The UK site for KS3 and KS4 Physics

KS4 Efficiency

If you are at this section of the AQA KS4 Physics syllabus then you should already have completed Energy transfers in a system.
In that section we learnt that not all energy transfers are Useful. Some are Wasteful.
And we mentioned the law of conservation of energy and tentatively mentioned the idea of Efficiency.

In this section we are going to explore the idea of Efficiency in more detail. In particular we are going to learn how we can calculate a value for it whenever an Energy Transfer takes place.

Let's start by repeating the energy flow diagram for the "car system" that we showed on the previous page:

It shows that of the 100 J of input energy in the Chemical store (from the fuel) only 25 J are transferred Usefully to the Kinetic store to bring about movement of the car; the remaining 75 J are transferred via a Thermal store (the car engine getting unnecessarily hot) to the Internal store of the surrounding air particles, so this energy is Wasted.

So, as we stated on the previous page, this energy transfer is not 100% efficient.

But, exactly how efficient is it?

To calculate the Efficiency of any energy transfer we use the following equation:

There is another version of the efficiency equation. In place of energy input and energy output it refers to Power input and Power output but the basic form of the equation is the same:

Increasing the efficiency of energy transfers

In the real world, designers, scientists and engineers are never content to say "this machine or this appliance is X % efficient", unless the X % is 100 %, of course!

They constantly strive to improve, to raise the efficiency of their machines and appliances.

The light bulb is a great example:
Not many years ago, the common household light bulb consisted of a tungsten filament wire inside glass enclosed vacuum; it was the familiar "bulb". But these bulbs produced far more heating than lighting. They were less than 5 % efficient, which is dreadful, especially when you consider how many of these bulbs were in use in the world.

Then, in 2012 they were outlawed (literally, it was illegal to sell them) because an alternative became available. This was the "compact fluorescent lamp" or CFL, which used the same "technology" as the fluorescent strip or tube lights that you usually see on ceilings in kitchens or in school classrooms, except in a much smaller package. These were much more efficient; about 30 %. But still, 70 % of energy is wasted filling the Thermal store of its surroundings!

But the story did not stop here.
Scientists continued to push for greater efficiency and from around 2014 a new type of bulb has been available, the LED bulb. This makes use of multiple LED elements and each one produces very little heating whilst producing a lot of light. The bulb has an efficiency of about 80 %.

Ways to increase efficiency

1. The development of the "bulb" suggests one major way to increase efficiency:
To reduce unneeded heating (ie allowing energy to "leak" into Thermal or Internal stores, especially of the surrounding air).
This can be done, depending on the machine or appliance, by reducing friction (using a lubricant, smoothing surfaces), or as in the case of the LED bulb, choosing electrical components that do not produce excessive heat when a current flows through them.

2. To prevent "leaking" of energy to sound or to external Thermal/Internal stores when it should stay within the appliance, eg within a washing machine or tumble drier. This can be done with correct use of insulation (see previous page).