Heat transfer - CCEA

• Heat can be transferred from place to place by conduction, convection and radiation.

• The transfer of energy by conduction and convection involves particles.

• Metals are the best conductors of heat because they contain free electrons.

• Convection occurs in liquids and gases.

• The transfer of energy by radiation does not require particles.

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

• Heat energy can be lost from homes mainly through conduction and convection, but these losses can be reduced by methods of insulation.

Temperature and heat are not the same thing because:

• Temperature is a measure of how hot something is.

• Heat is a measure of the thermal energy contained in an object.

• Temperature is measured in ^oC.

• Heat energy is measured in J.

Key fact

• 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.

Metals are good conductors of heat energy.

Non-metals and gases are usually poor conductors.

Poor conductors are called insulators.

Heat energy is conducted from the hot end of an object to the cold end.

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 so.

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 move much faster.

As they move through the metal, free electrons crash into metal ions.

Some of the kinetic energy of the free electron is absorbed by the ions and it vibrates faster and with greater amplitude.

This process is very much faster than conduction caused by just passing vibrations from atom to atom.

Hence, conduction in metals is faster than in non-metals.

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 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 very much faster than conduction by passing vibrations from atom to atom.

• Metals are good conductors because they contain free electrons.

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.

Method

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 Vaseline to melt and the drawing pin to drop off.

The fastest time shows the best conductor of heat.

In this experiment:

Independent variable: the material of the metal rod.

Dependent variable: the time taken for the Vaseline to melt and the drawing pin to drop off

Control variables: length of metal rod, diameter of rods, position in the Bunsen flame, mass of drawing pin and quantity of Vaseline.

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

From this we can conclude that copper conducts better than aluminium, while aluminium conducts better than iron and brass.

Heat can be transferred in liquids or gases by convection.

Fluids

Liquids and gases are fluids because they can be made to flow.

The particles in these fluids can move from place to place.

Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy.

Convection in a liquid

Convection in a liquid can be seen by putting a crystal of potassium permanganate in a beaker of water and gently heating it with a Bunsen flame.

Results

• Heat is initially transferred 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.

Convection currents can be seen in lava lamps.

The wax inside the lamp warms up, becomes less dense than the liquid and so rises.

Convection in a gas

How do radiators generate convection currents?

• 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 hence through the room.

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

Most of our winds are caused by convection currents occurring on a big scale in air.

Many ocean currents are also due to convection currents.

Heat can be transferred by infrared radiation.

Unlike conduction and convection - which need the vibration or movement of particles - infrared radiation is a type of electromagnetic radiation.

When infrared radiation is absorbed (taken in) by an object it is heated and its temperature rises.

When infrared radiation is emitted by an object it loses heat energy and its temperature decreases.

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.

The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called 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

1. Measure the temperature a fixed distance from each side of a Leslie's cube using four identical thermometers.

2. Heat the Leslie’s cube by filling it with boiling water.

3. 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.

4. Compare the four graphs obtained.

In this experiment:

Independent variable: the type of surface of the side

Dependent variable: the temperature rise of each surface

Control variables

Distance of each thermometer from the sides of the cube, the type of thermometer used and the time taken for each reading.

Results

The temperature of the thermometer opposite the dull, black side reaches the highest temperature in the same time interval.

From this we can say that dark matt surfaces are better at radiating heat energy than light shiny surfaces.

Key point

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

Radiators are often painted with gloss paint, but they would be better at radiating heat if they were painted with matt black paint instead.

They are painted white to make them look nicer.

However, despite their name, radiators actually transfer most of their heat to a room by convection, not radiation.

Normally radiant heaters must be at least red hot before they are effective.

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.

Heat energy is lost from buildings through their roofs, windows, walls, floors and through gaps around windows and doors.

However, there are ways that these losses can be reduced.

What are the main heat escape routes in a house?

Take a look at this thermogram of a house.

The roof and windows are the hottest, showing that most heat is lost from the house through those parts.

Heat energy is transferred from homes by conduction through the walls, floor, roof and windows.

It is also transferred from homes by convection.

For example, as hot air in rooms rise, cold air can enter the house through gaps in doors and windows to replace it.

These convection currents can transfer heat energy into the loft.

Heat energy also leaves the house by radiation - through the walls, roof and windows.

Where does heat loss occur in a house?

Key fact

• Trapped air is a natural insulator and because it is trapped, convection currents cannot be set up easily. So, trapped air reduces heat loss by conduction and convection. Many insulating materials incorporate trapped air.

There are several different ways to reduce heat loss:

• Simple ways to reduce heat loss include fitting carpets, curtains and draught excluders. It is even possible to fit reflective foil behind radiators.

• Heat loss through windows can be reduced by using double glazing. These windows have dry, trapped air between two panes of glass. This reduces heat loss by conduction and convection. Air is a poor conductor and by trapping it between the panes, convection currents cannot be set up.

• Heat loss through walls can be reduced using cavity wall insulation. This involves blowing insulating material into the gap between the brick and the inside wall. Insulating materials are bad conductors and so this reduces the heat loss by conduction. The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.

• Heat loss through the roof can be reduced by laying loft insulation. Air is trapped between the fibres of the insulation, reducing heat loss by conduction.

Heat energy is lost from the human body because body temperature is higher than room temperature.

However, there are ways that these losses can be reduced.

This is a thermogram of a man, woman and child.

The white and red parts are the hottest and so are losing most heat.

Orange and green represent medium temperatures, and blue and purple are the coldest parts.

It is easy to see that the heads of all three people are the hottest parts, followed by their torsos.

Their arms and legs show medium temperatures, while their hands and feet are among the coldest parts.

It is in these places that we sometimes get frostbite in very cold conditions.

Hair on our head reduces heat loss because air is trapped between individual hairs.

To reduce heat transfers from our bodies, we can wear insulating clothing - such as gloves, scarves, hats and thermal socks - in cold weather.

The more layers we wear, the more air can be trapped and the warmer we will remain.

Some clothes, such as wool, are excellent insulators, because they trap lots of air between individual fibres.

Vacuum flask

Refrigerator

A refrigerator works by transferring heat energy from inside the fridge to the room outside and therefore lowering the temperature on the inside of the fridge.

Heat energy inside the cabinet is absorbed by a coolant which circulates through pipes from inside to outside.

When the coolant passes through the pipes on the outside of the fridge it transfers this heat energy to the room.

The pipes on the back of the fridge containing the coolant are normally black, thin and have metal fins attached.