Exercises: Heat transfer: conduction, convection and radiation

You leave a metal spoon in hot soup and, after a while, the handle is also hot even though it does not touch the liquid, as shown in the diagram.

What is the name of the mechanism by which heat travelled along the spoon? Explain it briefly.

The Sun heats the Earth even though there are about $150$ million kilometres of empty space between them, as shown in the diagram.

What heat transfer mechanism makes it possible for its energy to reach us? Why can't it be the other two?

When turning on the radiator, the air in the room gradually warms up, starting with the air next to the radiator, which rises towards the ceiling, as shown in the diagram.

What mechanism distributes heat through the air in the room? Describe it.

Classify these materials as good thermal conductors or thermal insulators, and justify your choice:

1. a) An aluminium frying pan.
2. b) The wooden handle of the frying pan.
3. c) A woollen jumper.
4. d) A steel spoon.
5. e) A cork stopper.

Why do saucepans and frying pans have wooden or plastic handles instead of being made from a single piece of metal? Explain this using what you know about heat conduction.

At the beach at midday, a breeze can be felt blowing from the sea towards the sand: the air above the hot sand rises and the cooler air from the sea comes to take its place, as shown in the diagram.

What heat transfer mechanism is acting here? Explain it.

On a cold but sunny winter's day, you stand in the sun and feel warmth on your face, even though the air remains cold. By what mechanism does this heat reach you? Does it need air to travel?

Complete the sentences with the missing heat transfer mechanism:

1. a) In $\dots$, heat is transferred by direct contact, without the movement of matter.
2. b) In $\dots$, heat travels through the movement of a fluid, such as air or water.
3. c) In $\dots$, heat travels without the need for matter, as occurs with the energy from the Sun.

When you bake a pizza, all three mechanisms of heat transfer are involved. Identify them:

1. a) The glowing heating element heats the pizza without touching it.
2. b) Hot air circulates inside the oven.
3. c) The metal tray heats the base of the dough.

You touch the metal leg of your desk and then the wooden surface, and the metal feels colder to you, even though both have been in the same room for hours.

1. a) Are they really at different temperatures?
2. b) Explain the difference in feeling using heat conduction.

Sitting in front of an open fire, you feel the heat on your face while the smoke and hot air escape up the chimney flue, as shown in the diagram.

1. a) Which mechanism is heating you?
2. b) Which mechanism makes the hot air rise up the chimney?

Justify both answers.

With a toaster turned on, you perform three tests:

1. a) You place your hand above it, without touching it, and feel hot air rising.
2. b) You place your hand next to it, without touching it, and also feel heat.
3. c) You touch it with a finger and burn yourself.

Identify the mechanism of heat transfer acting in each case.

Down jackets keep us warm because feathers trap a lot of air between them.

1. a) Is trapped, still air a good or poor conductor of heat?
2. b) Explain why this trapped air causes our body to lose less heat.

A frying pan has an aluminium base and a handle made of a special plastic, a very poor conductor of heat, as shown in the diagram.

Justify the choice of each material by thinking about what you want the heat to do in each part of the pan.

During the day, at the beach, the breeze blows from the sea towards the land; at night, it usually blows in the opposite direction. Knowing that sand heats up and cools down much faster than sea water, explain both situations using convection currents.

You wrap a cold drink can in a thick woollen cloth and leave another identical can unwrapped on the worktop.

1. a) Predict which of the two will warm up first.
2. b) Explain the role of wool as a thermal insulator: does it give heat to the drink or prevent heat from reaching it?

You place an ice cube held down by a wire gauze at the bottom of a test tube filled with water, and heat the top part of the tube with a burner until the water at the top boils. The ice at the bottom barely melts, as shown in the diagram.

1. a) What does the experiment demonstrate about water as a heat conductor?
2. b) Why does convection not carry heat downwards to the ice?

A spacecraft is heated by the Sun and must get rid of excess heat, but it is surrounded by a vacuum, as shown in the diagram.

1. a) Explain why it cannot cool down by conduction or convection.
2. b) What mechanism remains for it to lose heat?
3. c) Cooling panels on spacecraft are large, black, and flat: explain why these features favour this mechanism.

You heat water in a steel saucepan on a gas hob, as shown in the diagram. Describe the journey of heat from the flame to the water, identifying the mechanism acting in each stage:

1. a) From the flame and hot gases to the bottom of the saucepan.
2. b) Through the metal of the saucepan.
3. c) From the bottom of the saucepan throughout all the water.

You serve hot chocolate in two mugs: one made of thin steel and the other of thick porcelain.

1. a) Which of the two burns your fingers more when picking it up?
2. b) In which one does the hot chocolate cool down faster?
3. c) Justify both predictions using heat conduction, as well as the thickness and type of material of each mug.

Barefoot in the bathroom, the tile feels much colder to you than the mat, even though both are at room temperature.

1. a) Explain this difference in feeling.
2. b) Predict what you would feel if the bathroom had underfloor heating that kept the floor warmer than your foot: which material would then feel warmer when stepped on?
3. c) Reason your answer by considering which object transfers heat to which and at what rate it does so.

In winter, with the heating on, someone opens the window wide to ventilate the room.

1. a) Explain by which mechanism heat is lost from the room when opening the window.
2. b) What results in less heat loss: opening wide for five minutes or leaving the window ajar all morning? Justify your answer.
3. c) Propose a way to ventilate the room while losing less heat and explain why it works.

Emergency services wrap accident victims in a space blanket, a thin, silver sheet.

1. a) What happens to the radiation emitted by the body when it reaches the silver side of the blanket?
2. b) The air trapped between the body and the blanket can hardly move: how does this help reduce heat loss by convection?
3. c) The blanket does not "give heat", yet the person cools down much more slowly. Explain this.

A radiator works better installed at the bottom of a wall than near the ceiling. Observe the diagram, which compares the two installations:

1. a) Explain why using convection currents.
2. b) What would happen to the hot air if the radiator were near the ceiling? Would the room heat up properly?
3. c) Position barely affects the heat that the radiator transfers by radiation: can you explain why?