Conservation of Energy & Efficiency

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

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!

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

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.

Quick Practice Efficiency = 400 kJ = 0.8 %Efficiency = 80% An electric kettle uses 500 kJ of electrical energy and transfers 400 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!!

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)

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

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

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

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 1897-98. Part IV.