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2. Worksheet Links These slides are linked to a student worksheet on common mistakes and misconceptions about heat transfer. The sources and ideas are explained more fully in the original post. If useful in teaching, please comment here.

3. Misconceptions Seven Sins of Heat Transfer

4. Emit/Evaporate/Expand These words get mixed up because students are not clear about what they apply to. Stop and think. Are you talking about: • Thermal radiation (waves or IR) • Individual particles • The whole substance or sample

5. We say that thermal radiation is emitted or sent out by hot objects — what are the other two words that we can use to describe the behaviour of IR (infra-red) waves? absorb, reflect Some words describe specific changes of state, like melt (solid to liquid), evaporate(liquid to gas). Particles in a heated substance spread out because they have more energy, which means the material expands. The particles themselves stay the same size.

6. Contract/Condense • Contraction means getting closer together, the opposite of expand. It normally happens when a substance cools down and means it gets smaller (eg shorter). • Condensation is the change of state from a gas to a liquid (and what we call the liquid droplets formed from steam).

7. State Changes melting evaporating SOLID LIQUID GAS solidifying condensing

8. No Change of State expanding COLD SUBSTANCE HOT SUBSTANCE contracting

9. Heat Doesn’t Rise! • Heated particles have more energy… • …so move faster and spread out… • …so the substance density decreases… • …so substance rises, taking heat upwards. • Saying particles ‘move more’ isn’t enough. • The mass of the substance doesn’t change as it expands. • Heated substances rise, not heat by itself.

10. Heat Particles Don’t Exist It’s easy to think of lumps of heat energy being passed between atoms like tiny tennis balls. A better model for conduction and convection is that KE is only passed between snooker balls when they touch. If a cheat sets up magnetic balls then contact isn’t needed – this is thermal radiation.

11. Heat Particles Don’t Exist It’s convenient to think of energy in lumps (Joules), but there are no moving heat particles. In conduction, energy is passed by collisions and shared free electrons. In convection, heated particles move faster and take energy as they go, often upwards. In radiation, waves are emitted by hot objects and absorbed by colder ones.

12. Shiny Conductors • Metals have free electrons so are good at conduction. • Metals have shiny surfaces so are bad absorbers (and emitters) of radiation. Which fact matters more depends on the situation. We design the materials to make one kind of heat transfer more important.

13. Shiny Conductors • A thick layer of metal is a good conductor and if we paint it a dark colour then radiation won’t cancel this out. • A shiny material will be a good reflector and we make it very thin so conduction won’t matter too much.

14. Shiny Conductors • A thin shiny foil blanket saves more heat (reflecting it back to the person) than a thick layer of fleece would save by reducing conduction. • Foil blankets are also very small and light, making them much more practical for people doing sport.

15. Cold Can’t Get In… • Darkness is an absence of light. • Cold is the result of a lack of heat energy. If heat moves away (by any method), what is left behind is colder. • There are no cold particles and cold isn’t transferred. • Cold air can get in, usually when warm air is lost.

16. Particles and Vacuum • Conduction works because particles are touching, even without free electrons. • More spread out particles means less contact which means slower conduction. • Even gases have some contact between particles, but in a vacuum this doesn’t happen; think about how sound dies away inside a vacuum chamber.

17. Particles and Vacuum • Using a material eg foam which traps a gas in small bubbles is a compromise between reducing conduction and convection. • The material conducts some heat, more than just a gas would. • A gas by itself would be free to move because of convection, taking heat with it. heat lost by (2) > heat lost by (1)