Efficiency

Machines are devices for converting (transforming) one form of 'useful' energy to another form of 'useful' energy. Think about a car travelling along a level road. The useful energy input is, say, petrol and the useful energy output is kinetic energy as the car moves. But not all the petrol energy 'locked away' in petrol is converted to motion. A lot of it is converted to useless energy such as heat and sound.

How effective the machine is at converting one form of useful energy into another form of useful energy is called the efficiency of the machine. In the case of the car if it could convert ALL the energy in the petrol to kinetic energy then it would be 100% efficient.

Unfortunately no machine is perfectly efficient – in other words you don't get as much energy out of the machine as you put in and certainly not more. So we define efficiency as:

An alternative definition for efficiency is:

For example if you get out half the energy that you put in, the machine is 50% efficient.

Strictly speaking we should say useful energy output in a given time divided by the energy input in that same time. Efficiency could then be expressed as useful energy output per second divided by energy input per second. This would then be useful power output divided by power input.

A petrol engine is about 30% efficient, a diesel engine 40% efficient, and our bodies are 25% efficient - only one quarter of the energy produced goes to moving the muscles.

Sankey diagrams

A very useful way of showing the efficiency of a system is to use a Sankey diagram. An example of one of these for a nuclear power station is shown below and you can find further examples at: Sankey diagrams



The efficiency of this power station is 42%.

'Wasted energy'

What happens to all this wasted energy? It is usually lost by friction and this produces heat.

Heat is often the waste product of a machine. The machine gives off this heat energy and this goes to warming up the surroundings. Three people moving about in a room would give off about as much heat energy as a 1 kW electric fire and so warm up the room! This means that the energy is spread out and it becomes more and more difficult to use this energy for other transformations.

Think about this on a very large astronomical scale. At the moment the galaxy is made of stars – very hot objects that are giving off large amounts of heat energy. The energy in the stars is concentrated in each star but as time goes by the energy is radiated into space, the star eventually cools down and we have a sort of luke warm energy soup spread out through the galaxy. This is sometimes referred to as the 'heat death' of the universe.