(in degrees Celsius)
Ask the students to plot a graph of temperature against time for each cup.
What is the difference in temperature between the two cups after five minutes? How do the results explain
conduction in hot liquids?
Discuss melting point of solids and boiling point of liquids in the light of the kinetic theory.
Reducing energy waste
Most of the energy used by us comes from fossil fuels—coal, oil and natural gas. The supplies of these are
limited and we are using them up at an alarming rate.
Explain that energy conservation means making the best use of our energy supplies. It is important that we
do not waste fuel in the home, cars, or in industries.
In the home most of the energy is used to heat rooms. When this energy escapes, it goes to heat up the
atmosphere outside. It is spread out among many air particles and so is wasted.
The way we use fuels in our homes has changed very much over the past hundred years or so. Originally
candles and oil lamps were used for lighting. They were dirty, dangerous and did not give much light.
Eventually electricity took their place and now electric light bulbs and fluorescent tubes seem to be everywhere.
Electricity has changed our lives because it can be used for lighting, heating, and to operate motors inside
domestic appliances like mixers, juicers, hair driers, etc. Electricity is not a fuel but we produce most of our
electricity by using fossil fuels.
When a kettle is put on to boil, the temperature of the water steadily rises until it reaches 100°C. At this
temperature it starts to boil. Once the water has begun boiling, the temperature remains constant at 100°C,
but at the same time, heat is being steadily absorbed by the water from the gas flame. This heat, which is
not increasing the temperature of water, is the energy needed to convert the water from the liquid state to
the vapour state. Experiments show that 22600000 J of energy are required to convert 1 kg of water at its
boiling point to steam at the same temperature. This is known as the specific latent heat of steam, where
‘latent’ means hidden. This extra heat goes into the vapour, but does not indicate its presence by producing
a rise in temperature. When steam condenses to form water, the latent heat is given out. This is one reason
why a scald from steam does more harm than one from boiling water.
Just as latent heat is taken in when water changes to vapour at the same temperature, a similar process
happens when ice melts to form water. However, in this case the latent heat is not so great. It requires only
336000 J to convert 1 kg of ice at 0°C to water at the same temperature. Likewise when water freezes into
ice at 0°C, the same quantity of heat is given out for every 1 kg of ice formed. This is called the specific
latent heat of ice.
Heating and cooling: p 24
1. A spark from a firecracker may be very hot but it does not contain much heat energy. Comparatively,
a cup of hot tea would cause more damage owing to the amount of heat energy it contains.
2. Heat is the amount of energy that something possesses. It is measured in Joules(J) or kilojoules(kJ).
3. Temperature is a measure of how hot or cold something is. It is measured with a thermometer in
4. The particles of a substance move with respect to the amount of energy they have. At higher
temperatures, the particles move about very vigourously. With the decrease in temperature, this energy
is lost slowly until the time when the particles stop moving altogether.
5. The temperature at which the particles of a substance stop moving is called absolute zero.
6. The Kelvin scale is a temperature scale starting from absolute zero.
Heat is about moving particles: p 25
1. In a liquid, the particles move faster as the temperature rises. At the surface the faster particles break
free to form a gas above it.
2. a) evaporation b) melting c) condensation d) freezing
3. From the atmosphere.
Moving heat (1): conduction: p 26
2. Substances that do not allow heat to pass through them are called insulators. For example, glass,
plastic, and wood.
3. If one end of a metal bar is heated, the particles there gain energy and vibrate faster. This causes the
particles next to them to vibrate faster as well. The increased vibration of particles is passed along
the bar until the whole bar is hot.
4. Take a plastic container and make three holes near the base at one side of the container. Insert three
metallic rods of the same length and diameter into the holes at the side of the container. Attach
drawing pins to the ends of the rods using wax. Pour hot water into the container and observe
what happens to the drawing pins. The drawing pins fall, one by one, according to how fast heat
was conducted through the rods. The results show that different materials conduct heat at different
5. a) Heat energy flows quickly through the metal base of the saucepan to reach the food item that
needs to be cooked or warmed.
b) Plastic is a poor conductor of heat so it will not burn our hand when we hold it.
Moving heat (2): convection: p 27
1. Liquids and gases can carry heat because their particles are free to move.
2. The heating element in the kettle only warms the water touching it. Warm water being less dense
than cold water rises to the top. Cold water replaces the rising warm water and this in turn becomes
3. Warm air is less dense than cold air, so the warm air rises and is replaced by the colder denser air.
In this way, a convection current is set up and air circulates around the room.
4. In rooms with high ceilings, it takes longer for the warm air to rise, and for the colder air to come to
take its place.
5. The warm air rises by convection from the heater and makes the paper decorations flutter about.
6. Convection cannot happen in solids because the particles are held in a framework and they cannot
move around as they do in liquids and gases.
Moving heat (3): radiation: p 28
1. a) White or silvery surfaces are poor absorbers because they reflect most of the radiation. That is
why in hot sunny countries houses are often painted white to keep them cool inside.
b) Black absorbs heat more quickly than white.
c) The silvery surface of aluminium is a poor emitter of thermal radiation so the food remains
d) A dark surface absorbs heat more quickly than a shiny one.
Reducing energy waste: p 29
1. Energy for our homes is produced from fossil fuels, which were formed over millions of years. Since
these energy sources are being used up at a fast pace, we should try to reduce heat loss from
2. Air is a good insulator because it is a poor conductor of heat.
3. Insulating materials such as foam and fibre contain a lot of trapped air.
4. A layer of air is trapped between two glass panes which slows down the rate at which heat energy is
5. Heat loss through draughts can be stopped by filling the gaps with strips of rubber or plastic draught
Solar power is free: p 30
1. Most of the thermal energy that comes to the Earth from the Sun during the day is lost into space as
radiation so the Earth cools down at night.
2. Solar heating is expensive because the cost of making the equipment is very high.
3. a) The blackened layer at the back of solar panels absorbs the heat radiation from the Sun and
warms the water.
b) The network of pipes helps to carry water throughout the house.
4. They are economically rich countries so they can afford to install solar heating equipments.
5. Combining solar power with the skilful use of insulating building materials could help to make an
ideal energy-saving home.
Some of the steps are:
• Solar panels on the roof should be positioned to absorb as much sunlight as possible.
• Large double glazed windows should be positioned on the side of the house with most sunlight
and small windows elsewhere.
• A greenhouse or conservatory should be built on the sunny side of the house, from where the
warm air can be circulated around the house.
• Loft insulation should stop heat loss through the roof.
• Thick cavity walls filled with cavity wall insulation would heat up during the day and radiate some
of this heat into the house at night.
• Shutters on the windows would help the double glazing by preventing heat loss at night.
Test yourself: p 32–33
1. Ice is a solid. When it is heated, it melts and becomes water. Water is a liquid which boils at 100°C.
At this temperature, it turns into a gas called steam. When steam cools, it condenses and turns back
into water. At 0°C, water freezes and turns back into ice. These are examples of changes of state.
2. a) Celsius
b) Add 273 to the degrees on the Celsius scale to convert into the Kelvin scale.
c) i) 573 K ii) 423 K iii) 223 K
d) Absolute zero is the temperature at which a material has no heat energy.
3. a) i) The red blobs represent molecules.
ii) They are vibrating.
b) i) It is a solid metal.
ii) The molecules are arranged in a regular grid or lattice.
c) As a metal is heated, its molecules vibrate more quickly. Some of this energy is passed on to
neighbouring molecules which pass it on to their neighbours and so on.
4. a) The particles are gaining kinetic energy. Some of them are also able to escape from the
d) Drying clothes in the Sun.
5. a) convection
Workbook 2, Chapter 3
1. a) i) B
iii) 30 degrees
b) 15 degrees
When a solid is warmed the particles move faster, until some
break and become part of the liquid.
When heat is applied to a liquid, the particles move faster. At the
surface, the faster particles break free to form a gas above it.
When heat energy is removed from a gas, its temperature falls.
The gas particles slow down and move closer together. Eventually
they will come close enough to form a liquid.
As a liquid cools, its particles slow down. Eventually they will start
to form a framework which is typical of a solid.
Mass is conserved
In a change of state, only the behaviour of the particles changes.
The actual particles remain the same.
What is it
What is its job?
prevents heat from a hot utensil to reach the
allows heat to get from burner to food
handle of fish slice
stops heat to reach the hand from the heated
base of iron
allows heat to pass from iron to clothes
feet of kettle
prevents heat from kettle to reach the table
hot water cylinder
allows heat to get from burner to water in boiler
c) i) To hold hot pans or the hot oven tray.
ii) An insulator.
iii) An insulator would not allow heat to pass through it.
4. a) Cold water being denser than hot water settles to the bottom of the tank.
b) i) B
ii) Hot water is less dense and would rise to the top of the tank. Therefore, point B would be
c) Hot air rises pulling along with it dust particles from the floor.
5. a) i) Screw top made of plastic stops heat loss by conduction.
ii) Vacuum between the inner and outer layers of the bottle stops heat loss by convection.
iii) Silvery inside surface would prevent heat loss by radiation as the heat rays are reflected
b) It prevents heat loss to the outside and stops outside heat from coming in.
6. a) An insulator is a substance that does not allow heat to pass through.
b) i) By using insulating materials such as foam or fibre.
ii) By filling cavity walls with insulating materials.
iii) By covering the floor with carpets or making wooden floors.
iv) Double glazing.
c) Double glazing and insulating materials.
d) Trapped air does not let heat to pass through, therefore, proving to be a good insulator.
7. a) i) White reflects radiations and is a poor absorber of heat. It helps to keep houses cool.
ii) Black is a better emitter of thermal radiations than a shiny surface.
iii) Hot air rises from the bonfire causing cool air to take its place. A convection current is set
up and the person can feel a draught.
iv) Cold air being denser than hot air settles at the bottom of the refrigerator. This escapes as
the refrigerator door is kept open.
v) By fluffing up, birds trap air between their feathers forming an insulation layer.
vi) A saucepan has a copper bottom (a conductor) to absorb heat from the fire, but it has a
plastic handle (an insulator) so that our hands do not get burnt.
vii) Air gets trapped between the hole of the string vest and the tight shirt.
viii) Convection currents of air are produced by the warm Sun shining on the field. This helps the
glider to gain height.
ix) Black is a good absorber of heat.
x) Convection currents of air are produced by the heater. As the warm air rises, it makes the
paper decorations flutter.
vacuum to prevent
heat loss by radiation
fill the space with
an insulating material
plastic body to prevent heat loss by conduction
tight-fitting to prevent
heat loss by convection
shiny metallic inner
surface to prevent heat
loss by radiation
Documents you may be interested
Documents you may be interested