1. Energy efficiency & Sankey diagrams
AS Physics

2. Efficiency
Nothing is 100% efficient in converting energy from one form to another.
A 40W light bulb is only around 2% efficient at converting electrical energy into light energy – what happens to the other 98%?
To calculate efficiency you must know the input energy and the useful output energy.
Efficiency = useful energy out
total energy in Or
Efficiency = work done
energy input

3. Try these quick questions
1) Work out the efficiency of a machine if it does 1050J of work with an energy input of 1234J. Write your answer as a decimal to 2d.p.
2) A machine gets 3500J of electrical energy. Work out its % efficiency to the nearest % if it did 759J of work.
3) A machine must do 5990J of work. Calculate to the nearest joule the energy that must be supplied if the machine’s efficiency is 35%.
4) A pulling machine is 6% efficient. How much work can be done for an energy input of 6800J?
5) A milk float uses 20000J to perform the task of driving over speed bumps for 90m, with a force of 80N. Calculate how much work is being done then calculate the milk floats efficiency to the nearest %.

4. How about these ones…
1) Work out the efficiency as a percentage to the nearest % of a speaker that is supplied with 2530J and gives out 1810J of sound energy
2) A guitar amp is 35% efficient. Work out the energy input, if the sound energy given out is 4050J.
3) Work out the efficiency of a motor that gives out 6050J of sound and 3950J of kinetic energy when it has been given 10000J of electrical energy.

5. 3 scientists are studying Kath’s decks
A) The scientists say that the turntables are defiantly either 105% or 78% efficient. How efficient must they be, and how can you tell?
B) Her headphones are 50% efficient. Listening to ABBA, they waste 1000J as heat. How much total energy does she need to supply them?
C) A speaker gives out 50150J as sound, and 46000J as heat. A garden lamp gives out 48484J as heat, and 60589J as light. Which is more efficient?

6. Sankey Diagrams
Sankey diagrams are a type of flow diagram. They can be used to represent energy transfer. The area of the arrows show the amount of energy being transferred.