220

Science Alive for VELS Level 5

Heat transfer: radiation

Transmitted

heat

Radiated

heat

Absorbed

heat

Reflected heat

he Sun provides energy to the Earth. Without heat from the Sun, the Earth

would be far too cold for humans to live on. Heat from the Sun must travel

through space to reach the Earth. The heat does not travel by conduction or

convection because there are too few particles in space to vibrate or to move

between the Sun and the Earth. Heat from the Sun reaches the Earth by radiation.

T

A different way to travelHeat that travels by radiation is

called radiant heat. Radiant heat

travels very quickly because it does

not rely on the movement of

particles to move energy from one

place to another. The heat from

the Sun takes about eight minutes

to reach the Earth, but would

never reach us by conduction or

convection.

Place your hand near the base of the globe

of a lamp. Turn on the lamp. You feel the

heat from the globe almost instantly. Heat

does not travel through air easily by

conduction. So, the heat does not reach

your hand by conduction. Convection

currents travel upwards, so the heat does

not reach you by convection. The heat has

reached your hand by radiation.

Striking a surfaceRadiant heat is similar to light. When radiant heat strikes a surface, it can

be reflected, transmitted or absorbed. Most surfaces do all three, but

some surfaces are better reflectors than others. Some surfaces are better

absorbers of heat and others transmit more heat.

Transmitted radiant heat

Clear objects, like glass, allow light and radiant

heat to pass through them. The temperature of

these objects does not increase quickly when

heat reaches them by radiation.

Absorbed radiant heat

Dark-coloured objects tend to absorb light and

radiant heat. Their temperatures increase

quickly when heat reaches them by radiation.

Reflected radiant heat

Shiny or light-coloured surfaces tend to reflect

light and radiant heat away. The temperature of

these objects does not change quickly when heat

reaches them by radiation.

Infra-red

scannersAll objects, including the human

body, radiate some heat. The human

body usually radiates more heat

than the environment around it.

That’s why infra-red scanners are

useful for finding people lost at sea,

in bushland or even buried under

a collapsed building. Infra-red

scanners detect the radiant heat

coming from the human body.

 

 

22110. Heat, light and sound

Police use infra-red scanners to locate criminals at night. Criminals can’t hide from the scanners. The scanners can distinguish between objects that are only 0.2 °C different in temperature. Police choose to use scanned images in black and white rather than colour because it is difficult to tell the people from the objects in the coloured images.

The image of a criminal formed by an infra-red

scanner. Each shade of grey represents a different

temperature. The criminal looks white because he

is the hottest object in the area.

Radiant heat

You will need:

bar radiator or microscope lamp

3 metal cans of the same size and shape

lids to fit the three cans

black and white paint

3 thermometers (or 3 temperature probes and a data logger).

• Paint one can white, one black and

leave the third with an unpainted,

shiny surface.

• Pour equal amounts of cold tap

water into each can.

• Place lids on the cans, with holes

for the thermometers to stand in.

Make sure the lids are the same

colour as the cans.

• Measure the starting

temperature of the water in

each can. Record your results.

• Place the three cans at the

same distance from the

radiator or lamp. Turn on the

power to the heat source.

• Record the temperature of

the water in each can every 2 minutes for 14 minutes.

1. Why was the temperature of the water measured before starting to

heat the water?

2. How did the temperature of the water in each can change during the

experiment?

3. Which cans were better absorbers and which were better reflectors of

radiant heat? How can you tell?

4. Why was it important to use cans that were the same size?

Radiator

ShinyWhiteBlack

I CAN:explain, in basic terms, how heat

travels from the Sun to the Earth.

describe similarities in

the way heat and light

behave when they

strike different surfaces.✓ lea

rnin

g

Go to worksheet 10.3:

Make a solar oven

REMEMBER

1. Which method of heat flow

is the only one for which

particles are not needed?

2. List three different things

that can happen to radiant

heat when it reaches a

surface.

3. List two uses of infra-red

scanners.

THINK

4. How do we know

that heat travels

from the Sun to

Earth by

radiation?

5. Why is it almost

impossible for

criminals to hide

from infra-red

scanners?

INVESTIGATE

6. Look at the back

surface of a

radiator like the

one used in the experiment

on this page.

(a) What colour is the

surface?

(b) Why is the back surface

this colour?

7. The change in temperature

of water inside different

containers was investigated

in the experiment on this

page. The surfaces were

shiny, black and white.

Design and perform an

experiment to investigate

the heat-absorbing

properties of different

colours. Which colours

absorb more heat? How can

you tell?

Chec

kpoint

   

222

Science Alive for VELS Level 5

Insulationnsulators, such as a thermos flask or a thick jacket, control the transfer of heat. Some materials are

better at controlling heat that is moving by conduction; others slow heat transfer by convection, and some by radiation.

I

Slowing down the flow of heat is the key to keeping drinks cool in the summer and warm in the winter. On a hot day, heat flows from the hot environment to a cold drink, until they are both at the same temperature. On a cold day, heat flows from a warm drink to the cooler environment, until their temperatures are equal.

Thermos flask

Heat does not travel into or out of a thermos flask very easily. For this reason, a thermos flask can be used to keep cold drinks cold and warm drinks warm. A thermos flask slows heat transfer by conduction, convection and radiation.

Protective

case

Air gapAir does not

allow much

heat to travel

through it by

conduction.

StopperThe stopper is

made from

materials that

do not allow

much heat to

move through

them by

convection or

conduction.

Foam pads keep the

glass bottle in place

and absorb impacts.

Empty spaceNearly all particles are

taken out from between

these layers. Without

particles, heat cannot

move through the walls by

conduction or convection.

Silver surfaceThe silver surfaces facing

the inside of the container

reflect radiant heat back

into the container. Silver

surfaces facing the outside

of the container reflect

radiant heat away from

the container.

Shock absorber

Heat and the body

The temperature of the human body is about 37 °C. When the air temperature is much less than this, heat moves from your body to the environment. In very hot weather, heat moves from the environment to your body.

Losing radiant heatAll objects give off some

radiant heat. The amount

depends on the

temperature around the

object. On a hot day, an

object does not transfer as

much radiant heat away as

in the cold weather.

Absorbing radiant heatWhen sunlight

strikes any

object,

including the

human body,

the object

absorbs some

radiant heat.

Convection Convection

currents form when

the heat from your

body warms the air

next to it. The air

rises, taking some

of your body heat

away with it.

Convection currents

can form only in air

that is free to move.

Unless your body is touching a very hot or a very cold object, you won’t gain or lose heat by conduction very easily. Most of the movement of heat near your body happens by convection and radiation.

 

22310. Heat, light and sound

✓ le

arn

ing I CAN:

describe how insulators control

the transfer of heat

explain how a thermos flask stops

the transfer of heat to (and from)

the liquid inside it.

Fabric choices

Clothes keep you warm in winter by stopping your

body heat from escaping.

Fabrics made from natural fibres, like wool, are

good insulators of heat. Natural fibres contain small

pockets of air. Air allows heat to travel through it by

convection only if it is free to move. By having only

small pockets of air, convection currents cannot be set

up. However, air can flow easily through thin material,

keeping you cool in summer. Loose fitting clothes

allow more convection currents to form. The

convection currents help heat to escape from

your body.

Go to worksheet 10.4:

Antarctic fashion designer

REMEMBER

1. How does a thermos flask stop heat escaping by

radiation from a hot drink?

2. List two features of a thermos flask that reduce heat

loss by:

(a) conduction

(b) convection.

3. In what two main ways does heat move between

your body and the environment?

THINK

4. It is especially important for the stopper at the top of

a thermos flask to stop heat loss by convection. Why?

5. How does a thermos keep liquids cool?

6. What is wrong with this statement: ‘A thick coat

keeps the cold out’?

CREATE

7. Design and test a container that will keep ice from

melting as long as possible. The container needs to

be easily carried and built from items found around

your home.

8. Use the information on these two pages to design:

(a) a summer outfit that keeps you cool

(b) a winter outfit that keeps you warm.

40 ˚c

50 ˚c

60 ˚c

70 ˚c

80 ˚c

90 ˚c

100 ˚c

ThermometerPlasticineholdingthermometerin place

Reducing heat loss

You will need:

4 identical soft-drink cans 4 thermometers

plasticine foam ‘stubby’ holder

polystyrene packaging sticky tape

lamb’s wool or woollen cloth.

• Draw up a table like the one below.

• Put one can in the ‘stubby’ holder, wrap one in wool

and another in polystyrene. Make sure the covers are

equal thicknesses. Leave the fourth can bare.

• Pour equal amounts of

hot water into each can.

• Place a thermometer in each

can, standing it up with the

plasticine.

• Quickly read the temperature

of the water inside each can.

• Record the temperatures of

the water every 2 minutes for

12 minutes.

1. Draw a line graph of your results. Plot time on the

horizontal axis and temperature on the vertical axis.

Use a different coloured line for each can cover.

2. Which can cover is the best insulator? Explain your

answer.

3. The covers used in this experiment slow the transfer

of heat by two processes.

(a) What are the two processes?

(b) Explain how each of the covers slows down heat

transfer in the two methods from part (a).

4. Why was a can left without a cover?

5. Why should the thickness of each cover be the same?

Cover

Temperature (°C)

0

min

2

min

4

min

6

min

8

min

10

min

12

min

No cover

‘Stubby’ holder

Polystyrene

Wool