Physics
Cambridge IGCSE

TOPIC 2: THERMAL PHYSICS

2.3 Heat transfer

Global warming is a major area of science research at present. The Earth is warming, almost certainly due to the use of fossil fuels. However, how this affects the Earth depends on how heat from the Sun is transferred to the Earth. Scientists need to have a detailed understanding of heat transfer to predict the outcomes from continued global warming.

When heat, or thermal energy is is transferred, it can travel in different ways. In this section, we will look at conduction, convection and radiation. (Evaporation is not required in the syllabus).

Conduction

When you put a saucepan on an oven top to heat food, the thermal energy passes through the metal in the pan. This movement of heat through the metal is called conduction. When a solid substance is heated, the particles vibrate more quickly. The hotter the substance, the faster the movement of the particles. However these particles produce forces on their nearby neighbours, passing some of this energy to these particles and making them vibrate faster. In this way, the thermal energy stored as kinetic energy of the particles is passed through the substance.

Figure 1. Conduction through a metal rod

Some substances like metals are particularly good thermal conductors. This is because metals have free (delocalised) electrons that can rapidly transfer the kinetic energy through the substance. The free electrons can basically hop from atom to atom, and they transfer kinetic energy rapidly and efficiently as they do this. In all solids, conduction is the transfer of kinetic energy using atomic or molecular lattice vibrations.

Gases and liquids

Liquids are poor conductors. This is because the forces are weak betwen particles in a liquid as discussed in section 2.1a. If particles gain energy and vibrate, it has less effect on the near neighbours because of this weak bonding.

Gases are very poor conductors. This is because the particles are so far apart that collisions are unlikely. In addition, the forces are negligible. If we heat one part of a gas, the particles do not affect other near neighbouring regions of the gas and conduction does not occur efficiently.

Insulators

Any poor conductor is called a thermal insulator. Air is an excellent thermal insulator, so clothing that traps air will prevent conduction. Examples include thick winter jackets, woolen clothing such as hats, and down feather sleeping bags. They all have trapped air, so insulate us in cold weather  - they prevent thermal conduction taking place. Double glazing in cold countries uses a trapped layer of air (or a vacuum with no particles of air at all) to prevent heat conduction.

Figure 2. Insulated clothing with trapped air
Image by Alexandra_Koch

Note that note all materials are classed as good conductors or insulators. There are some materials that are not particularly good at conducting or insulating - for example marble floor tiles. They are used as they are hard and durable on floors, but not for any particular thermal properties.

Required Practical: Investigating conduction

There are many ways to demonstrate or investigate conduction. This method is a standard practical to investigate conduction through a variety of substances. You will need:

• A selection of identical rods of the same dimensions, made of different substances (for example, copper, aluminium, brass, steel, graphite).
• A wax candle to produce liquid wax.
• Drawing pins.
• A heat source - a candle or bunsen burner.
• A tripod or wooden block to support the rods.
• A stop watch.

Figure 3. Investigating conduction

Method:

Drop some hot wax on to the end of one rod and stick the drawing pin to the end of the rod with the wax as it cools. If the other end of the rod is heated, thermal energy will pass down the rod through conduction. Eventually the wax will melt and the pin will drop off. Time how long it takes between applying heat and the pin dropping. Repeat this for a variety of rods.
The best conductor will be the rod that takes the least time to melt the wax. Metals are generally good conductors, particularly copper.
Safety - you will need goggles for this experiment, and take care not to touch the hot rods during and immediately after the experiment.

Convection

Why does a tennis ball float on water? If you push it under water, it will rapidly rise back to the surface. This is because the ball is less dense than the surrounding water, and there is a force called an upthrust making it rise upwards to the surface.

This is how hot air balloons work. The air in the balloon is heated, and this makes the air expand. The air in the balloon is now less dense than the surrounding cold air, and there is now an upthrust on the balloon. All hot air rises in this way, carrying thermal energy upwards. In fact, any gas or liquid can behave in this way. Solids cannot, since the particles are fixed in position.

This upward movement of thermal energy in gases and liquids is called convection. It happens all around us, as shown in figure 3:

Figure 4. A convection current in a room

In this diagram, the hot air rising above the heater and the cold air sinking near the window creates a circulation of air (a convection current). This happens all the time, and convection currents caused by heat from the Sun drive our weather, causing wind, clouds and movement of warmer air from one area to another.

Consider a large island. On a hot sunny day, the land quickly warms up, whilst the sea stays relatively cool. The air above the land rises just like in figure 4, and the air above the sea falls, creating a convection current as shown above.

Required Practical: Investigating convection

There are many ways to demonstrate and investigate conduction. One of the most simple ways is the following:

You will need:

• A small coloured crystal of potassium permanganate.
• A beaker of cold water.
• Tweezers to pick up a crystal or two.
• a heat source such as a candle or bunsen burner.

Method:
Carefully drop a crystal or two of potassium permanganate into the side of the beaker. it will sink to the bottom, and start dissolving, releasing a strong purple colour. Then heat the water directly below the crystal. The water will expand a little, becoming less dense, and start to rise. The purple-coloured water can be seen rising and then sinking on the other side as a convection current is started.

Figure 5. Investigating a convection current

The Sun produces a huge quantity of infrared, visible and ultraviolet radiation. This electromagnetic radiation passes across the vacuum of space and warms the Earth. The heat radiation we feel from all hot objects is actually infrared radiation. (See section 3.5 for more on electromagnetic radiation and infrared light). This radiation, like light can pass freely through a vaccum, through the air and other gases, but not through solids where it is either reflected, scattered or absorbed. It can pass a short distance into water and other liquids where it is then absorbed.

Some surfaces are better at absorbing and emitting infrared radiation, and this can be investigated with a simple experiment:

You will need:

• Several identical boiling tubes.
• Temperature probes (or thermometers).
• cotton wool to make a plug for the boiling tubes.
• Different colours of paper, all of the same type/thickness (e.g., black, white, yellow, blue).
• Several bright desk lamps, or strong sunlight.

Method:

Place all of the boiling tubes in direct sunlight, or under desk lamps, so that all are being equally heated by the heat radiation. (For example, make sure the lamps are the same distance from each tube). Record the temperature of the air in each tube at the start of the experiment, and then leave the tubes for 10 to 20 minutes. Then record the increase in temperature in each tube.

Results:

What you will find is that the black tube will heat up quickly, and the white will only increase a little. This is because black surfaces absorb heat radiation, and white and other light colours mostly reflect the radiation. In fact, silver reflects very effectively, just as it does with visible light. Shiny surfaces, even if black, also reflect some radiation. Dull surfaces (also called 'matt' e.g. Matt black) absorb infrared more readily.

Alternative methods:
You could also try metal plates, with wax attached as per the investigation described above for conduction. Each metal plate is painted a different colour, and held a fixed distance from a heater / lamp or similar source of heat radiation. The time it takes for the wax to melt and a pin to drop with give a measurement of how well the coloured plate absorbs radiation.

In this experiment, a cube called a 'Leslie' cube is required. This cube can be filled up with hot water. Each side is of the metal cube is painted a different colour. The outside of the cube shows different temperatures when tested with an infrared temperature sensor as shown in figure 7:

Figure 7. A Leslie cube emitting heat radiation

Results

The matt ('matt' = not shiny) black surface shows the highest temperature, and the shiny silver surface (not shown) has the lowest temperature. This demonstrates that matt black surfaces are the best heat radiation emitters. Shiny surfaces are less effecive, and silver surfaces are extremely poor at emitting infra red radiation.

Absorbing Heat

In summary, experiment #1 above shows that:

• black surfaces absorb infrared heat radiation
• white and other light colours mostly reflect the radiation.
• Silver reflects very effectively, just as it does with visible light.
• All shiny surfaces, even if black, reflect some radiation.
• Dull surfaces (also called 'matt' e.g. matt black) absorb infrared more readily.

The quantity of heat radiation emitted by any object is dependent on several factors:

1. The temperature of the object. A hot object will radiate a lot more heat than a warm one.
2. The surface area of the object. If the surface area is large, more heat will be radiated and the object will cool faster. This is why elephants have large ears - the large surface area helps them to radiate heat and cool down.
3. The colour of the surface. As well as absorbing radiation efficiently, a black surface is also an excellent radiator of heat - it releases a large quantity of infrared radiation. Silver and white surfaces are not good radiators.

Why is the wood burning stove in figure 8 painted black?

Figure 8. A multi-fuel stove.
(Windyswindy - Wikimedia GNU free license)

The answer is that - as described in the practical above - the matt black surface is an excellent emitter of radiation, releasing the heat energy into the room efficiently.

Summary:

• Matt black (not shiny) surfaces absorb and emit the most heat radiation.

The transmission of heat radiation acts in exactly the same way as the absorption does - it is equivalent to dull/matt black surfaces being an 'open door' to heat radiation, allowing it in and out easily.

Complex Energy Transfers

The fire shown above is an example of a system where thermal energy is transferred in many ways. Heat is radiated from the black surface as described above. In addition, the metal case allows for efficient conduction so that the outside surface is very hot. The air above the fire expands and rises, and carries heat upwards through convection.

Another example is a car radiator. This contains water that extracts heat from the engine and prevents it overheating. The water is pumped into the car engine and back to the radiator. Convection currents make the hot water rise to the top. The radiator itself is painted black and has a large surface area to aid radiation. A fan blows air across the radiator to cool it. The radiator is metal to allow for effective conduction, and again the large surface area means that heat can be transferred to the air being blown across the radiator.

The picture here shows a dull black ball placed near to a heat lamp. The lamp has just been switched on. What do you think will happen to the temperature of the ball?

Figure 9. A ball heated by a strong heat lamp

In this experiment, as you may think obvious, the ball heats up as it absorbs heat radiation. However, it will not continue getting hotter and hotter. Eventually the temperature reaches a hot, upper limit. Why is this?

Figure 10 below shows that as the ball gets hotter, it emits infrared radiation of it's own. Eventually, at 140°C in this experiment, the ball radiates heat as fast as it absorbs heat from the lamp. The ball has reached an equilibrium, and the temperature remains constant.

Figure 10. A surface reaching thermal equilibrium

Note that if the ball radiated thermal energy at a greater rate than being absorbed, the ball would cool down again. This is what happens when the heat source is turned off.

The greenhouse effect

The principle of thermal equilibrium is really important for the Earth's climate. We receive heat from the Sun all year round at an almost constant rate, and have done for billions of years. The Earth therefore has reached a thermal equilibrium, where the heat received from the Sun balances the heat radiated back out into space. However the situation for the Earth is complicated by the Earth's atmosphere, which we now know traps heat. This is called the greenhouse effect. The gases responsible for this - CO2, methane and water vapour for example - are known as greenhouse gases. They are actually really important in maintaining a balance above freezing. However scientists are increasingly concerned as we burn fossil fuels and increase the level of greenhouse gases, which will disturb this natural thermal equilibrium, and increase global average temperatures.

Figure 11 shows the natural balance and thermal equilibrium we currently have. It shows the important role the atmosphere has in trapping and redirecting heat energy. The pre-industrial figures have about 150 W m-2 (watts per metre squared) trapped by the atmosphere and directed back to the ground. We do not want to increase this number by adding to the greenhouse effect.

Figure 11. Thermal equilibrium for the Earth
This diagram is a bit complicated for IGCSE Physics and you are not expected to learn this

Heat Transfer Questions:

1. Why are many houses in hot countries painted white?

A white surface is a poor absorber of heat radiation, so the house will not absorb as much heat from the sun making it too hot for comfort.

2. Explain why water that is cooled at the surface of a lake on a cold day sinks to the bottom of the lake.

The water contracts a little as it cools, becomes more dense than the surrounding water, and so sinks towards the bottom.

3.Explain why most saucepans have a metal base and a plastic handle.

The metal base is a good conductor of heat, allowing heat to transfer from the cooker to the food inside the pan.
The plastic handle is a good insulator (or poor conductor), preventing the user from burning their hand when they pick up the pan - a risk on pans with metal handles.

4. Figures 9 and 10 describe an experiment to find how the temperature of a ball changes with time as it is heated. Sketch a graph to show the results for this experiment. Choose suitable axis labels, but no numbers are needed.

The graph should have time on the x-axis and temperature on the y-axis.

The line plotted should:

• increase initially
• then the temperature remains constant at thermal equilibrium

Preventing heat loss

Imagine you have a nice, hot cup of your favourite hot drink. How can you keep it hot for a long time? How would you design a new container for the drink? In order to do this, you need to stop conduction, convection and radiation losses.

To stop conduction you need to insulate the cup. Polystyrene foam has air bubbles trapped in it, and air is an excellent insulator. The insulation should be thick.

To stop convection, you need a lid or a screw top to stop the hot air above the drink from rising. This should also be an insulator.

To stop radiation, the outer and inner layer need to have a silver layer. The outer silver will be a bad radiator of heat, and the inner one will reflect the heat back inwards.

A vacuum flask has a vacuum around the inner container, and this stops all conduction and convection. Both the inner and outer layers are silvered as described above. These features make heat transfer out (or in) to the container extremely difficult.

Questions:

5. Describe and explain one feature found in a home that prevent heat loss by:

• a) Conduction
• b) Convection

• Carpets - the fibres trap air which is a good insulator.
• Fibre glass insulation in the roof - the fibres trap air which is a good insulator.
• Insulation between/on the walls of the house - trap air which is a good insulator.
• Double glazed windows - air is trapped between the panes of glass, and air is a good insulator.
• Thick curtains - made of insulating materials.

• Double glazing - air is trapped between the panes of glass preventing convection currents.
• Draught excluders under doors - preventing the movement of air into/out of the house.
• Thick curtains - prevent cold air from the windows flowing down into the room.