Science

Heat transfer and efficiency

Appliances transfer energy but not all the energy is transferred in useful ways. The choice of appliance is influenced by their efficiency and cost effectiveness.

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:

  • magnetic
  • kinetic (movement energy)
  • heat (thermal energy)
  • light
  • gravitational potential
  • chemical
  • sound
  • electrical
  • elastic potential
  • nuclear.

Types of energy explained

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 charges 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 diagrams

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 energy is transferred.

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.

This Sankey diagram for an electric lamp 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

Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. Notice that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as heat energy.

The energy transfer to light energy is the useful transfer. The rest is ‘wasted’: it is eventually transferred to the surroundings, making them warmer. This ‘wasted’ energy eventually becomes so spread out that it becomes less useful.

Efficiency

The efficiency of a device is the proportion of the energy supplied that is transferred in useful ways. You should be able to calculate the efficiency of a device as a decimal or as a percentage.

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 and LEDs (light-emitting diodes) 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 from the energy-saving lamp. It's more efficient than the filament lamp.

Calculating efficiency

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

efficiency = useful energy out ÷ total energy in (for a decimal efficiency)

efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)

The efficiency of the filament lamp is 10 ÷ 100 = 0.10 (or 10 percent).

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

The efficiency of the energy-saving lamp is 75 ÷ 100 = 0.75 (or 75 percent). This means that 75 percent of the electrical energy supplied is transferred as light energy (25 percent is transferred as heat energy).

Note that the efficiency of a device will always be less than 100 percent. You might be given the power in W instead of the energy in J. The equations are the same – just substitute power for energy.

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