Energy transfer by conduction
Key idea: In conduction, energy passes from hotter to cooler parts through vibrating particles, without the material itself moving. Metals conduct well because their free electrons also carry energy.
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Guided steps
- Follow energy transfer between vibrating neighbours.
- Compare lattice vibrations with energy carried by mobile electrons.
- Follow both energy-transfer mechanisms in copper.
- Explain conduction in metal and compare it with glass.
- Compare the conductors and insulators.
- Explain why the handle warms more slowly.
- Explain how trapped air slows conduction and convection.
- Explain which energy paths remain across and around the vacuum.
- Compare starting temperature with the rate of skin cooling.
- Explain why metal feels colder at the same temperature.
- Explain why the hottest water stays near the top.
- Explain both convection and conduction in the ice experiment.
- Predict the pin order and plan a fair rod comparison.
- Compare matched pins and explain the model’s dropping order.
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Put a metal spoon in hot tea and soon the handle is hot too, although no part of the spoon has moved. Energy has travelled along it by conduction: energy transfer through a material from a hotter region to a colder one, without any bulk movement of the material. Conduction is how energy passes through solids. Convection and radiation, the other two ways energy travels, have lessons of their own.
How a solid conducts
In a solid, the particles are packed closely and held in place, vibrating about fixed positions. When one end is heated, the particles there vibrate more vigorously. Because each particle is linked to its neighbours, it pushes on them and makes them vibrate more too. The extra vibration is passed from particle to particle along the solid, from the hot end towards the cold end.
The solid particles vibrate about their positions rather than flowing along the solid. Interactions transfer energy through the material. The rate depends on the material: vibration-based transfer is relatively slow in many insulators, but some non-metals, such as diamond, conduct well.
Why metals conduct so well
A metal conducts in the same way, but it also has free electrons that move through the metal’s lattice. Near the hot end they gain kinetic energy. They move rapidly between collisions, carry energy to the cooler parts of the metal and pass it on in collisions with ions and other electrons. This is much faster than passing vibrations from neighbour to neighbour, which is why metals are good thermal conductors.
The electrons move in many directions, but their net energy transfer is from hotter to cooler regions. This does not require an electric current.
Worked example 1
Conduction along a copper rod
Problem
One end of a copper rod is heated. Explain how energy reaches the cooler end, even though no part of the rod moves along it.
Show full solution
Vibrations passed through the lattice
Method
The ions at the hot end vibrate more and make their neighbours vibrate more.
Reason
The ions are linked to one another, so extra vibration is handed along.
Working
Answer
More vigorous vibration at the hot end → passed from ion to ion towards the cooler end
Free electrons carry energy quickly
Method
Free electrons gain kinetic energy at the hot end, move into cooler regions and give it up in collisions.
Reason
Mobile electrons provide an efficient additional route for energy transfer, beyond the lattice vibrations alone.
Working
Answer
Lattice vibrations + free electrons → rapid conduction in copper
Quick check
Check your understanding 1 Explain particle motion and energy transfer
Use the figure to explain how energy reaches the cooler end of a metal. What moves locally, what travels along the rod, and which transfer mechanism is absent from glass?
Show answer
Check three distinctions. Ions vibrate near fixed positions; they do not flow from hot to cold. Energy passes between neighbouring particles, from hotter to cooler regions. Mobile electrons move in many directions, gain energy in hotter regions and give it up in cooler regions through collisions. Glass lacks the freely moving electrons that carry energy in a metal and conducts more slowly.
Good and poor conductors
| Material | Conducts | Why | Used for |
|---|---|---|---|
| metals | fast | free electrons carry energy | pans, kettles, heat sinks |
| plastic, wood, glass | slowly compared with metals | energy transfer through particle interactions, without mobile electrons | handles, mats |
| water | slowly compared with metals | particle interactions, without mobile electrons | — |
| ordinary air | very slowly compared with these solids | low particle density; energy carried between molecular collisions | insulation, clothing |
Ordinary air is a much poorer thermal conductor than metals. Its particles are far apart, and their motion and collisions transfer energy less effectively than in most solids. Many good insulators, such as fleece and foam, work by trapping air in small pockets, where it cannot move about to carry energy by convection. Note that “non-metal” does not always mean “poor conductor”: diamond is a non-metal that conducts very well.
Guided practice 1
Choose a handle material
Problem
Explain why saucepan handles are often made of plastic or wood rather than metal.
Match the material to its job
Hints
Hint 1
follow the energy from the pan
Energy is conducted from the hot pan towards your hand.
Hint 2
slow that path down
Plastic and wood lack the mobile-electron energy-transfer route found in metals.
Show solution step by step
Identify the property
Method
Plastic and wood are poor conductors.Reason
They have no freely moving electrons to carry energy. Energy spreads through particle interactions more slowly than in metals.
Working
Answer
Slow conduction through the handleLink the property to safety
Method
The end you hold stays cool for longer.Reason
Energy from the hot pan reaches your hand only slowly.
Working
Answer
Poor conductor → slower warming at the grip
Quick check
Try it yourself 1
Double glazing
Problem
Double glazing traps a thin layer of air between two panes of glass. Explain how the air layer reduces energy transfer by conduction through the window.
Link particle spacing to how fast energy is conducted
Hints
Hint 1
compare the materials
Which material in the path conducts energy most slowly?
Hint 2
use particle spacing
Air has low particle density and conducts much less well than the glass panes.
Show solution step by step
Identify air as a poor conductor
Method
Air conducts very slowly.Reason
Molecular motion and collisions transport energy, but air’s thermal conductivity is much lower than that of the panes.
Working
Answer
Air layer → slow conductionApply it to the window
Method
Energy has to cross the air layer to get from the inner pane to the outer pane.
Reason
The slowest part of the path limits the rate of transfer through the whole window.
Working
Answer
Trapped air layer → much less conduction through the window
Quick check
Check your understanding 2 Why a vacuum stops conduction
An ideal vacuum separates a flask’s two walls. Explain why conduction cannot cross that gap, and whether the vacuum also stops radiation or conduction through the neck and supports.
Show answer
Conduction needs matter. An ideal vacuum has no material to transport energy across the gap. A practical evacuated gap greatly reduces gas conduction, but infrared radiation can still cross it, and conduction can still occur through solid connections such as the neck and supports.
Why metal feels colder than plastic
On a cold morning, a metal door handle feels colder than a plastic one, even though both have been in the same place all night and are at the same temperature.
Both handles are colder than your hand, so energy flows from your hand into each. The metal is a good conductor: it carries energy away from the skin it touches quickly. The plastic carries energy away slowly, so the skin under it stays warmer. Touch tells you how quickly your skin gains or loses energy, so use a thermometer to compare temperatures.
Spot the mistake 1
Does the metal handle feel colder because it is colder?
Learner response
A metal handle and a plastic handle have both reached room temperature, which is below skin temperature. A student says the metal feels colder because its temperature is lower. Find the error.
Separate temperature from transfer rate
Explain the full cause before using the optional starting-temperature choice.
Show solution step by step
Check the starting temperatures
Method
Both handles are at about room temperature.Reason
They have been in the same surroundings long enough to reach thermal equilibrium.Working
Answer
T metal ≈ T plastic before you touch themExplain the feeling
Method
The metal conducts energy away from your hand faster.Reason
The skin touching the metal loses energy faster, so it cools faster and feels colder.Working
Answer
Faster energy loss from the skin → feels colder
Common mistake
Water is a poor conductor
In the experiment above, a piece of ice is held at the bottom of a test tube of water by a weighted gauze, and the water near the top is heated. The water at the top can boil while the ice at the bottom stays frozen.
With little stirring and little direct heating of the bottom through the glass or gauze, the hottest water stays above the cooler water. A large convection loop does not carry it down to the ice. Conduction through water is slow, so the ice can remain while the upper water boils.
Exam-style question 1
Ice at the bottom of a test tube
Examination question
Weighted gauze holds ice at the bottom of a test tube of water while the water near the top is heated. Assume little stirring and little direct heating of the bottom through the glass or gauze. Explain why the ice melts only slowly. [3 marks]
Deal with convection and conduction separately
Show solution step by step
No convection downwards
2 marksMethod
The heated water is less dense, so it stays above the cooler, denser water.
Reason
The hottest, less-dense upper layer is stable above the cooler water; no large convection loop carries it down to the ice.
Working
Answer
Heating at the top → no convection current to the ice
Slow conduction through the water
1 markMethod
The main downward transfer through the water in this model is slow conduction.
Reason
Water is a poor conductor.Working
Answer
Slow conduction → slow melting
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Self-mark the density arrangement, the absence of downward convection and slow conduction.
Common mistake
“Heat rises” alone earns no marks: warmer, less dense water stays at the top, and conduction through water is slow.
Common mistakes
- Saying the solid lattice flows from hot to cold. Its particles vibrate near fixed positions; mobile electrons in a metal can travel and carry energy.
- Explaining metal conduction without the free electrons.
- Saying a metal “feels colder because it is colder” when both are at room temperature.
- Saying “cold flows into” an object. Net energy transfer by heating is from the hotter region to the colder one.
- Mixing up conduction (no bulk movement) with convection (the fluid itself moves).
Before you move on
Check your understanding 3 Plan the rod comparison before running the model
The model heats one end of equal-length copper, aluminium, iron and glass rods. All start at 25 °C; wax-fixed pins drop at 50 °C. Predict which matched pin drops first and how distance affects dropping time. Describe the controls needed for a fair practical comparison.
Show answer
In this model, matched metal pins drop first on copper, then aluminium and iron. The glass pins stay attached during the model’s run. Pins farther from the heated end warm later; some may not reach 50 °C during the observation.
Compare pins at the same distance. Keep rod length and cross-section, starting temperature, hot-end temperature, wax amount and pin type the same. Keep heating contact and surrounding conditions alike, and repeat the timings. A dropped pin marks the wax reaching 50 °C at that position.
Metals conduct rapidly through lattice interactions and mobile electrons. Glass lacks these freely moving electrons. The model uses thermal diffusivity: spreading temperature depends on density and heat capacity as well as conductivity, so pin timing alone is not a direct conductivity measurement.
Four rods heated at one end. Pins dropped so far: copper 0 pins, aluminium 0 pins, iron 0 pins, glass 0 pins.
- Temperature
- — °C
- State
- —
- Energy supplied, E = P t
- — kJ
- Dull black can
- — °C
- Shiny silver can
- — °C
Try this
0 of 4 doneHeat the ice until the water boils, and compare the two flat parts of the graph. (not done yet)
While ice melts or water boils, latent energy changes the arrangement and interparticle potential energy instead of raising temperature. Ice melting does not require increased particle separation. For the same mass of water, boiling takes far more energy than melting.
Record a cooling curve for stearic acid from above 72 °C to below 66 °C. (not done yet)
The flat part of your curve is the melting point, 69 °C: as the liquid solidifies it gives out latent heat, which balances the heat lost.
Find out which rod conducts heat best. (not done yet)
Copper's pins drop first. In metals, free electrons carry energy along the rod quickly; glass relies only on vibrating particles.
Compare how quickly the dull black and shiny silver cans warm up. (not done yet)
The dull black can warms faster: dull black surfaces are good absorbers (and good emitters) of infrared radiation; shiny surfaces reflect it.
You’ve tried every task.
Your readings
| # | t / min | θ / °C | Remove |
|---|---|---|---|
| No readings yet. Set up a measurement, then record it. | |||
Check your understanding 4 Compare the model with your prediction
Select Play in Conduction along rods and compare the nearest pin on each rod. Record which pins drop and which stay attached, then compare a near pin with a farther pin on copper. Explain the order, then say why pin timings alone cannot give thermal conductivity.
Show answer
The nearest pins drop first on copper, then aluminium and iron. The glass pins stay attached throughout the 1800 s model run because they never reach the 50 °C threshold. On copper, a farther pin warms later and may not drop during a short run. Metals have mobile electrons as an additional energy-transfer route; glass lacks that route. Compare your observation with your prediction using matched positions. The model’s temperature-spreading rates depend on thermal diffusivity, which includes density and specific heat capacity as well as conductivity. Pin timings therefore need more information before they can be converted into conductivity.
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