6 Investigating

heat
The photo shows a glassworker. The molten glass inside the furnace is at a temperature of more than 1000C. The furnace and glass give off a huge amount of heat. What do you notice about the end of the metal rod in the furnace? List the items of protective clothing the worker is wearing. Suggest why the glassworkers clothing is silver-coloured. Describe two ways in which the heat moves from the furnace to the glassworker.

116

ScienceWorld 8

6.1 Heat and temperature

Heat is very important in our lives. Our body functions best at a temperature of about 37C. If we get too hot or too cold we feel uncomfortable. If our body temperature rises too far above normal or too far below normal we can die. We use fans, heaters and air conditioners to keep us comfortable. The walls and ceilings of our homes are insulated to keep heat in during winter and out during summer. We use heat for cooking food and for heating water. Heat is used by industries to make new materials such as glass, steel and plastics. Our cars produce heat when they burn petrol. Heat from burning coal is used to generate electricity. But what is heat? And how is it different from temperature? Several hundred years ago, people thought of heat as a special fluid called caloric which flowed in and out of objects as they were heated or cooled. An American named Benjamin Thompson, who later moved to Germany and became Count Rumford, showed that this caloric idea was incorrect. He observed that when holes were drilled in brass to make cannons, so much heat was produced that water had to be poured over the cannons to cool them. From this

Rumford inferred that it was the movement of the drills that made the cannons hot. The kinetic energy of the drill had been converted into heat energy. People soon realised that some heat is always produced when energy changes from one form to another. In other words, heat is a form of energy (page 99). For this reason it is measured in joules (J). Heat and temperature are not the same, but there is a connection. Temperature is a measure of how hot or cold something is. It is measured in degrees Celsius (C) using a thermometer. You have probably used sparklers. Each spark is actually a tiny piece of white-hot metal, and its temperature may be as high as 800C. (The temperature of boiling water is only 100C.) However, if a spark falls on your hand you dont even feel it. This is because each spark contains only a small amount of heat energy. Some of this heat energy is transferred to your skin, but the resulting temperature rise is so small that you usually cannot detect it.
Fig 3 Each tiny spark has a high temperature but contains very little heat.

Fig 2

Drilling brass cannons produced considerable heat. From this, Count Rumford inferred that heat is a form of energy.

Fig 3

Chapter 6 Investigating heat Heat and the particle theory

We can use the particle theory to explain heat. When you heat an object, the particles in it move more rapidly and therefore have more energy. This is why the temperature is higher. When the particles lose energy and move more slowly, the temperature is lower. Look at the diagram below. When a hot object comes into contact with a cold object, heat flows from hot to cold until both objects are at the same temperature. The rapidly moving particles in the hot object transfer some of their energy to the particles in the colder object. The larger the temperature difference, the faster the transfer. Cool objects in warm places take in energy from their surroundings. For example, an ice block melts quickly on a hot day. Warm objects such as a cup of hot coffee lose heat energy to their cooler surroundings.
The particle theory states that all matter is made up of tiny little particles....

117

....and that these tiny particles are in a constant state of motion.

Did you know?

If the temperature of a substance was lowered to 273C, its particles would have no energy at all and would therefore be completely still. This temperature is called absolute zero, and scientists have come close to this in some experiments.

The more energy the particles have, the faster they move.

hot metal

cold water

Heat transfer has stopped. Water and metal are at the same temperature.

direction of heat transfer

118

ScienceWorld 8

14 HEATANDTEMPERATURE
Aim
To find answers to these questions: A Does the mass of a substance influence how much its temperature rises? B Does the type of substance influence how much its temperature rises?

Investigate

hotplate

Materials
hotplate or burner, tripod and gauze 250 mL beaker thermometer measuring cylinder, 100 mL stopwatch Flammable olive oil paper towel

Planning and Safety Check

Read through both parts of the investigation. Discuss with your teacher the safest way to handle the hot beaker. How is Part B different from Part A? Suggest why a hotplate is used in this experiment instead of a Bunsen burner. Draw up a data table like the one below. Temperature of Rise in the liquid (C) temperature before after (C) heating heating 50 mL water 100 mL water 60 mL olive oil 120 mL olive oil

2 Adjust the hotplate or the burner to medium heat. Leave it at the same setting throughout the experiment. This is to make sure that the heater supplies heat at a constant rate. 3 Place the beaker of water on the hotplate for exactly 2 minutes. Then remove the beaker from the hotplate, stir the water gently with the thermometer and read the temperature. Record this temperature in the data table. Calculate and record the rise in temperature.

4 Empty the beaker, cool it under running water, and dry it. 5 Add 100 mL of water to the same beaker and measure the temperature before and after heating for 2 minutes. Record your results in the data table.

Discussion

PART A
Method
1 Use the measuring cylinder to add exactly 50 mL of water to the beaker. Use the thermometer to measure the temperature of the water, to the nearest degree. Record this in the data table.

1 Which variable did you change in this investigation? 2 Which variables did you keep the same?

Conclusion
Write an answer to the question How does the mass of a substance influence how much its temperature rises?

Chapter 6 Investigating heat

119

PART B
Method
Repeat Part A, but this time use olive oil60 mL and 120 mL. (Sixty millilitres of olive oil has the same mass as 50 mL of water.) Record all results in the data table.

Discussion
Which variable did you change going from Part A to Part B? Which variables did you keep the same?

Conclusion
Write an answer to the question How does the type of substance influence how much its temperature rises?

From Investigate 14 you can conclude that A The same amount of heat will raise the temperature of 50 mL of water twice as much as it raises the temperature of 100 mL of water. B The same amount of heat will raise the temperature of olive oil more than it raises the temperature of an equal mass of water. In other words, olive oil heats up more quickly than water does. The bar graph below shows the amounts of heat needed to 4.2 J warm 1 gram of various materials by 1C. You could also predict that if you supply twice as much heat to water or olive oil you raise 2.0 J the temperature twice as much.
0.7 J 0.1 J
u rc ry c

Skillbuilder
Using maths equations
The amount of heat energy needed to raise the temperature of 1 gram of a substance by 1C is called its specific heat capacity. For example, the specific heat capacity of water is 4.2 joules per gram per C. To calculate the heat needed to change the temperature of something, you can use this mathematical formula: heat (J) = mass (g) x specific heat capacity x change in temperature (C) So, the heat needed to raise the temperature of 50 mL (50 g) of water by 10C can be calculated as follows: heat = = 50 g x 4.2 x 10C 2100 joules

0.9 J

0.4 J
er ss a m lu m o li oi l er

Use the specific heat capacities from the bar graph on the left to answer these questions.

p op

a gl

u ni

ve

at w

Fig 7

The heat needed to raise the temperature of 1 gram by 1C. You will notice that solids generally heat up more easily than liquids.

To summarise, the amount of heat gained or lost by an object depends on three variables: its mass the temperature change what it is made of. You can calculate the amount of heat that is transferred if you know these three variables.

1 How much heat is required to: a raise the temperature of 100 mL of water by 10C? b raise the temperature of 60 mL of olive oil by 10C? 2 A 5 gram block of aluminium was heated from 30C to 100C. How much heat energy was needed? 3 How much heat is given out when 60 g of copper cools from 100C to 20C? 4 Using your results from Investigate 14, calculate the amount of heat transferred to 50 mL water, 100 mL water, 60 mL olive oil and 120 mL olive oil. Are they all the same?

120

ScienceWorld 8

Check!
1 Decide which of the following statements are true and which are false. Rewrite the false ones to make them true. a Heat is a form of energy. b When you strike a match, you convert kinetic energy into heat energy. c When an energy change occurs, some heat energy is always produced. d A block of ice contains no heat energy. e Heat is measured in degrees Celsius. f As an object becomes hotter, its particles move more rapidly. g Heat travels from cold objects to hot objects. When Faith used an electric hair dryer to dry her hair, the hair dryer became quite hot. What energy change has occurred?

Josh heats two identical iron nails together until they are red hot. He drops one into 50 mL (50 g) of water and the other into 60 mL (50 g) of olive oil. If both liquids are at the same temperature to start with, predict which will be hotter one minute after the nails are dropped in? Explain your answer. Explain why heat energy can be considered a form of kinetic energy. Eva had four identical beakers containing different amounts of water, as shown. She heated them for different lengths of time and none of them boiled. A
half full 5 minutes

6 7

B
half full 8 minutes

C
full 5 minutes

D
full 10 minutes

a b c 8 3 a b 4 Which is hottera cup of water at 50C or a bathtub full of water at 50C? Which contains more heat energy?

Which beaker of water received the most heat? Which beaker would you predict had the highest temperature after heating? Which beaker would have the lowest temperature after heating?

A cold saucepan is put into a sink containing hot dishwashing water. a What will happen to the temperature of the saucepan? b What will happen to the temperature of the water? c Does heat flow from the water into the saucepan, or from the saucepan into the water?

Samples of 50 g of aluminium, copper, glass and water all initially at 20C are heated for 5 minutes on a hotplate with a constant setting. Predict the order (from highest to lowest) of the final temperatures of each sample. See the bar graph on the previous page. Suggest why mercury is used in thermometers. (Hint: see the bar graph on the previous page.)

10 On a hot summers day the dry sand at the beach can be almost unbearable to stand on, while the water is cool. Try to explain this temperature difference.

Chapter 6 Investigating heat

121

challenge
1 Ramone was making some ice blocks from fruit juice, and decided to investigate their temperature as they cooled in the freezer. He put a thermometer in one of them and measured the temperature every 10 minutes. His results are in the table below. a Was heat being added to the ice blocks during Ramones experiment, or being taken away from them? b Plot Ramones results on a line graph. c Suggest a reason for the flat part of the graph between 40 min and 60 min. d Suggest a reason for the flat part between 90 min and 100 min. e At what temperature did the ice blocks freeze?
Time (min) 0 10 20 30 40 50 60 70 80 90 100 Temperature (oC) 25 15 8 1 1 1 1 4 8 10 10

2 Harry did an experiment and drew these diagrams to show his method.
thermometers

100 mL water methylated spirits kerosene Heat for 5 minutes.

a Which variables did Harry control in this experiment? b Which variable did he purposely change? c Which variable did he measure? d What do you think was the aim of Harrys experiment? 3 Nicky measured the temperature of a saucepan of hot water as it cooled. She plotted her results as shown in the graph. What was the temperature of the water after a 10 minutes? After 40 minutes? b Why is Nickys graph steep to start with but flatter near the end? c What do you think the temperature of the room was when Nicky did her experiment? Explain your answer.

X
Temperature (C)

X
50

Temperature as hot water cools (Challenge 3)

X X X

20

Time (min)

40

60

122

ScienceWorld 8

6.2 Heat transfer

Heat energy can be transferred in three different ways. The direction in which it flows is always from a higher temperature to a lower temperature.
1 Conduction This is how heat energy is transferred through solids. Some solids conduct heat better than others. 2 Convection This is how heat energy is transferred in liquids and gases.

3 Radiation This is how heat energy is transferred from the sun to Earth. No matter is necessary.

Activity
1 You will need a glass rod about 20 cm long, and a metal rod the same length and thickness as the glass one. 2 Use wax or grease to stick a paperclip about 5 cm from the end of each rod. Then lay the rods across a tripod so that the paperclips hang down as shown. 3 Heat the end of both rods equally. Which paperclip falls off first? What does this mean?
wax glass rod

Conduction
A metal rod in contact with a hot flame quickly becomes hot. The heat is transferred along the rod by the process of conduction. The particles in the end of the rod gain energy from the flame. This causes them to vibrate faster and collide more energetically with each other. This process continues like a chain reaction from particle to particle along the rod. As a result, heat energy is transferred from the hot end of the rod to the cooler end.
Heat energy is transferred along the rod.

To see how meat is cooked by conduction, open the Cooking animation on the CD.

metal rod paperclip

tripod

As you saw in the activity, some solids conduct heat better than others. Substances that conduct heat well are called good conductors, and most metals are good conductors. Substances like glass, which are poor conductors of heat, are called insulators. Most plastics are poor conductors of heat, so those that do not melt easily are used to make handles for saucepans, kettles, frying pans and irons.

Chapter 6 Investigating heat

Examples of conduction
Insulating handles allow you to pick up hot objects without the heat being conducted to your hand. Plastic foam is a good insulator, and is used in the walls of refrigerators to keep heat out.
aluminium saucepan (good conductor) bakelite plastic handle

123

You may have seen birds fluffing up their feathers on cold days. This is to trap air between their feathers. Because air is an insulator, it slows down the loss of heat from the birds skin to the surrounding cooler air. Woollen jumpers, sleeping bags and the batts used to insulate houses also work by trapping air. The wetsuits worn by surfers and divers are made of foam rubber. A thin layer of water warmed by body heat is trapped between the suit and the divers skin. Being a poor conductor, this water helps to prevent the divers body heat from escaping.
waterproof neoprene wetsuit body of diver

heat from stove

Liquids do not conduct heat very well. The set-up below shows that water is a poor conductor of heat. Even though the water boils at the top of the test tube, the ice at the bottom melts only slowly. If water was a good conductor of heat, the ice would melt quickly.

water caught under wetsuit

body heat

water

boiling water

tongs

iceblock weighted with wire

Gases are also poor conductors of heat. You can demonstrate this by holding your hand beside a burner flame. If air was a good conductor of heat, your hand would quickly be burnt, but it isnt.

124

ScienceWorld 8

Activity
1 Fill a large beaker with water and allow it to stand until the water is completely still. 2 Carefully drop half a teaspoon of used tea leaves down one side of the beaker, making sure not to disturb the water. 3 Heat underneath the tea leaves as shown. Suggest why the tea leaves rise. Draw a diagram showing the movement of the tea leaves.

Hot water systems work by convection. The heater at the bottom warms the water which moves upwards as the cool water takes its place, setting up convection currents. The hot water is drawn off from the top. When a hot water tap is turned on, more cold water flows in at the bottom.

Hot water rises...

hot water taps

water tea leaves

...and cool water takes its place. heater cold water

Convection
The movement of the tea leaves in the activity above demonstrates the movement of heat energy by the process of convection. This process can be explained using the particle theory. When water particles at the bottom of the beaker are heated, they gain more energy and move more rapidly. Because of this, they move further apart than the particles above them. Hence the warm water near the bottom is less dense than the water above it. This warmer water therefore rises, and colder water moves in to take its place. This movement of particles is called a convection current. It continues until all the water in the beaker is at the same temperature.

Convection currents also occur in air. When you turn on a heater in winter, the warm air rises above the heater. A convection current is then set up as the cooler air sinks. The same thing happens on a larger scale to form a sea breeze. During the day the land is warmer than the sea, because the sea takes a long time to warm up. Warm air rises above the land, and cooler air blows in from the sea to take its place.

Warm air rises.

cooler air (sea breeze)

Land warmer than sea

Fig 21

How a sea breeze is caused by convection

Chapter 6 Investigating heat Radiation

The Suns rays heat the Earth. However, the space between the Sun and the Earth does not contain matter, so heat energy cannot be transferred by the processes of conduction or convection. Instead, the Sun transfers heat energy by the process of radiation. All objects transfer some heat by radiation. The hotter the object, the more heat it radiates. The radiation itself is not hot, but when it is absorbed by an object it causes the particles in the object to move more rapidly, thus heating it.
radiation absorbed infra-red radiation

125

TRANSMITTED (passes through transparent material)

REFLECTED (bounces off)

ABSORBED (taken in)

Fig 23

Heat and light can be transmitted, reflected or absorbed.

Fig 22

The curved silver mirror at the back of an electric radiator reflects the radiation.

Warm objects radiate heat mainly in the form of infra-red radiation, which we cannot see but which can be detected by special infra-red scanners. People are usually warmer than their surroundings and give off more infra-red radiation. This is why infra-red scanners are used at night by airsea rescue helicopters to help find people who are lost. If a metal object becomes hot enough it will glow, giving off visible light as well as infra-red radiation. Light, infra-red and other forms of radiation all travel as extremely high-speed waves which can pass through a vacuum, such as the vacuum of space. In a vacuum the speed of radiation is 300 million metres per second or 3 108 m/s. Different types of radiation have different wavelengths. The waves travel in straight lines, and they can be reflected, absorbed or transmitted by matter. All of these properties of radiation are applied in microwave cooking. Microwaves are reflected off metals, so they reflect from the

inside of the oven onto the food being cooked. The microwaves penetrate the food to a depth of between two and four centimetres, where they are absorbed. This causes the molecules in the food (mainly water molecules) to move more rapidly, and hence the food heats up. The heat is then transferred to other parts of the food by conduction. This process may continue for a short time after the food is removed from the oven. The microwaves are transmitted through the glass dish, which remains relatively cool because it absorbs very little radiation. (Microwaves will also pass through paper and most plastics.)
stirrer to reflect waves waveguide source of microwaves

glass dish food

turntable

Fig 24

How a microwave oven works. You can see through the glass door, but the microwaves cannot pass through the metal lattice behind the glass.

126

ScienceWorld 8

Investigate

15 WHICHABSORBSMORERADIATION?
Aim
To compare the amount of radiation absorbed by a shiny silver can and a dull black can. 5 Plot the temperature for both cans on a single graph. (A datalogger will do this for you.) You could use a different colour for each can, but make sure you label the two curves.

Materials
2 thermometers or datalogger and 2 temperature probes portable spotlight or electric radiator 2 metal cansone shiny silver and one dull black

Discussion and conclusion

1 Which was the independent variable, and which was the dependent variable? 2 Which variables did you control? 3 Which can absorbed more radiation? How do you know? Was your prediction correct? 4 Look at your graph. What does the slope of each line tell you about the warming rate of the can? 5 Based on the results of this experiment, write a generalisation saying how the amount of heat absorbed by an object depends on the type of surface. 6 Could the experiment be improved? If so, how?

NOTES
1 Instead of using a spotlight, you could use a microscope lamp, or you could put the cans in direct sunlight. 2 If you use empty food cans, you could blacken one by holding it in the smoke from a burning candle. Painted soft drink cans work well. 3 To cut down on heat loss by convection, you need lids.

Planning and Safety Check

Read the Method carefully and discuss with your teacher what equipment you will use. What safety precautions will be necessary? Which can do you predict will absorb more radiation? Why? In your notebook design a data table in which to record the temperature of each can every minute for 15 minutes.

Design a similar experiment to find out which can cools more quickly.

thermometer hole in lid

dull black can

Method
1 Add equal volumes of cold water to both cans. 2 Position the spotlight or radiator at an equal distance from each can. 3 Record the initial temperature of the water in each can. (These should be the same.)
shiny silver can portable spotlight

4 Turn on the lamp and at the same time start timing. Record the temperature in each can every minute for 15 minutes.

Chapter 6 Investigating heat Absorbing and emitting radiation

Dark-coloured surfaces are better absorbers of radiation than light-coloured ones. This is because light-coloured surfaces reflect more of the radiation. Bright shiny surfaces are the best reflectors and the poorest absorbers. This is why aluminium foil is used in ceilings and walls of houses to reflect heat. On the other hand, the absorbing panels of solar water heaters are painted black so that the copper pipes inside them absorb as much of the suns radiation as possible. Dark-coloured cars become much hotter than light-coloured cars when left in the sun. And dark-coloured clothes are hotter in summer than light-coloured clothes.
light car (good reflector, poor absorber) dark car (good absorber)

127

cooling fins

Fig 28

The cooling fins on an air-cooled motorcycle engine have a large surface area to increase radiation of heat.

Controlling heat transfer

An object that is warmer than its surroundings will lose heat until it is the same temperature as its surroundings. Similarly, an object that is cooler than its surroundings will gain heat from its surroundings. We use insulators to control this transfer of heat. Eskys and thermos flasks (see page 132) are insulated containers to keep food and drink at the temperature we want iteither hot or cold. Pizza delivery people put their pizzas in special insulated boxes to keep them warm.
Is the pizza still hot? Why did I ask for triple chilli?

All objects emit (give out) infra-red radiation if they are at a higher temperature than their surroundings, but some radiate heat more readily than others. Dark-coloured objects radiate heat more effectively than light-coloured objects. Rough surfaces also radiate heat more effectively, due to their greater surface area.

silver teapot (poor emitter)

black teapot (good emitter)

We insulate the walls and ceilings of our homes. This keeps them cool in summer by preventing heat from coming in from outside. It also keeps them warm in winter by preventing heat from escaping. (See page 134.)

128

ScienceWorld 8

Experiment

WHICHISTHEBESTINSULATOR?
The problem to be solved
Your task is to design an experiment to solve the problem Which type of material keeps you warmest in winter? If possible, repeat your experiment to improve the accuracy of your measurements. If you get the same results, then you can be more confident your conclusion is correct. If someone else repeats your experiment and gets the same results you can be even more confident. Results like this are said to be reliable.

Designing your experiment

The design of the experiment is up to you, but here are some questions to guide you. What will you use as a model for a human body? One idea is to use a can filled with hot water. How will you clothe your model bodies? What materials will you use? Some possibilities are wool, cotton, nylon, polyester, flannelette. How many model bodies will you need? How will you measure the temperature? How often will you do this, and how long will you continue the experiment? How will you make your test fair (as in Chapter 2)? You are varying the type of clothing, but what other variables are there? How will you keep these other variables constant? It would be a good idea to use an experimental controla model body with no clothes. You can then compare the clothed bodies with it to see how effective the different types of clothes are.

Results
How will you record and display your results? If a datalogger is available you could use it. Would a graph be useful?
Computer programs such as Excel can be used for drawing graphs. Open the ICT skillsheet on the CD to see how this can be done.

Writing your report

Look carefully at your results and write a report of your findings, giving your answer to the problem. You could take a digital photo of your set-up to include in your report. Could you improve your design? How?

Instead of keeping something warm you often want to keep something cold. Design an experiment to find out which is the best insulator for this.

Chapter 6 Investigating heat

129

Check!
1 2 What is the advantage of a copper bottom on a saucepan? The four beakers shown are identical and contain the same volume of water at 80C. After 10 minutes the temperature of each is measured again.

Refrigerators and freezers are painted white. Yet the coils at the back are painted black. Why is this? Predict the effect that a chocolate coating would have on the rate at which an icecream melts. How could you test your prediction? Polar bears have white fur and black skin. In winter their fur is fluffed up, and in summer it sits down flat. Suggest how these adaptations allow the bears to control their temperature.

plastic foam

a b c 3

Which beaker do you think will be the hottest after 10 minutes? Why? Explain why the water in B will probably be a little warmer than that in A. What happens to the heat energy lost from the beakers?

An ordinary gas or electric oven is more correctly called a convection oven. Why? Draw a diagram showing how it works by convection.

In a supermarket the doors of vertical refrigerators must be kept shut, yet the freezer unit in the foreground of the photo has no lid. How can you explain this?

heating element

You put a can and a glass bottle of ginger beer into the refrigerator at the same time. They both contain 375 mL and are both at room temperature. Predict which one will cool more quickly. Why?

There are similarities and differences in the way light, heat and sound are transmitted, reflected and absorbed. a Can heat travel through space where there is no air? What about light and sound? b Can heat, light and sound be reflected? Give examples. c Can heat, light and sound be absorbed? Give examples.

130

ScienceWorld 8

challenge
1 hat colour would you paint large petrol W storage tanks? Why? 2 Look at the cartoon showing Jason thinking about how heat travels. Is he right? How would you answer him?

Wool is a very good insulator. So, why not wear a woollen jumper on a hot day? Surely this would reduce the amount of heat reaching your body.

3 Ansa and Tammy filled two paper cups, one with water and the other with soil. They placed them in the refrigerator overnight. The next morning they took both cups, put them in the sun, and measured their temperature every 15 minutes. Here are their results. Time 9.00 am 9.15 am 9.30 am 9.45 am 10.00 am 10.15 am 10.30 am Water 10C 10C 12C 13C 14C 15C 15C Soil 10C 11C 13C 16C 20C 25C 30C

a Plot these results on a graph. b Which absorbs heat more readilywater or soil? During the day, which becomes hotterthe c land or the sea? Where would a glider pilot look for thermals d (rising air)above land or above a lake?

4 Hot-air balloons work by using a burner that heats the air below the balloon. How does this make the balloon rise? 5 Use the particle theory to explain the following. a Conduction occurs much more rapidly in solids than in gases. b Convection currents can occur in liquids and gases, but not in solids. 6 Design an experiment to compare the insulating properties of four different house bricks. Try it if you have time. 7 One end of a long glass rod is heated to 100C and the other end is cooled to 0C. What will happen to the temperature at a each end if the rod is left at room temperature? Sketch graphs to illustrate the temperature b changes at the two ends of the rod. 8 Using what you have learnt in this chapter suggest: a four ways of preventing heat loss from your house in winter b four ways of preventing your house from getting hot in summer.

Chapter 6 Investigating heat

131

6.3 Heat in everyday life

This section is different from other sections of the book. Instead of working through it page by page, you can select any of the six activities on the following pages. You will need to apply what you have learnt in the first two sections and work things out for yourself. In some of the activities you will be designing your own experiments to solve a problem. If you need help with this, see Chapter 2.

1 Firewalking
each coal is actually burning. When a firewalkers foot touches a burning coal, a small amount of heat is transferred to the foot by conduction. This loss of heat is enough to temporarily reduce the surface temperature of the coal below ignition temperature, causing it to stop burning. The secret to firewalking is that charcoal is a poor conductor of heat, and it takes about a second before enough heat is transferred through the dead outer layer of skin on the foot to the living tissue beneath, thereby causing a burn. So, provided that the foot is in contact with any one hot coal for less than a second, it will not be burned. Despite all this, firewalking is still dangerous, and you should not try it yourself! Firstly, burns can occur where the skin is thinnest; for example, under the arch and between the toes. Secondly, if there is any burning wood mixed with the coals it may produce hot gas jets capable of burning. Thirdly, small bits of coals can sometimes stick to the firewalkers feet. When this occurs the coal is in contact with the foot for longer than a second and a burn will result.

Exercises
You may have seen firewalking on TV, where people walk barefoot across a pit of red-hot coals. Some people think that this shows how the mind can influence the body. But it can be explained in terms of heat transfer. Even when you walk barefoot on a hot bitumen road your feet can be burned as heat is transferred to them by conduction. So how can you walk on red-hot coals at about 800C? The coals are charcoalformed by the partial combustion of wood. Only the outer layer of 1 What is the temperature of red-hot coals? 2 What are coals made of? 3 Are coals good conductors of heat or poor conductors? 4 What is the main way heat is transferred in firewalking? 5 What is meant by the term ignition temperature?

>

132

ScienceWorld 8

6 About how long does it take before living tissue beneath dead skin is burned? 7 Why is it a problem if small bits of coals stick to the firewalkers feet? 8 Given the maximum contact time of one second, is it safe to walk across the coals at normal walking pace? Explain. 9 Suggest why the maximum contact time is slightly different for different people. 10 How does firewalking illustrate the difference between temperature and heat?

2 Howdoesathermoswork?
Study the labelled diagram. Then write an explanation of how a thermos keeps liquids hot or cold. Make sure you explain how the various parts work to prevent heat flow by conduction, convection and radiation. Your teacher may be able to show you the inside of a thermos.
cover

well-fitting plastic stopper double-walled glass or stainless steel container

silvered walls heat radiation

hot or cold liquid

air space

air pumped out to create a vacuum

rubber, plastic or cork supports

Chapter 6 Investigating heat

133

3 Whichisthecoolestcolourtowear?
Which is the coolest colour to wear in summer? Design an experiment to find out. You could use a method similar to that in Investigate 15 on page 126, or you could work out your own design. A datalogger with several temperature probes would be very useful here. Write your report, giving your conclusion and commenting on the accuracy and reliability of your method and results. Include a recommendation to people wanting to keep cool in summer. Phew! Its so hot! You must be absolutely roasting in those dark clothes.

4 Doeswhitecoffeecoolfasterthanblackcoffee?
One evening as Mahdi was making coffee for Kyle and herself the telephone rang. Mahdi was just about to add the milk to Kyles coffee when he said, The coffee will probably stay hotter if you add the milk after Ive finished on the phone. Mahdi knew Kyle would be on the phone for ages, so she said, Wouldnt it be better if I added the milk now? I learnt at school that darkcoloured things like coffee give off more heat and cool faster than light-coloured things like milk. Who is right? Write a hypothesis about the cooling of coffee. Then design and carry out an experiment to test your hypothesis. You will need to make careful measurements and record your results on a graph. (You may be able to use a datalogger with temperature probes.) Is Mahdis explanation correct? Is it to do with colour, or is it to do with the relative temperatures of the coffee and milk? How could you find out?

Dont add the milk to my coffee til Ive finished on the phone.

Wouldnt it be better if I put it in now?

134

ScienceWorld 8

5 Whyusealid?
You have been glancing through a book called 101 ways to save energy in the home. One of the tips is to always put a lid on a saucepan when cooking. You wonder whether this is in fact true. Based on what you have learnt in this chapter, write a hypothesis that you think is correct. Make sure that the hypothesis is written in such a way that you can test it. For example, you need to say what will be measured. Now go ahead and test your hypothesis. If possible, repeat your experiment to make sure your conclusion is reliable.
lid

steam boiling water food

stove

6 Designingahouse
Your task is to design a house for your area that is cool in summer and warm in winter, using what you have learnt in this chapter about heat transfer. Take into account how heat is gained and lost by an average house, as shown in the diagram. In your design you should consider: the position of the house the type of building materials used for the floor, walls and roof
windows roof 25%

walls 35%

10% design features such as a flat or sloping roof, types of windows (eg single- or double-glazed) and ventilation

the surrounds of the house, including the types of trees

< WEB watch>

floor 15%

draughts & ventilation 15%

To find out more about energy-efficient house designs, go to www.scienceworld.net.au and follow the links to Sustainable energy info (fact sheets on building) and Energy Smart house design.

Fig 43

The percentages of the total heat transfer in various parts of an average house

Chapter 6 Investigating heat

135

Copy and complete these statements to make a summary of this chapter. The missing words are on the right.

absorb change conduction convection energy high insulators low mass particles radiation temperature transfer

1 Heat is a form of ______ which can raise the ______ of an object. 2 The temperature of an object depends on how fast its ______
are moving. The faster they move, the higher the temperature.

3 The amount of heat gained or lost by an object depends on its

______, the temperature ______ and what it is made from.

4 ______ is the transfer of heat through a material by the collision

of particles. Metals are the best conductors of heat. Poor conductors are called ______.

5 Heat energy flows from places where the temperature is ______

to where it is ______. Insulators are used to reduce the amount of heat ______.

6 ______ is where heat is transferred by circulating currents in

liquids or gases.

7 Heat energy can be transferred across empty space by means of ______. 8 Dark-coloured surfaces ______ and emit radiation better than light-coloured
surfaces.
Try doing the Chapter 6 crossword on the CD.

REVIEW

1 If one end of a copper rod is held in a burner flame, heat travels quickly along the rod to the other end. Substances like copper which behave in this way are called good: A absorbers B insulators C radiators D conductors 2 A building is heated by running hot water through a number of radiators. The most efficient colour for these radiators would be: A silver B white C black D red

Copy and complete the diagrams below to show the convection currents you would expect to form in the water.
a b

136
4

ScienceWorld 8
Which of the following statements are true, and which are false? Rewrite the false ones to make them correct. a A cold object eventually heats up to the same temperature as its surroundings. b Conduction is fast in insulators. c Heat transfer by conduction is very slow in liquids and gases. d The sun transfers heat energy to the Earth by the process of convection. e The hotter an object is, the less radiation it emits. f When an object absorbs radiation its temperature rises. g Heat radiation travels at the speed of light. If two objects are at different temperatures, what can you say about the movement of the particles in the hotter one? Which has more heat energya teaspoon of water at 80C or a bucket of water at 80C? Explain. If you hold your hand above a burning candle, you will burn yourself. Yet you can quite comfortably hold your hand beside the flame. Why is this? Look at the diagram of a toasted cheese sandwich being cooked in a griller. a How does heat travel from the heating element to the sandwich? b Why cant the heat travel by conduction or convection?

REVIEW

9 Rory and Trent poured equal volumes of cold water into two identical styrofoam cups, then put identical thermometers in each. They put one cup in the sun and the other in the shade, and recorded the temperatures every 10 minutes. Here are their results.
Times (minutes) Temperature (C) in sun in shade

0 10 20 30 40 50 60

15 16.5 18 20 21 23 24

15 16 17 17.5 18.5 19.5 20.5

a Plot the results on a graph. b What conclusion can you draw from the graph? c What variables did Rory and Trent control in this experiment? d Which method of heat transfer caused the increase in temperature of the water in the cups? e What would be the effect of painting the cup in the sun black? 10 A manufacturer claims that a certain insulating material is good to keep the cold out. Is this expression accurate? Explain using a diagram. 11 Do sheep get colder when it is raining and their wool is wet? Design an experiment to find out, listing the steps you would need to take to make it a fair test.

heating element toasted cheese sandwich

Check your answers on pages 256258.