WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.254 Unit S P 3: Conservation of energy. 00:00:09.933 --> 00:00:11.981 Energy is measured in joules. 00:00:12.131 --> 00:00:17.497 It cannot be created or destroyed: total energy in a closed system remains constant. 00:00:20.167 --> 00:00:29.193 The eight stores are kinetic, thermal or internal, chemical, gravitational potential, elastic potential, nuclear, magnetic and electrostatic. 00:00:31.867 --> 00:00:41.092 Energy transfers occur mechanically (forces doing work), electrically (moving charges), by heating and by radiation such as light or infrared. 00:00:41.242 --> 00:00:43.589 Sound can also carry energy away. 00:00:46.267 --> 00:00:54.573 A battery-powered motor transfers energy electrically from the battery’s chemical store to a moving object's kinetic store and thermal stores. 00:00:57.233 --> 00:01:03.077 A falling object transfers energy from its gravitational potential store to its kinetic store. 00:01:03.227 --> 00:01:08.397 Air resistance also transfers energy to thermal stores in the object and surroundings. 00:01:11.067 --> 00:01:14.019 Useful transfers achieve the intended purpose. 00:01:14.169 --> 00:01:18.132 Unwanted transfers often heat the surroundings or produce sound. 00:01:18.282 --> 00:01:23.114 Dissipated energy is spread out and becomes less useful, but has not been destroyed. 00:01:25.767 --> 00:01:30.589 A Sankey diagram represents transfers with arrow widths proportional to energy. 00:01:30.739 --> 00:01:36.029 Input equals total useful and unwanted output; read values and units carefully. 00:01:36.179 --> 00:01:41.969 Energy conservation: 100 joules equals 25 joules plus 75 joules. 00:01:42.119 --> 00:01:44.402 Arrow shafts show the proportions. 00:01:49.067 --> 00:01:56.723 Change in gravitational potential energy equals mass times gravitational field strength times change in vertical height, 00:01:56.873 --> 00:02:02.322 change in gravitational potential energy equals M times G times delta H. 00:02:02.472 --> 00:02:06.553 Use kilograms, newtons per kilogram and metres to obtain joules. 00:02:06.703 --> 00:02:10.152 Use the vertical height change, not the length of a slope. 00:02:14.833 --> 00:02:21.189 A 2 kilograms object raised 3 metres in G equals 10 newtons per kilogram gains 60 joules. 00:02:21.339 --> 00:02:26.501 The reference level is chosen for the calculation; energy depends on a change in height. 00:02:29.167 --> 00:02:36.098 Kinetic energy equals one half times M times V squared, with mass in kilograms and speed in metres per second. 00:02:36.248 --> 00:02:41.676 Squaring speed means doubling speed quadruples kinetic energy at unchanged mass. 00:02:44.333 --> 00:02:53.227 A 4 kilograms object moving at 5 metres per second has kinetic energy one half times 4 times 25 equals 50 joules. 00:02:55.900 --> 00:03:00.991 Efficiency equals useful energy transferred divided by total energy supplied. 00:03:01.141 --> 00:03:04.496 It has no unit and is a fraction between 0 and 1. 00:03:04.646 --> 00:03:08.405 Multiply by 100 to express it as a percentage. 00:03:11.067 --> 00:03:20.271 A lamp supplied 100 joules and transferring 25 joules by useful light has efficiency 0.25, or 25 percent. 00:03:20.421 --> 00:03:24.405 The remaining 75 joules is transferred in unwanted ways. 00:03:27.067 --> 00:03:31.633 Lubrication reduces friction and unwanted heating in moving machinery. 00:03:31.783 --> 00:03:35.289 Insulation reduces unwanted thermal transfers. 00:03:35.439 --> 00:03:38.455 Both can improve efficiency for the intended task. 00:03:41.133 --> 00:03:45.540 Heating normally transfers energy from a hotter region to a cooler region. 00:03:45.690 --> 00:03:51.801 Temperature is not the same quantity as energy; heating can raise temperature or cause a change of state. 00:03:54.467 --> 00:04:01.772 In conduction, energy passes between neighbouring particles; free electrons also transfer energy efficiently in metals. 00:04:01.922 --> 00:04:05.057 Material does not flow along the conductor as a whole. 00:04:07.733 --> 00:04:10.888 Convection happens in fluids (liquids and gases). 00:04:11.038 --> 00:04:14.469 A warmer region usually becomes less dense and rises. 00:04:14.619 --> 00:04:19.279 Cooler, denser fluid sinks to take its place, setting up a convection current. 00:04:19.429 --> 00:04:22.871 Convection transfers energy by bulk movement of fluid. 00:04:27.533 --> 00:04:33.431 Infrared radiation transfers energy without needing a material to travel through (a medium). 00:04:33.581 --> 00:04:37.562 It can travel through a vacuum, unlike conduction and convection. 00:04:40.233 --> 00:04:44.865 A low thermal conductivity makes a material a good thermal insulator. 00:04:45.015 --> 00:04:49.793 For otherwise identical walls, greater thickness reduces the rate of conduction. 00:04:52.467 --> 00:05:00.998 A larger temperature difference across a wall increases the rate of energy transfer; a higher thermal conductivity also increases that rate. 00:05:03.667 --> 00:05:10.311 Trapped air in cavity-wall insulation and double glazing reduces conduction; small pockets limit convection. 00:05:10.461 --> 00:05:13.839 Unfilled large air spaces can still allow convection. 00:05:13.989 --> 00:05:17.196 Small air pockets inhibit large convection currents. 00:05:21.867 --> 00:05:26.933 Loft insulation, thick low-conductivity walls and double glazing reduce cooling. 00:05:27.083 --> 00:05:30.143 Draught proofing reduces transfers by moving air. 00:05:30.293 --> 00:05:33.979 Shiny surfaces can reduce infrared absorption and emission. 00:05:36.633 --> 00:05:39.733 Fossil fuels are coal, oil and natural gas. 00:05:39.883 --> 00:05:45.406 They form over very long times and are used faster than they are replaced, making them non-renewable. 00:05:48.067 --> 00:05:57.046 Combustion transfers energy from chemical stores; electricity generation often uses heating to make steam that drives turbines and generators. 00:05:59.700 --> 00:06:04.915 Burning fossil fuels releases carbon dioxide, contributing to climate change. 00:06:05.065 --> 00:06:09.656 Some fuels also cause air pollution and habitat damage during extraction. 00:06:12.333 --> 00:06:15.573 Nuclear fuels such as uranium are non-renewable. 00:06:15.723 --> 00:06:23.252 Nuclear reactions release energy for heating and electricity generation with low operational carbon dioxide emissions. 00:06:25.933 --> 00:06:35.446 Nuclear generation produces radioactive waste requiring secure management; accidents, decommissioning costs and lifecycle emissions must be considered. 00:06:35.596 --> 00:06:39.831 Low operational emissions do not mean no environmental impacts. 00:06:42.500 --> 00:06:46.676 Renewable resources are replaced naturally quickly enough for continued use. 00:06:46.826 --> 00:06:55.015 Examples include wind, solar, tides, hydroelectricity, geothermal energy and biofuels grown and replaced sustainably. 00:06:57.667 --> 00:07:06.999 Wind turbines transfer kinetic energy of air; hydroelectricity uses water dropping from a higher level; tidal systems use tidal water movements. 00:07:07.149 --> 00:07:09.282 These can drive generators. 00:07:11.933 --> 00:07:17.117 Solar cells transfer energy electrically from sunlight; solar heating warms water. 00:07:17.267 --> 00:07:19.891 Their output varies with sunlight and weather. 00:07:22.567 --> 00:07:25.808 Geothermal heating uses energy from hot rocks. 00:07:25.958 --> 00:07:36.226 Biofuels release energy by combustion; plant growth can take up carbon dioxide, but farming, processing, transport and land-use changes affect net emissions. 00:07:38.900 --> 00:07:42.697 Resource reliability differs: wind and solar are variable, 00:07:42.847 --> 00:07:44.533 tides are predictable, 00:07:44.683 --> 00:07:47.968 stored-water hydroelectricity can respond quickly, 00:07:48.118 --> 00:07:52.145 and geothermal can provide steadier output in suitable locations. 00:07:54.800 --> 00:08:01.475 Compare cost, start-up time, availability, land use, habitat impacts, pollution, waste and reliability. 00:08:01.625 --> 00:08:04.791 No single resource is best in every location. 00:08:04.941 --> 00:08:08.265 Use several criteria rather than a single advantage. 00:08:12.933 --> 00:08:18.266 Energy use changes with demand, price, technology and environmental priorities. 00:08:18.416 --> 00:08:24.783 Use the data in a question to describe trends without inventing causes or treating a correlation as proof. 00:08:27.433 --> 00:08:29.908 That completes Conservation of energy. 00:08:30.058 --> 00:08:33.403 Revisit the notes and test yourself on the revision website.