# Efficiency To be able to calculate the efficiency of energy transfers Sunday, May 10, 2015.

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Efficiency To be able to calculate the efficiency of energy transfers Sunday, May 10, 2015

Efficiency Rating All modern appliances (and even homes) now come with efficiency ratings – this is important because not only does wasted energy cost money, but generally it leads to global warming as the vast majority of this energy comes from the burning of fossil fuels…

Efficiency Rating Modern combustion engines are extremely well developed in comparison to 100 years ago when they were first developed – however they are still very in-efficient (at most 40%) 1. Intake 2. Compression 3. Power 4. Exhaust

Energy Transfer Many everyday objects are designed to transfer energy from one form into another useful form What energy transfer is an electric fan designed to carry out? Electrical  Kinetic

Energy Transfer Many everyday objects are designed to transfer energy from one form into another useful form What energy transfer is a speaker designed to carry out? Electrical  Sound

Energy Transfer Many everyday objects are designed to transfer energy from one form into another useful form What energy transfer is a wind farm designed to carry out? Kinetic  Electrical

Energy Transfer Many everyday objects are designed to transfer energy from one form into another useful form What energy transfer is a hydro-electric power station designed to carry out? Gravitational  Electrical

Sankey Remember, Sankey diagrams show the relative amounts of each type of energy They summarise all the energy transfers taking place in a process – the thicker the line or arrow, the greater the amount of energy involved Which is the energy efficient bulb?! Normal bulb – lots of wasted energyEnergy efficient bulb – little wasted energy

Wasted Remember, wasted energy spreads out into the surroundings – this is called dissipated energy This dissipated energy is too spread out to do useful work and so cannot be re-used which is why it is important to reduce the amount of wasted energy Energy is usually wasted as heat and sound

Efficiency Efficiency (J) is a measure of how good a device is at changing energy from one form to another – no device is 100% efficient: the wasted energy is dissipated (usually as heat and sound) Efficiency = useful energy output total energy input EfficiencyEnergy input Energy output

Efficiency To work out the efficiency as a percentage divide the useful energy output by the total energy input and multiply by 100 Efficiency = useful energy output x 100 total energy input

Efficiency What is the efficiency of the normal light bulb and the energy efficient light bulb? Normal bulb – lots of wasted energy Efficiency = (10 ÷ 100) x 100 Efficiency = 10% Energy efficient bulb – little wasted energy Efficiency = (75 ÷ 100) x 100 Efficiency = 75%

Efficiency Questions A filament bulb is supplied with 100 J of electrical energy which it converts to 45 J of light energy How much energy is wasted? In what form is this energy wasted? What is the efficiency of the bulb?

Efficiency Questions A radio is supplied with 300J of electrical energy which it converts to 96J of sound energy How much energy is wasted? In what form is this energy wasted? What is the efficiency of the speaker?

Efficiency Questions A television converts 2000J of electrical energy into useful energy at an efficiency of 65% What useful energy does a television produce? In much useful energy is produced?

Efficiency Questions A filament bulb is supplied with 100 J of electrical energy which it converts to 45 J of light energy How much energy is wasted? Wasted energy = total energy – useful energy 100J – 45J = 55J In what form is this energy wasted? Heat What is the efficiency of the bulb? Efficiency = (useful energy ÷ total energy) x 100 (45 ÷ 100) x 100 = 45%

Efficiency Questions A radio is supplied with 300J of electrical energy which it converts to 96J of sound energy How much energy is wasted? Wasted energy = total energy – useful energy 300J – 96J = 204J In what form is this energy wasted? Heat What is the efficiency of the speaker? Efficiency = (useful energy ÷ total energy) x 100 (96 ÷ 300) x 100 = 32%

Efficiency Questions A television converts 2000J of electrical energy into useful energy at an efficiency of 65% What useful energy does a television produce? Light and sound In much useful energy is produced? Useful energy = efficiency x total energy 0.65 x 2000J = 1300J

Efficiency Experiment How could you test the efficiency of a light bulb? An experiment can be set up to test the efficiency of a light bulb by placing it next to a known quantity of water – the vast majority of wasted energy comes in the form of heat which will then heat the water…

Efficiency Experiment Light bulbs are measured in watts (1 watt is the 1 joule per second) 418.6 joules of energy is needed to raise the temperature of 100ml of water by 1 o C Watt of bulb (W) Energy transferred per minute (J) Temp. rise in water ( o C) Energy transferred to water as heat (J) Energy transferred as light (J) Efficiency (%) Multiply watt by 60 Multiply rise by 418.6 (Useful energy out ÷ total energy in) x 100

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