What is a useful energy transfer?

2. What is a useful energy transfer? electrical energy kinetic energy 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?

3. What is a useful energy transfer? electrical energy sound energy What energy transfer are these speakers designed to carry out?

4. What is a useful energy transfer? electrical energy kinetic energy What energy transfer are wind turbines in a wind farm designed to carry out?

5. What is a useful energy transfer? gravitational potential energy electrical energy What energy transfer is a hydroelectric power station designed to carry out?

6. Energy transfer in a radio What are the main energy transfers in a radio? (Don’t forget the wasted energy.) sound electrical heat

7. 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. 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

9. Useful energy transfers

10. 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. Filament light bulb 100J electrical energy (input) 10J light energy (output) A Sankey diagram uses arrows to represent all the output energies. 90J heat energy (wasted) The thickness of each arrow is proportional to the amount of energy involved at that stage. Energy efficient light bulb 20J electrical energy (input) 10J light energy (output) 10J heat energy (wasted) How does the energy use in these light bulbs compare?

12. Energy cannot be created or destroyed, just changed in form. Law of conservation of energy Allenergytransfersfollowthelawofconservationofenergy: • This means that energy never just ‘disappears’. • The total amount of energy always stays the same, i.e. total input energy = total output energy. • In most energy transfers, the energy is transferred to several different forms, which may or may not be useful. • Energy that is transferred to unwanted forms of energy is wasted.

13. 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 15000J of GPE at the top of the slope, how much KE will it have gained when it reaches the bottom? 15000J, assuming that no energy is lost by air resistance and friction.

15. 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 wastedenergy. 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.

16. 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:

17. Which type of energy is wasted?

18. For example, the heat and light energy produced by this light bulb are too spread out to be reused. 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.

20. 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.

21. 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.

22. useful output energy total input energy energy efficiency = How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: • Useful energy is measured in joules (J). • Total energy is measured in joules (J). • 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.

23. Calculate the efficiency of a bulb This filament bulb is supplied with 100J of electrical energy, which it converts to 45J of light energy. • How much energy • is wasted? • In what form is the energy wasted? • What is the efficiency • of the bulb? Wasted energy = Total – Useful = 100J – 45J = 55J 55J Efficiency= Useful Total = 45 J 100 J = 0.45 or 45% heat 0.45 or 45%

24. Calculate the efficiency of a television This television converts 2000J of electrical energy into useful energy at an efficiency of 65%. • What useful energy does a television produce? • How much useful • energy is produced? light and sound Efficiency = Useful Total Useful = Efficiency x Total = 0.65 x 2000J = 1300J 1300J

25. Energy efficiency calculations – activity

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

28. Anagrams

29. Energy transfers and efficiency – summary

30. Multiple-choice quiz