate → slow conduction.

  2. Apply the trapped layer

    Method

    Insert this slow-conducting region between indoor and outdoor panes.

    Reason

    The layer increases resistance to the conduction pathway.

    Working

    Trapped air → reduced conduction through the window.

7. Mind Stretchers

Mind stretcher 1: Clothing and trapped airExtension

Wool and fleece keep you warm partly because they trap air. Explain why trapped air helps.

Show Answer

Air is a poor conductor, so trapped air reduces energy transfer by conduction from your body to the surroundings. It can also reduce convection because the air is trapped and cannot circulate easily.

Mind stretcher 2: Why a vacuum reduces energy transferExtension

A vacuum flask has a vacuum between two walls. Explain how the vacuum reduces conduction.

Show Answer

Conduction needs particles to collide and transfer energy. In a vacuum, there are (almost) no particles, so conduction is greatly reduced.

8. Practice and next step

In the Thermal Physics Explorer, identify which changes affect conduction without confusing it with bulk fluid motion. Next, compare the density-driven mechanism in convection.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027