Transfer of Thermal Energy — Cambridge O Level Physics 5054 Revision Notes & Topical Questions

Section 1 of 10Thermal energy and why it moves

• Thermal energy transfer
  • thermal energy flows from a hotter region to a cooler region until the temperatures are equal.
• Three methods
  • conduction, convection and radiation, which can occur together.
• Rate of transfer
  • it increases with a larger temperature difference, a larger surface area and a better conducting material.
  • A large difference in temperature makes the energy flow faster.
• Link to the particle model
  • in solids and fluids the particles pass on kinetic energy, and in radiation energy travels as waves.

Section 2 of 10Conduction

• Conduction
  • the transfer of thermal energy through a material without any movement of the material as a whole.
• In non-metals
  • hotter particles vibrate more and pass energy to neighbouring particles by collisions.
• In metals
  • free electrons gain energy, move through the metal and collide with other particles, so metals conduct faster.
  • This is why metals feel coldthey conduct heat away from your hand quickly.
• Conductors and insulators
  • good conductors transfer energy quickly, and insulators transfer it slowly.

Good thermal conductors	Good thermal insulators
Copper, aluminium, iron, silver	Wood, plastic, glass, wool
Free electrons carry energy	No free electrons
Used for cooking pans	Used for pan handles and oven gloves
Used for radiators	Used for lagging pipes

• Gases and liquids
  • they are poor conductors because the particles are far apart; trapped air is an excellent insulator.
  • Fur, feathers and woollen jumpers trap air, which keeps the body warm.
• Experiments for conduction
  • compare how well different rods conduct.

1. Fix identical rods of different metals to a heating source with wax and a pin on each rod.
2. Heat one end of each rod equally.
3. Observe which pin falls first as the wax melts.
4. The metal whose pin falls first is the best conductor.

Section 3 of 10Convection

• Convection
  • the transfer of thermal energy by the movement of a fluid (liquid or gas) due to differences in density.
• Mechanism
  • it works in the following sequence.

1. The fluid is heated and its particles move apart so the fluid expands.
2. The warmer fluid is less dense and rises.
3. Cooler, denser fluid sinks to take its place.
4. A circulating convection current is set up and carries energy through the fluid.

• Not possible in solids
  • the particles in a solid cannot move from place to place.
• Examples
  • sea breezes, central heating, a kettle heating water and cold air sinking from a freezer.
  • By day the land is hotter, air rises over it and cool air blows in from the sea.
• Demonstration
  • place a crystal of potassium permanganate at the side of a beaker of water and heat below it;
    • the dye rises on one side and sinks on the other.
• Heater position
  • a room heater is placed low and a freezer compartment high so convection currents spread the effect.

Section 4 of 10Radiation

• Thermal radiation
  • the transfer of energy by infrared electromagnetic waves, which can travel through a vacuum.
  • It is how the Sun's energy reaches the Earth through space.
• Emission
  • every object above absolute zero emits infrared radiation, and hotter objects emit more.
• Surface effects
  • the type of surface changes how much radiation is emitted and absorbed.

Surface	Emission	Absorption	Reflection
Dull black	Best emitter	Best absorber	Poorest reflector
Matt (dull)	Good	Good	Poor
Shiny white or silver	Poorest emitter	Poorest absorber	Best reflector

• Surface area
  • a larger surface area emits more radiation, which is why elephants have large ears and radiators have fins.
• Temperature
  • a hotter object radiates energy at a higher rate.
• Practical: comparing emitters
  • use a Leslie cube with four faces of different surface type filled with hot water.

1. Fill the cube with hot water so all four faces are at the same temperature.
2. Place an infrared detector the same distance from each face in turn.
3. Record the reading for each surface.
4. The matt black face gives the largest reading and the shiny silver face the smallest.

• Practical: comparing absorbers
  • attach wax to the back of two metal plates, one black and one shiny, at equal distances from a heater.
  • The wax melts first on the black plate.

Section 5 of 10Applications: vacuum flask and houses

• Vacuum flask
  • it reduces energy transfer by all three methods.

Part	Reduces
Vacuum between double walls	Conduction and convection
Silvered (shiny) surfaces	Radiation, by reflecting it
Plastic or cork stopper	Conduction and convection from the top
Glass walls (a poor conductor)	Conduction

• House insulation
  • loft insulation, cavity wall foam, double glazing and draught excluders reduce energy loss.
  • Trapped air in the material and in a cavity acts as an insulator.
• Solar heating
  • a solar panel uses a black surface to absorb radiation and heats water that flows through pipes.
• Cooking and clothing
  • pans conduct energy to the food, and light-coloured clothes reflect radiation in hot weather.
• Cooling a hot object
  • fins, a dull black surface and a draught increase the rate of cooling.

Section 6 of 10Comparing the methods

Feature	Conduction	Convection	Radiation
Medium needed	Yes, a solid mostly	Yes, a fluid	No, travels through a vacuum
Particle movement	Vibrations and electron collisions	Bulk movement of fluid	None, waves
Speed	Slow	Moderate	Fastest, at the speed of light
Example	Metal spoon in hot tea	Hot air rising	Sun warming your face

• Which method applies
  • in a solid it is conduction; in a liquid or gas it is convection; through a vacuum it must be radiation.
• Several methods together
  • a fire warms a room by radiation directly and by convection currents of air.

Section 7 of 10Everyday consequences of thermal transfer

• Kitchen pans
  • the base is metal, such as copper or aluminium, so conduction carries energy quickly from the hob to the food.
  • The handle is plastic or wood, which is an insulator, so it stays cool enough to hold.
  • A dull black base absorbs radiation from the flame or element better than a shiny one.
• Heating a room by convection
  • a radiator warms the air touching it.

1. The warm air expands, becomes less dense and rises.
2. Cooler air from lower in the room flows toward the radiator to replace it.
3. The warm air spreads across the ceiling, cools and sinks on the far side.
4. A circulating current of air heats the whole room.

• Infrared thermometer
  • it detects the infrared radiation emitted by a surface and displays the temperature.
  • It works without touching the object, so it can be used on moving, very hot or dangerous objects, and it gives a reading quickly.
  • A hotter surface emits more infrared radiation, so the detector signal is larger.
• Insulating a hot drink
  • a lid reduces convection and evaporation, a double wall with a vacuum or foam reduces conduction, and a shiny surface reduces radiation.
• Insulating buildings
  • they reduce thermal energy transfer in several ways.

Measure	Reduces	How
Loft insulation (fibreglass)	Conduction and convection	Traps air in small pockets
Cavity wall foam	Conduction and convection	Stops air circulating in the cavity
Double glazing	Conduction and convection	A layer of air or gas between panes
Reflective foil behind radiators	Radiation	Reflects infrared back into the room
Draught excluders	Convection	Block air movement through gaps

Section 8 of 10Worked examples

1. Explain why a metal spoon feels colder than a wooden spoon at the same room temperature. [3]3 marks

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2. Describe how convection currents heat a room using a radiator. [4]4 marks

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3. Explain two reasons why the inside surface of a vacuum flask is silvered. [2]2 marks

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4. Describe an experiment to show that a black surface is a better absorber of radiation than a white surface. [4]4 marks

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5. Why are the fins on a car radiator painted black? [2]2 marks

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6. Explain why a metal pan with a plastic handle is better than a pan made wholly of metal. [3]3 marks

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Section 9 of 10Key terms

Term	Meaning
Conduction	Transfer of thermal energy through a material by particle vibrations and electrons.
Convection	Transfer of thermal energy by the movement of a fluid due to density differences.
Radiation	Transfer of thermal energy by infrared waves, needing no medium.
Insulator	A material that conducts thermal energy poorly.
Convection current	The circulation of a fluid caused by warm fluid rising and cool fluid sinking.
Emitter	A surface that gives out thermal radiation.
Leslie cube	A hollow metal cube with different faces, used to compare radiation emitted.

Section 10 of 10Exam technique

1. Saying heat rises — it is hot fluid that rises because it is less dense; heat flows from hot to cold.
2. Thinking radiation needs a medium — it travels through a vacuum.
3. Saying metals feel cold because they are cold — they conduct heat from your hand faster.
4. Using convection to explain energy transfer in solids — it only occurs in fluids.
5. Forgetting that shiny surfaces are poor emitters — silver reflects instead of emitting or absorbing.