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1 of 35 © Boardworks Ltd 2006

2 of 35 © Boardworks Ltd 2006

What is a useful energy transfer? Many household 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 energy 3 of 35 kinetic energy © Boardworks Ltd 2006

What is a useful energy transfer? What energy transfer are these speakers designed to carry out? electrical energy 4 of 35 sound energy © Boardworks Ltd 2006

What is a useful energy transfer? What energy transfer are wind turbines in a wind farm designed to carry out? kinetic energy 5 of 35 electrical energy © Boardworks Ltd 2006

What is a useful energy transfer? What energy transfer is a hydroelectric power station designed to carry out? gravitational potential energy 6 of 35 electrical energy © Boardworks Ltd 2006

Useful energy transfers 7 of 35 © Boardworks Ltd 2006

Energy transfer in a television An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy. For example, in a television: light electrical sound heat 8 of 35 © Boardworks Ltd 2006

Energy transfer in a radio What are the main energy transfers in a radio? (Don’t forget the wasted energy. ) electrical 9 of 35 sound heat © Boardworks Ltd 2006

Energy transfer in a car engine What are the main energy transfers in a car engine? (Don’t forget the wasted energy. ) kinetic chemical sound heat 10 of 35 © Boardworks Ltd 2006

Identifying energy transfers 11 of 35 © Boardworks Ltd 2006

How can we represent energy transfers? All the energy transfers (useful and wasted) that are associated with a device can be represented by a Sankey diagram. A Sankey diagram uses arrows to represent all the output energies. Filament light bulb 100 J 10 J electrical light energy (input) (output) 90 J heat energy (wasted) The thickness of each arrow is proportional to the amount of energy involved at that stage. Energy efficient light bulb 20 J 10 J electrical light energy (input) 10 J (output) heat energy How does the energy use in (wasted) these light bulbs compare? 12 of 35 © Boardworks Ltd 2006

13 of 35 © Boardworks Ltd 2006

Law of conservation of energy All energy transfers follow the law of conservation of energy: Energy cannot be created or destroyed, just changed in form. l This means that energy never just ‘disappears’. l The total amount of energy always stays the same, i. e. total input energy = total output energy. l In most energy transfers, the energy is transferred to several different forms, which may or may not be useful. l Energy that is transferred to unwanted forms of energy is wasted. 14 of 35 © Boardworks Ltd 2006

What happens to the energy of falling objects? A falling object loses gravitational potential energy and gains kinetic energy. This is an example of the law of conservation of energy. gravitational potential energy (GPE) lost equals kinetic energy (KE) gained If the rollercoaster has 15 000 J of GPE at the top of the slope, how much KE will it have gained when it reaches the bottom? 15 000 J, assuming that no energy is lost by air resistance and friction. 15 of 35 © Boardworks Ltd 2006

Gerald the Human Cannonball 16 of 35 © Boardworks Ltd 2006

17 of 35 © Boardworks Ltd 2006

How is energy wasted? This cordless drill transfers chemical energy in the battery into movement, sound and heat energy. The movement energy can be used to do useful work, but the sound and heat energy cannot. Energy that does not do useful work is called wasted energy. Appliances usually waste energy as sound and heat. In fact, all electrical devices produce a small amount of heat energy when they are used, which is normally wasted. 18 of 35 © Boardworks Ltd 2006

Which devices waste energy as sound? If a device makes noise but is not designed to make noise, then energy is being wasted as sound. Examples of devices that waste energy as sound include: 19 of 35 © Boardworks Ltd 2006

Which devices waste energy as heat? If a device gets warm but it is not designed to get warm, then energy is wasted as heat. Examples of devices that waste energy as heat include: 20 of 35 © Boardworks Ltd 2006

Which type of energy is wasted? 21 of 35 © Boardworks Ltd 2006

What happens to wasted energy? What happens to the wasted energy produced when energy is changed from one form to another? Remember that energy cannot be created or destroyed. 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 reused. For example, the heat and light energy produced by this light bulb are too spread out to be reused. 22 of 35 © Boardworks Ltd 2006

23 of 35 © Boardworks Ltd 2006

What is energy efficiency? Efficiency is a measure of how good a device is at changing energy from one form to another. All devices waste energy, so the efficiency of a device is never 100%. The more efficient a device is the less energy is wasted. An energy-efficient light bulb is more efficient than a filament bulb because it wastes less energy. It converts a greater proportion of the energy that it is supplied with into light energy and not heat. 24 of 35 © Boardworks Ltd 2006

Why is energy efficiency important? New electrical appliances display information about how energy efficient they are. This helps consumers to identify which appliance will use the least energy and cost the least to run. Energy efficiency is not just about saving money, it’s also about saving the planet. Appliances that are more efficient need less electricity, and so less fossil fuel needs to be burnt in fossil-fuel power stations. This is important for reducing pollution and the effects of climate change. 25 of 35 © Boardworks Ltd 2006

How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: useful output energy efficiency = total input energy l Useful energy is measured in joules (J). l Total energy is measured in joules (J). l Energy efficiency does not have any units. It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100. 26 of 35 © Boardworks Ltd 2006

Calculate the efficiency of a bulb This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy. a) How much energy b) is wasted? 55 J Wasted energy = Total – Useful = 100 J – 45 J = 55 J b) In what form is the energy wasted? heat c) What is the efficiency of the bulb? 0. 45 or 45% 27 of 35 Efficiency = Useful Total = 45 J 100 J = 0. 45 or 45% © Boardworks Ltd 2006

Calculate the efficiency of a radio This radio is supplied with 300 J of electrical energy which it converts to 96 J of sound energy. a) How much energy 204 J b) is wasted? Wasted energy = Total – Useful = 300 J – 96 J = 204 J b) In what form is the energy wasted? heat c) What is the efficiency of 0. 32 the or 32% radio? 28 of 35 Efficiency = Useful Total = 96 J 300 J = 0. 32 or 32% © Boardworks Ltd 2006

Calculate the efficiency of a television This television converts 2 000 J of electrical energy into useful energy at an efficiency of 65%. a) What useful energy does a television produce? light and sound b) How much useful c) energy is produced? 1 300 J 29 of 35 Efficiency = Useful Total Useful = Efficiency x Total = 0. 65 x 2 000 J = 1 300 J © Boardworks Ltd 2006

Energy efficiency calculations – activity 30 of 35 © Boardworks Ltd 2006

Glossary l dissipated energy– Energy that spreads out into its surroundings so that it is no longer useful. l efficiency – A measure of how good a device is at changing energy from one form to another. l energy transfer – The process of changing energy from one form to another. l input energy – The energy going into a device. l output energy – The forms of energy produced by a device. l useful energy– Energy in a form that is wanted. l wasted energy – Energy in a form that is not wanted or is not used. 32 of 35 © Boardworks Ltd 2006