Heat Transfers Booklet

This booklet covers:

1. Conduction

2. Convection

3. Radiation

4. Insulation

Introduction

Heat does not simply travel in one direction but flows from hot objects to cold ones.

Substances hold energy in the thermal store due to random movement of their molecules. Hotter objects have more thermal energy than cooler ones.

1. Conduction

Particles in a hotter area have more energy and vibrate more than particles in a colder area. When they collide, the hotter particles pass on this extra energy to the colder particles.

This can occur in solids, where there are many bonds, and in liquids, where there are less bonds.

All metals and some other materials, like graphite, conduct heat well.

Materials that do not conduct heat are known as insulators (wood, plastic, rubber).

Example

Hot toast heating butter on top of it.

Questions:

1. Which energy store is related to how hot or cold an object is?

2. What happens to the particles in a solid when you heat it?

3. Why are gasses very poor conductors of heat?

4. Name 2 good conductors of heat and 2 insulators?

2. Convection

Key word: Fluid

A fluid is any substance that is either a liquid or a gas.

Hot fluids are less dense than cool fluids. This means the hotter part of a fluid rises up and the cooler part sinks down.

This causes the hotter and cooler parts to mix, this is called a convection current.

1. The water at the bottom of the pan is heated.

2. This makes it less dense.

3. The hot water then rises to the top of the pan and the cold water at the top sinks to the bottom.

4. The hot water at the top starts to cool and the cooler water at the bottom starts to heat up.

5. The cycle then repeats.

Lava lamps are a good example of a convection current. When the ‘lava’ is at the bottom of the lamp, it is heated. It then rises to the top, cools down, and then sinks back to the bottom again.

Questions:

5. What is a fluid?

6. What happens to the density of a fluid when it cools down?

7. In a convection current, which direction does the hotter part of the fluid move?

8. If you gently pour cold water into a pan of hot water, will the cold water sink to the bottom or stay at the top?

3. Radiation

All objects give off (emit) some of their thermal energy as infra-red radiation. Hotter objects give off more radiation than colder objects.

Infra-red radiation can travel through solids, liquids, gasses and through a vacuum.

Key word: vacuum

A vacuum is an area with no particles in it at all. Space is a vacuum.

We can use cameras that measure the amount of infra-red radiation coming from an object to tell how hot something is. These are called Infra-red cameras.

Questions:

9. What is the name of the radiation given off by all objects that reduces their thermal energy?

10. Can infra-red radiation from the sun reach the earth even though space is a vacuum? Why?

11. Which areas of the persons face are giving off the most infra-red radiation?

12. What does this tell you about the temperature of those areas?

4. Insulators

An insulator is a material that is a bad conductor of heat.

We use insulators a lot to keep things cold and to keep other things warm. Insulators prevent warm things losing heat to their surroundings and prevent cold things from being heated by their surroundings.

Air is a good insulator because, as a gas, heat can only be transferred by convection and radiation.

Warm clothes and the insulation in houses trap air in them, it is this trapped air that makes them good insulators.

A vacuum is an even better insulator than trapped air. This is because radiation is the only way that heat can be transferred through a vacuum. We use vacuums to make flasks and water bottles.

Questions:

13. What is an insulator?

14. What do we use insulators for?

15. Rank the following in order of how good an insulator they are from best to worst: metal, trapped air, a vacuum, plastic.

16. Look at the previous infra-red photo, is the temperature of the glove higher or lower than the temperature of the person’s arm?

Extension Task – Optional

Create an information sheet on the different methods of heat transfer. Include examples and diagrams.