1. Conservation of Energy & Efficiency

2. Principle of Conservation of Energy
Energy cannot be created or destroyed, only transferred from one form into another.

3. Efficiency The amount of useful energy output from a device.
The more the useful energy output the more efficient the device is!
E.g. Filament light bulb
100 J of electrical  9 J Light + 91 J Heat
Therefore a filament light bulb is 9% efficient!

4. Calculating efficiency
We know that efficiency is the amount of useful energy output from a device, what ‘values’ do you think we will need to know to calculate the efficiency of a device?
Amount of useful energy output
Total amount of energy input

5. Calculating efficiency
Efficiency = useful energy output
total energy input
Always a number between 0 and 1.
% Efficiency = useful energy output x 100
total energy input
Always a number between 0 and 100.

6. Quick Practice Efficiency = 400 kJ = 0.8 %Efficiency = 80%
An electric kettle uses 500 kJ of electrical energy and transfers kJ of energy to the water in the kettle. What is the efficiency?
Efficiency = 400 kJ
500 kJ
= 0.8
%Efficiency = 80%
It doesn‘t matter what units are used once they are the same as each other!!

7. 80 J/s heat energy (given to the surroundings)
Sankey diagrams
Consider a light bulb. Let’s say that the bulb runs on 100 watts (100 joules per second) and transfers 20 joules per second into light and the rest into heat. Draw this as a diagram:
“Input” energy
“Output” energy
20 J/s light energy
100 J/s electrical energy
80 J/s heat energy (given to the surroundings)

8. Which is the energy-saving lightbulb and which is the filament bulb?

9. Coal Power Station
What is the approximate efficiency of a coal-fired power station?

10. Example questions Consider a computer: Consider a kettle:
150 J/s electrical energy
10 J/s wasted sound
20 J/s wasted heat
Useful light and sound
How much energy is converted into useful energy?
What is the computer’s efficiency?
Consider a kettle:
Work out each energy value.
What is the kettle’s efficiency?
Sound energy
Wasted heat
Heat to water
1000 J/s electrical energy

12. A scan of one of the first – if not the first ever – published Sankey diagrams, published by Captain Henry R. Sankey in 1898 in the Minutes of Proceedings of The Institution of Civil Engineers. Vol. CXXXIV, Session Part IV.