8 i heating & cooling (boardworks)

KS3 Physics

8I Heating and
Cooling
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© Boardworks Ltd 2004
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Contents

8I Heating and Cooling
Heat and temperature
Conduction and convection
Radiation
Changes of state and evaporation
Summary activities
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What is heat?
Heat is a type of energy.
Heat is the name for the type of kinetic energy possessed
by particles.
Heat energy is measured in joules (J).
How many joules are there in a kilojoule (kJ)?

If something gains a lot of heat energy, it becomes hot
– so what is temperature?

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What is temperature?
Temperature is a measure of how hot or cold an object is.
(It is not the total amount of energy contained in the object.)
Temperature is measured in degrees Celsius (ºC).
The freezing point of water is defined as 0ºC (at 1 atm.).
The boiling point of water is defined as 100ºC (at 1 atm.).
Temperature can be measured by a variety of different
thermometers. These include liquid in glass, digital,
thermocouple and bimetal strip thermometers.

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Heat transfer
The correct phrase for heat transfer is ‘thermal transfer’.
Heat energy can be transferred (moved) by four processes:

1. conduction
2. convection
3. evaporation
4. radiation
In each process, thermal energy is also transferred in the way:

HOT

COLD

Heat energy only flows when there is a temperature difference.
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The particle model
The differences between solids, liquids and gases can be
explained by the particle model:

1. All substances are made up of particles (atoms, ions or
molecules).

2. These particles are attracted to each other, some strongly
and others weakly.

3. These particles move around (i.e. have kinetic energy).
4. The kinetic energy of particles increases with temperature.

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Temperature and energy
What happens to the gas as the temperature increases?
Would this be the same for solids and liquids?

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Contents

Radiation
Summary activities
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Conduction experiment
Conduction of heat in different materials can be investigated
in an experiment.
Apparatus:

8 cm strip of copper
8 cm piece of wood
Bunsen burner
stopwatch
blob of wax

Take care whilst doing this experiment:
 Wear safety glasses.
 Do not touch mot metal objects!
 Wash any burn with cold water for ten minutes.
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Conduction experiment

 Add a blob of wax to one end of the
copper and hold the other end in the
yellow Bunsen flame.
 How long does it take for the wax to
melt and drop off the metal strip?
 Now do the same for wood.
 Why are the times very different?
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Conduction in metals

Metals are good conductors of heat because:
Metals have atoms inside them and lots of
free electrons.
The free electrons can move around
and vibrate.
The heat energy is passed on by neighbouring
particles vibrating along the metal.
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Conduction – compare a metal and a non-metal
Which material feels warmer if you touch a piece of wood
and a piece copper metal, both at room temperature
(i.e. both at 25ºC)?

The wood feels warmer because it is a poor conductor.
It cannot conduct heat away from your hand as quickly as
the copper which is a good conductor.
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Conduction in non-metals
Non-metals are poor conductors of heat.
In a non-metal, heat energy is only passed on by neighbouring
particles vibrating along the non-metal (no free electrons).
This allows a flow of energy from hot to cold.

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Conduction in liquids
Metals are good conductors of heat and non-metals are
poor conductors of heat (insulators).
Are liquids good at conducting heat?
Use some gauze to hold an
ice cube at the bottom of a
tube of water.
Carefully heat the
water at the top of the tube
only, until this water is
boiling.
If the liquid is good at

100ºC

0ºC

conducting, the ice should
quickly melt – it doesn’t.
Liquids are poor conductors of heat (i.e. good insulators).
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Conduction in gases
Liquids are poor conductors of heat (good insulators).
Are gases good at conducting heat?
Carefully hold a safety match
1 cm away from a Bunsen
burner flame. (Do not put the
match in the flame!)
If a gas is a good conductor,
the air between the flame and
the match should conduct heat
and light the match – it doesn’t.
Gases are poor conductors of heat (i.e. good insulators).
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Conduction – summary table
Material
metals

very good conductors

non-metals

insulators

liquids

insulators

gases

good insulators

vacuum

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Conductor or Insulator?

excellent insulator

2005

Convection and density
To understand how heat can be transferred by convection,
the idea of density is important.
If water, oil and air are mixed
up, they will settle out in order
of density – which one will
rise to the top?
The air is least dense and rises
to the top, the water is the
most dense and sinks to the
bottom – it depends on how
far apart the particles are.

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Convection current in a liquid
The movement of hotter areas in a liquid can be seen using
potassium permanganate as a dye:
This cycle is called a
convection current.

heat

heat

Can you explain how the convection current moves using
the idea of density?
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Convection current in a gas
Convection currents cannot occur in solids because the
particles are held in fixed positions – but can they occur
in gases?
Place a candle at one side
of a litre beaker.
Place a piece of card down
the centre, leaving a gap of
2cm at the bottom.
Make some smoke with
smouldering spills and
watch the path of the smoke.
Can you explain why this happens?
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Convection currents in coal mines
When shaft mining was first used to mine coal,
convection currents caused by an underground fire
were used to ventilate the shafts:

Why do you think miners don’t use this method anymore?
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Convection currents in a pan of boiling water

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Contents

Radiation
Summary activities
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Radiation
Heat can move by travelling as infrared waves.
These are electromagnetic waves, like light waves,
but with a longer wavelength.

This means that infrared waves act like light waves:
 They can travel through a vacuum.
 They travel at 300,000,000 m/s.
 They can be reflected.
 They cannot travel through opaque
materials.
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Radiation experiment – instructions
Paint four thermometer bulbs with the following colours:

black

white

silver

red

 Place each thermometer into very hot water
for one minute.
 Take it out of the water, start the stopwatch
and read the temperature.
 Read the temperature every 30 seconds and
record the results in a table.

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Radiation experiment – results table

0

0.5

1

1.5

2

2.5

3

3.5

4

Temp

Colour min min min min min min min min min change

black
white
White

silver
red
Which colour of the thermometer bulb radiated most heat?
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Radiation experiment – results analysis
Either:
a) Draw a bar chart of your results.
Put temperature change on the y axis
and colour on the x axis.
Or:
b) Draw a line graph of your results.
Put temperature on the y axis
and time on the x axis.
Draw four lines on the graph, one for each colour.

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Radiation investigation
A cup of tea takes up to 30 minutes to go cold.
This depends on the colour and shape of the cup.

Design an investigation to find out the best shape and
colour of a cup to keep tea warm for longer.

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Contents

Radiation
Summary activities
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States of matter and temperature
Water can be a solid, liquid or gas.

 At a cold enough temperature, even substances that
are normally gases will become solid.
 At higher temperatures, solids change to become
liquids or gases – as long as they don’t catch fire or
decompose first.
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Changes of state
Each change of state is given a different name:

melting

solid
freezing

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liquid

boiling

gas

condensing

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Changes of state activity

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Changes of state – heating curve
If a solid is heated, its temperature rises until it reaches
the melting point of the solid.
At the melting point, the temperature stops rising whilst the
solid melts. This is because heat energy is going into
separating the particles rather than raising the temperature.
Once all the solid has melted, the temperature starts to
rise again until it reaches the boiling point.
At the boiling point the temperature again stays the same
as energy goes into further separating the particles.

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Changes of state – heating curve

temperature

boiling ⇒

as
g

liquid ⇔ gas

melting ⇒

liq

id
u

condensing⇐

solid ⇔ liquid

li d
so

freezing⇐

time
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Changes of state – cooling curve activity

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What is evaporation?
Evaporation occurs when the particles in a liquid escape
to form a vapour.
Evaporation can occur at any temperature but it occurs
most rapidly at a liquid’s boiling point.
The particles that escape take some energy from the
remaining particles and so the temperature of the liquid falls.
Evaporation is another method of heat transfer.

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Evaporation experiment
Evaporation from different liquids can be investigated
using this experiment:
 Take four equal masses of cotton
wool and soak each one in a different
liquid – ethanol, water, propanol and
octanol.
 Wrap each piece of cotton wool
around the bottom of a thermometer
and secure it with a rubber band.
 Read the temperature every
0.5 minutes and record the
results in a table.
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Evaporation experiment – results table

Liquid

Temp.
change
0 0.5 1.0 1.5 2.0 2.5 3.0
(ºC)
min min min min min min min
Temperature (ºC)

ethanol
water
propanol
octanol

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Evaporation experiment – results analysis
Either:

1. Draw a bar chart of your results.
Put temperature change on the y axis.
Put type of liquid on the x axis.
Or:

2. Plot a line graph of your results.
Put temperature on the y axis.
Put time on the x axis.
Plot four lines, one for each liquid.
Are any of the results anomalous?
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Contents

Radiation
Summary activities
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Glossary
conduction – The way that heat energy travels through

solids because their particles are close together.
conductor – A material that allows heat energy to flow
through it.
convection – The way that heat energy travels through
liquids and gases because their particles are free to move.

heat – A form of energy, measured in joules (J).
heating – The transfer of heat energy.
insulator – A material that does not allow heat energy to

flow through it.
radiation – The transfer of heat energy by electromagnetic
waves, and which does not need a medium.
temperature – How hot or cold an object is, measured in
degrees Celsius (°C).
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Anagrams

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Heat transfer questions
1. How does a cup of tea lose heat
by conduction, convection,
evaporation and radiation?

2. Why does take-away food often
come in aluminium containers?

3. Why do elephants have big ears?

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Heat transfer – energy losses
Name the three processes
that cause energy to be
lost from the home.

Which areas of the house
lose energy?

What can be done to
prevent this energy loss?

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Multiple-choice quiz

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8 i heating & cooling (boardworks)

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