Chapter 10 – Heat Transfer

1. What is heat?

Heat is a form of Energy.

It travels from a region of higher temperature to a region of lower temperature.

2. Name the three methods of heat transfer.

Heat energy can be transfer by conduction, convection and radiation.

Method 1: passing book from one pupil to another without pupil moving – conduction.

Method 2: student walk with the book from front to the back row – convection

Method 3: student A throwing the book from front to the back – radiation.

Activity 1: Conduction

Get a piece of stiff copper wire about the same length as a match. Strike the match and

hold the copper wire in the flame.

What happens? The copper wire is heated up.

Does the energy get to your hand quicker through wood

or through copper? Through the copper

Conclusion:

Copper is a better conductor than wood. The energy has traveled from atom to atom

through the copper.

1

Activity 2: Convection

Hold your hand over and then under the flame of a match.

What do you notice? You can feel the heat above the match and no heat under the

match.

_______________________________________________________________

Conclusion:

The hot air expands and then rises. Heat is convected upward.

Activity 3: Radiation

The heat energy of the Sun is radiated to us in the same way

that the light reaches us from the Sun.

CONDUCTION

1. What is conduction?

Conduction is the process by which heat is transmitted through a medium from one

particle to another.

Conduction is heat flow through solids without any visible movement.

Atoms vibrate when heated. The vibration will also cause the neighbouring atoms to

vibrate and thus heat is transferred from the hot end to the cold end.

2. Experiment to investigate the flow of

heat through solids.

a) Get some rods (all of the same size) of

different substances.

2

Cold endHot end

heat

b) The rods can be copper, iron and glass.

c) Rest them on a tripod and fix a small nail at one end of each rod.

d) Heat the other ends of the rods equally with a Bunsen Burner.

e) What happens?

The nails from different materials dropped off from the rod at different time.

Conclusion:

From the experiment it shows that nail from the Copper drops off first at 14 seconds.

This is because heat travels fastest through the copper than iron and glass.

We say that copper is a good conductor of heat.

5. What are the good conductors of heat ?

Good conductors of heat refer to object that can conduct heat very fast. All metals

are good conductors of heat. Copper, silicon, iron, magnesium are all good

conductors.

6. What are insulators?

Insulators are materials that Heat cannot travel through easily.

They are Poor conductors of heat.

Poor conductors are good insulators.

7. What are the examples of insulators?

Most of the liquid and gases are poor conductors of heat. Other examples such as

glass, wood, plastics, cotton and wool are all good insulators.

8. Is air a conductor or an insulator?

3

Materials Time taken for nails to drop glass did not drop copper 14 iron 73

Activity 3:

Hold a match about 1 cm away from a very hot Bunsen flame. Does the match get hot enough to burst into flame?

This shows that air is a very poor conductor – it is a very good insulator.

All gases are poor conductors.

9. What are the application of the good insulators?

a) Use as lagging for hot water tanks and pipes.

b) Use as coats and blankets to keep us warm.

c) Use to make kitchen utensils such as handles of pipes, and

pots etc.

4

Convection

10. What is convection?

Convection is the transfer of heat energy in the substance by movements of the heated

particles. It occurs only in fluids (gases and liquids ). It does not occur in solids and

vacuum.

11. Explain how convection currents are formed in the following experiments.

a) Air around the hot flame gains energy , they move faster. The air becomes less

dense and rises.

b) Cooler air from the top, which is denser, falls (sinks) to the bottom to replace the

hotter air.

c) In this way of hot air rises and cold air sinks, a convection current is formed.

Therefore heat is transferred from the fire to the surroundings by

convection(conduction/convection/radiation).

5

12. Convection in Liquids. To demonstrate convection in water, drop a few tiny crystals of potassium

permanganate into a flask filled with water. Gently heat the flask, purple streaks of water will rise upwards and

then fan outwards. The water becomes uniformly purplish after some time. The circulation of a liquid in this matter is called a convection

current.Why hot water rises and cold water sinks?When the liquid at the bottom is heated, it expands and becomes less

dense. The cooler liquid at the top, which is denser, will sink to replace

the hotter liquid.

This forms a convection current.

Water is heated up by convection.

13. Convection current in gases.

a) The air above the candle becomes hot and expands. The hot air is less dense and

rises straight up the chimney above the candle.

b) Cooler air is drawn down the other chimney

to replace the air leaving the room.

c) The movement of hot and cold air set up a

convection convection.

14. Milk is heated in a pan on a cooker.a) Explain how milk becomes hot near the top of the pan.

Hot milk at the bottom when heated expands and becomes less dense and rises to the top of the pan. Cold milk at the top is denser will sink to the bottom. Convection current is set up.

b) Why is the handle made of plastic?

Handle is make up of plastics as it is a poor conductor/ good insulator of heat. It prevents the user being burnt.

15. Usually it is better to install air conditioner in the higher portion of the wall. Explain why?

6

Cold air at the top which is denser will sinks, warm air which is less dense will rise. The room will be cools down by the convection current.

16. Why is the heating coil of an electric kettle placed near the bottom of the vessel?

So that hot water at the bottom will be heated becomes less dense and rises to the top. Cooler water at the top which is denser will sink to the bottom. Convection current is set up.

Radiation

17. What is radiation?

Radiation is a process where heat energy is transferred as waves and has the ability to

travel through vacuum.

Heat transfer by radiation does not need any medium.

18. Do our bodies or any objects emits heat or absorb heat?

Yes. All objects either emit / radiate or absorb heat . The heat is transferred in a form

of infra-red radiation.

19. What does the rate of emitting or absorbing heat for an object depends on?

The rate of emitting or absorbing heat for an object depends on

a) Temperature of the object.

The hotter the object, more heat is radiated.

b) type of surface of the object.

7

HOT

cold

COLD

hot

20. What is radiator or emitter?

When an object is hotter than it’s surrounding, heat will flow

out of the object. The heat is said to be

emitted, radiated or given out.

Objects that radiate heat very fast is said to be good radiator of heat.

21. What is an absorber?

When an object is colder than it’s surroundings, heat will flow into the object, the object absorbs heat.

Objects that absorbs heat very fast is good

absorber of heat.

24. Nature of surface to determine absorbers and radiators of heat.

Dull, black and rough surfaces are the best radiators and the best

absorbers of heat and the poorest reflector of heat.

Shiny, silvery and smooth surfaces are the poorest radiators, poorest

absorbers of heat but the best reflector of heat.

8

25. What type of surface would a teapot have if you wanted it to be able to stay hot for as long as possible?

Silvery white surface. It is a worst radiator of heat.

9

Everyday applications of Heat transfer

26.

27. piece of paper is pinned along its top edge to a notice board situated directly above a desk lamp as shown in the Figure 4.1 below.

10