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.
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:
|Type of energy||Description|
|energy in magnets and electromagnets|
|the energy in moving objects – also called movement energy|
|also called thermal energy|
|also called radiant energy|
|stored energy in raised objects|
|stored energy in fuel, foods and batteries|
|energy released by vibrating objects|
|energy in moving charges or static electric charges|
|stored energy in stretched or squashed objects|
|stored in the nuclei of atoms|
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.
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.
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 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.
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.
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.
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.
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.
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.
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.