Heat transfer - BBC Bitesize (2024)

Temperature and heat

Temperature and heat are not the same.

The temperature of an object is to do with how hot or cold it is. It is measured indegrees Celsius, °C, with a thermometer.

Thermal energy is to do with the movement of the particles inside matter.Heating matter makes the particles vibrate faster or move faster. The hotter the substance, the more its molecules vibrate or move, so the more they kinetic energy they have. Thermal energy comes from the total kinetic energy of all the particles.

It is measured in joules, J.

A joulemeter is a device that measures heat energy.

When thermal energy flows, we refer to it as heat energy.

Temperature and heat are not the same thing because:

• Temperature is a measure of how hot an object is.

• Heat is to do with the total kinetic energy of the particles of an object.

• Temperature is measured in °C.

• Heat is a form of energy and is measured in J.

A bowl of soup might have a temperature of 75°C. The sea at Portrush has a summer temperature of 17°C.

The soup is hotter than the sea water because its temperature is higher.The sea has more thermal energy because the sea contains many more particles than the soup.

Heat energy can flow by conduction, convection or radiation. It always flows from a region of high temperature to a region of low temperature i.e. from hot to cold.

Conduction is the flow of heat energy from a region of high temperature to a region of low temperature without overall movement of the material itself.

Metals are good conductors of heat energy. Non-metals and gases are usually poor conductors. Poor conductors of heat are called insulators.

Conduction in solids

The atoms of a solid are held together by chemical bonds. The atoms are fixed in place but are free to vibrate.

When part of a solid absorbs heat energy the atoms vibrate faster and with bigger amplitude. These vibrations pass from atom to atom, transferring heat energy as they do.

This process happens in all solids when heated but is a slow process.

Conduction in metals

Some of the electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions.

When the free electrons absorb heat energy, they gain kinetic energy and move much faster. As they move through the metal, they crash into the metal ions.

Some of their kinetic energy is absorbed by the ions and they vibrate faster, and with greater amplitude. We observe this as a rise in temperature of the metal.

This process is very much faster than conduction caused by just passing vibrations from atom to atom. So, conduction in metals is faster than in non-metals.

Metals are good conductors of heat energy because they contain free electrons.

Key points

• Conduction in insulators (non-metals) is only caused by passing vibrations from atom to atom.
• Conduction in good conductors (metals) is caused by two processes: collisions between fast moving free electrons and metal ions, and by passing vibrations from atom to atom.
• Conduction by collisions between fast moving free electrons is much faster than conduction by passing vibrations from atom to atom.
• Metals are good conductors because they contain free electrons.

Conduction in metals

An experiment can be used to investigate which metal is the best conductor of heat.

It involves some long thin strips of different metals (eg iron, aluminium and copper), Vaseline, drawing pins and a Bunsen burner.

1. Fix the drawing pin to the end of the metal strip using spots of Vaseline.

2. Position the other end of the metal strip into a Bunsen flame.

3. Record the time taken for the wax to melt and the drawing pin to drop off.

The fastest time shows the best conductor of heat.

It is important to keep the length of metal rods, diameter of rods and position in the Bunsen flame the same. These are called the control variables.

Safety

• Safety glasses should be worn when working with a Bunsen burner.
• Place equipment on a safety mat.
• Long hair should be tied back when working with a Bunsen burner.
• After heating do not lift the metal rods by hand they will be hot and could burn. Use tongs or a heat proof glove.

Results

The drawing pin falls off the copper rod first followed by the aluminium.

Conclusion

We can conclude that copper is a better conductor than aluminium, while aluminium conducts better than iron and brass.

Applications

You may have noticed that the handle of a metal spoon feels colder than the handle of a wooden spoon even though they are side-by-side in the kitchen.

Both spoons are at exactly the same temperature – they will both be at room temperature, which is about 20°C and your hand is warmer than both. As metal is a better conductor than wood, it will conduct heat away from your hand faster than the wooden spoon, making your hand holding the metal feel cooler.

Saucepan and frying pan handles are made from plastic or wood which are insulators so that heat does not conduct easily from the hot pan to your hand.

• Insert a lump of ice at the bottom of a boiling tube and put a piece of wire gauze on top of it.
• Fill the tube with water – the wire gauze will stop the ice floating to the top.
• Heat the boiling tube at the top with a Bunsen burner as shown in the diagram.
• Record your observations.

After a short time, the water at the top of the boiling tube boils but the ice cube does not melt.

This shows that water is a bad conductor of heat energy.

• Read the thermometers and record the temperatures at A and B in a suitable table.
• Switch on the heater and leave for eight minutes.
• Read the thermometers again and record the temperatures at A and B.

Results

Conclusion

Heat has not conducted easily through the trapped air to position B. This shows that trapped air is a poor conductor (or good insulator).

Applications

Trapped air is a good, natural insulator. That is why we wear layers of clothes when it is cold.

A string vest keeps you warm because it traps a layer of air between your skin and your shirt. Additional layers provided by shirt, fleece and coat help to trap more air, providing more insulation.

Hair is a good insulator too, trapping air between individual hairs on your head. Wearing a woollen hat on top helps to trap even more air. Each layer of trapped air helps us to stop losing heat energy by conduction.

Summary: conduction

• Conduction is the flow of heat energy from a region of high temperature to a region of low temperature without overall movement of the material itself.

• Occurs mainly in solids – most liquids are really poor conductors and hardly any conduction occurs in gases.

• A poor conductor of heat is called an insulator.

• Metals are good conductors of heat.

• Metals are good conductors because they contain free electrons.

• Trapped air is a very good natural insulator.

• Hair, fur, feathers and wool are good insulators because they trap air.

Fluids

Liquids and gases are fluids. They can be made to flow from one place to another. Convection occurs when particles with a lot of heat energy in the fluid move and take the place of particles with less heat energy.

• Heat is initially transferred from the Bunsen flame through the glass wall of the beaker by conduction.
• The water in the region of the Bunsen flame is heated.
• It expands, becomes less dense and rises.
• It is replaced by the cooler, denser water which surrounds it.
• This water is in turn heated, expands becomes less dense and rises.
• The process continues, a convection current is set up and heat is transferred through the liquid.

Since water is a bad conductor of heat it must be heated by convection. Kettles and saucepans heat water from the bottom so that convection currents are set up easily.

Convection currents can be seen in lava lamps.The wax inside the lamp warms up, expands, becomes less dense than the surrounding liquid and rises.

Tectonic plates

The Earth is a ball of molten rock with an iron core and a cool brittle crust.The crust is made up of plates which are constantly moving because of convection currents in the molten mantle.Areas along the edges of plates are prone to volcanoes and earthquakes.

Convection in gases

A convection current in air can be demonstrated using the following apparatus.

• A burning splint or piece of rope is blown out so that it is smoky.
• When the smoky rope is held in air the smoke rises.
• The rope is then held over both chimneys in turn as shown above.
• When the rope is held above the candle the smoke rises.
• When the rope is held above the other chimney the smoke is drawn down the chimney. It then passes across the horizontal section and up through the chimney above the candle.

What is happening?

• The candle flame heats the air around it.
• The hot air expands and increases in volume.
• The density of the air decreases, and it floats upwards, rising through the chimney.
• Cooler air is sucked in through the other chimney to replace the rising, warm air – a convection current has been set up.
• The smoke from the smouldering rope shows the path of the cool air.

Applications

Early coal mines were ventilated with fresh air using a similar method. Two shafts were dug down from ground level to the mine and a fire was lit beneath one of them. As hot air rose upwards, fresh, clean, cool air was sucked down the other shaft and across the coal mine. Mind you, lighting a fire in a coal mine did have some safety issues!

The heating of the Earth’s surface and atmosphere by the Sun causes convection in the atmosphere and oceans, producing winds, clouds and ocean currents.

For example, as the Sun heats the Earth’s surface, the air above it heats up, expands and rises. This air can continue to rise, cooling as it does so, forming fluffy, cauliflower-shaped cumulus clouds.

Sea breezes and land breezes

Temperature differences at the Earth's surface occur where there are different surfaces such as land and sea.

• During the daytime, along the coast, the land heats up more quickly than the sea water.
• Air above the land is heated, expands and begins to rise.
• The rising air is replaced by cooler air drawn in from the surface of the sea.
• This is called a sea breeze and can offer a nice way of cooling down on hot summer afternoons while further inland the heat may be too much to bear.

In the evening and at night the reverse happens:

• Land cools down more quickly than the sea.
• Air above the sea is heated, expands and begins to rise.
• The rising air is replaced by cooler air drawn off the land.
• This is called a land breeze.

• Air close to the radiator is heated.
• It expands, becomes less dense and rises.
• It is replaced by the cooler, denser air which surrounds it.
• This air is in turn heated, expands becomes less dense and rises.
• The process continues, a convection current is set up and heat is transferred through the air and so through the room.
• A radiator heats mainly by convection – not by radiation!

Convection currents enable hot air balloons to rise and explains why it is often hotter in houses upstairs rather than downstairs.

Summary: convection

• Convection is the flow of heat energy from a region of high temperature to a region of low temperature by movement of a fluid.

  • Convection only occurs in fluids - liquids and gases.
  • When part of a liquid or gas is heated it expands, becomes less dense and rises.
  • It is replaced by the cooler, denser fluid.
  • This liquid or gas is in turn heated, expands becomes less dense and rises.
  • This sets up a convection current.

• Wall heaters (“radiators”) heat mainly by convection.

• Trapped air prevents heat loss by conduction and convection.

Radiation is the transfer of heat energy from a region of high temperature to a region of low temperature by infrared radiation.

Unlike conduction and convection, radiation takes place without the need of any particles. Because no particles are involved, radiation can work through the vacuum of space. This is why we can still feel the heat of the Sun even though it is 150 million km away from the Earth.

Heat radiation investigation

Giving radiation out (or emitting radiation)

An object giving out or emitting radiation cools down, but the surroundings can be heated.

The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called a Leslie's cube.

This is a metal cube with four sides prepared in different ways: black, white, shiny, or dull.

It can be filled with hot water or heated on an electrical hot plate so that all four sides are at the same temperature.

Method

• Measure the temperature a fixed distance from each side of a Leslie's cube using four identical thermometers.
• Heat the Leslie's cube by filling it with boiling water.
• Continue to measure and record the temperatures every 30 seconds for five minutes, then plot a graph of temperature on the y-axis, against time on the x-axis, for each side.
• Compare the four graphs obtained.

Fair test

It is important to keep the following the same:

• the distance of each thermometer from the sides of the cube
• the type of thermometer used
• the time taken for each reading.

These are called the control variables.

Results

The temperature of the thermometer opposite the dull, black side reaches the highest temperature.From this we can say that dark matt surfaces are better at radiating heat energy than light shiny surfaces.

When an object absorbs radiation, it heats up and its temperature will rise. You feel the heat of the Sun when you absorb the infrared radiation from it. If a cloud passes in front of the Sun, you absorb less infrared, and it feels cooler.

How infrared radiation is absorbed can be investigated using the apparatus shown below.

Two squares of aluminium are arranged as shown above. One is painted dull black, the other is polished and shiny.To ensure a fair test they are the same area and thickness and are placed the same distance from the Bunsen flame. Two identical corks are stuck to the back of the plates using equal amounts of Vaseline or candle wax (control variables).The Bunsen is lit. Quite quickly the cork attached to the black plate falls off. The cork behind the polished plate takes much longer to fall off.

Conclusion

Both plates receive the same quantity of radiation, but the black plate heated up more quickly. This tells us that a dull, black surface is a better absorber of radiation than a shiny, polished surface.

Key points

• Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.
• Dark matt surfaces are better at radiating heat energy than light shiny surfaces.

Shiny silver blankets are used to stop heat loss from a person pulled from a cold sea or evacuated from a swimming pool.

Solar Panels should be black because black is a good absorber of radiation.

Infrared radiation reaches us from the Sun. A lot of it is reflected by the atmosphere back into space but some of it reaches the surface and is absorbed. This warms the Earth. The Earth then radiates infrared radiation back into the atmosphere and out into space.

However, because the Earth is cooler the radiation it emits is not as energetic as the Sun’s radiation and some of it is absorbed by carbon dioxide and other greenhouse gases in our atmosphere. The radiation is trapped and warms the atmosphere. The result is that the Earth and atmosphere are at a comfortable temperature, suitable for sustaining life as we know it. This is the normal greenhouse effect. Without the greenhouse effect the mean temperature on Earth would be about -18°C.

The problem is that as more fossil fuels are burnt; more carbon dioxide is released into the atmosphere. More of the radiation emitted by the Earth is absorbed by the atmosphere, which could become too warm. The result might be that the Earth and atmosphere are at too high a temperature which would not be so favourable for life. It is a very fine balance. This is called global warming.

Global warming may lead to climate change with some places becoming wetter than normal and others dryer than normal. Also, the polar ice caps may melt, leading to higher sea levels and flooding.

Summary: radiation

• Radiation is the transfer of heat energy from a region of high temperature to a region of low temperature by infrared radiation.

• Radiation can travel through a vacuum – it does not need a medium (material or particles) to travel through.

• The Sun radiates heat energy which travels to Earth through the vacuum of space.

• Objects which take in heat radiation are called absorbers of heat.

• Objects which give off heat by radiation are called emitters of heat.

• Dull black surfaces are the best absorbers and emitters of heat radiation.

• Shiny surfaces are the poor absorbers and emitters of heat radiation – they are good reflectors.

• Carbon dioxide absorbs infrared radiation.

• Radiators should be painted dull black.

A vacuum flask has the following features:

• A double walled glass bottle.
• A vacuum between the glass walls.
• Both glass walls facing the vacuum are silvered.
• A plastic stopper filled with cork or foam.
• Cork or sponge packaging and a plastic case to protect the glass bottle.

Your home loses heat through the walls, windows, doors and roof. The colder the outside temperature or the stronger the wind, the greater the amount of heat lost.

There are several ways to help reduce heat loss from your home, making it more comfortable and reducing energy bills.

Approximate cost and savings based on an average semi-detached house in Northern Ireland.