Science

Heat transfer and efficiency

Heat can be transferred from place to place by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ], convection [convection: The transfer of heat energy through a moving liquid or gas. ] and radiation [infrared radiation: Electromagnetic radiation emitted from a hot object. ]. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.

There are several different types of energy, and these can be transferred from one type to another. Energy transfer diagrams show the energy transfers in a process. More efficient devices transfer the energy supplied to them into a greater proportion of useful energy than less efficient devices do.

Heat transfer by conduction and convection

Heat is thermal energy. It can be transferred from one place to another by conduction, convection and radiation. Conduction and convection involve particles, but radiation involves electromagnetic waves.

Conduction

Thermogram of a pan being heated on a stove

Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.

The electrons in 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 charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.

Heat transfer by conductionStep 1 - metal is heated



Heat transfer by conductionStep 2 - atoms in the metal are energised and vibration increases



Heat transfer by conductionStep 3 - vibration is spread through the metal



Convection

Liquids and gases are fluids. 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. Heat energy is transferred from hot places to cooler places by convection.

Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.

Heat transfer by radiation

Radiation

All objects give out and take in thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.

Light from the sun reaching earth

Infrared radiation is a type of electromagnetic radiation that involves waves. No particles are involved, unlike in the processes of conduction and convection, so radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.

Some surfaces are better than others at reflecting and absorbing infrared radiation.

Comparison of surfaces abilities to reflect and absorb radiation

colourfinishability to emit thermal radiationability to absorb thermal radiation
darkdull or mattgoodgood
lightshinypoorpoor

If two objects made from the same material have identical volumes, a thin, flat object will radiate heat energy faster than a fat object. This is one reason why domestic radiators are thin and flat. Radiators are often painted with white gloss paint. They would be better at radiating heat if they were painted with black matt paint, but in fact, despite their name, radiators transfer most of their heat to a room by convection.

Reducing heat loss

You should be able to describe how heat energy is lost from buildings and to explain how these losses can be reduced.

Heat escape routes

Take a look at this image showing heat loss from a house.

red shows where most heat is lost - through the windows and roof

Most household heat is lost through the windows and roof

Heat is lost:

  • through the roof
  • through windows
  • through gaps around the door
  • through the walls
  • through the floor

Heat energy is transferred from homes by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ] through the walls, floor, roof and windows. It is also transferred from homes by convection [convection: The transfer of heat energy through a moving liquid or gas. ]. For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles. Heat energy also leaves the house by radiation through the walls, roof and windows.

Ways to reduce heat loss

There are some simple ways to reduce heat loss, including fitting carpets, curtains and draught excluders.

Heat loss through windows can be reduced using double glazing. The gap between the two panes of glass is filled with air. Heat loss through conduction is reduced, as air is a poor conductor of heat. Heat transfer by convection currents is also reduced by making the gap is very narrow.

Watch

You may wish to view this BBC News item from 2006 about energy efficiency in homes.

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, which 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. This works in a similar way to cavity wall insulation.

Forms of energy

You should be able to recognise the main types of energy. One way to remember the different types of energy is to learn this sentence:

Most Kids Hate Learning GCSE Energy Names

Each capital letter is the first letter in the name of a type of energy.

Types of energy

Type of energyDescription
Magnetic

Magnet

Energy in magnets and electromagnets
Kinetic

A bullet cutting a playing card

The energy in moving objects. Also called movement energy.
Heat

Burning match

Also called thermal energy
Light

Sunlight

Also called radiant energy
Gravitational potential
sky divers in freefall under gravity

Sky divers

Stored energy in raised objects
Chemical

Organic food

Stored energy in fuel, foods and batteries
Sound

Guitar

Energy released by vibrating objects
Electrical

Lightning

Energy in moving or static electric charges
Elastic potential
catapult stores elastic energy

Catapult

Stored energy in stretched or squashed objects
Nuclear

Nuclear fuel assembly

Stored in the nuclei of atoms

Energy transfer

You should recall from your Key Stage 3 studies how to draw and interpret an energy transfer diagram.

Different types of energy can be transferred from one type to another. Energy transfer diagrams show each type of energy, whether it is stored or not, and the processes taking place as it is transferred. Sankey diagrams also show the relative amounts of each type of energy.

Energy transfer diagrams

This energy transfer diagram shows the useful energy transfer in a car engine. You can see that a car engine transfers chemical energy, which is stored in the fuel, into kinetic energy in the engine and wheels.

chemical energy in the fuel is the energy input, the car engine is the process, and energy output is in the form of kinetic energy in the engine and wheels

Process of using chemical energy

This diagram shows the energy transfer diagram for the useful energy transfer in an electric lamp. You can see that the electric lamp transfers or converts electrical energy into light energy.

electrical energy is the input, the electric lamp is the process, and the light energy the energy output

Process of using electrical energy

Notice that these energy transfer diagrams only show the useful energy transfers. However, car engines are also noisy and hot, and electric lamps also give out heat energy.

Sankey diagrams

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.

total electrical energy is 100 j, 90 j is transferred as heat energy and 10 j transferred as light energy

Sankey diagram for a filament lamp

Efficiency

You should know that energy can be 'wasted' during energy transfers, and you should be able to calculate the efficiencyefficiency: The fraction of the energy supplied to a device which is transferred in a useful form. of a device.

'Wasted' energy

Energy cannot be created or destroyed. It can only be transferred from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.

Electric lamps

Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy. This is the Sankey diagram for a typical filament lamp.

total electrical energy is 100 j, 90 j is transferred as heat energy and 10 j transferred as light energy

Sankey diagram for a filament lamp

Modern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

total electrical energy is 100 j. 25 j is transferred as heat energy and 75 j transferred as light energy

Sankey diagram for a typical energy-saving lamp

From the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy.

Calculating efficiency

The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = ( useful energy transferred ÷ energy supplied ) × 100

The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.

This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).

The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. This means that 75% of the electrical energy supplied is transferred as light energy (25% is transferred as heat energy).

Note that the efficiency of a device will always be less than 100%.

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