WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:07.163 Unit C P 3: Conservation of energy. 00:00:09.833 --> 00:00:11.881 Energy is measured in joules. 00:00:12.031 --> 00:00:17.397 It cannot be created or destroyed: total energy in a closed system remains constant. 00:00:20.067 --> 00:00:29.093 The eight stores are kinetic, thermal or internal, chemical, gravitational potential, elastic potential, nuclear, magnetic and electrostatic. 00:00:31.767 --> 00:00:40.992 Energy transfers occur mechanically (forces doing work), electrically (moving charges), by heating and by radiation such as light or infrared. 00:00:41.142 --> 00:00:43.489 Sound can also carry energy away. 00:00:46.167 --> 00:00:54.473 A battery-powered motor transfers energy electrically from the battery’s chemical store to a moving object's kinetic store and thermal stores. 00:00:57.133 --> 00:01:02.977 A falling object transfers energy from its gravitational potential store to its kinetic store. 00:01:03.127 --> 00:01:08.297 Air resistance also transfers energy to thermal stores in the object and surroundings. 00:01:10.967 --> 00:01:13.919 Useful transfers achieve the intended purpose. 00:01:14.069 --> 00:01:18.032 Unwanted transfers often heat the surroundings or produce sound. 00:01:18.182 --> 00:01:23.014 Dissipated energy is spread out and becomes less useful, but has not been destroyed. 00:01:25.667 --> 00:01:30.489 A Sankey diagram represents transfers with arrow widths proportional to energy. 00:01:30.639 --> 00:01:35.929 Input equals total useful and unwanted output; read values and units carefully. 00:01:36.079 --> 00:01:41.869 Energy conservation: 100 joules equals 25 joules plus 75 joules. 00:01:42.019 --> 00:01:44.302 Arrow shafts show the proportions. 00:01:48.967 --> 00:01:56.623 Change in gravitational potential energy equals mass times gravitational field strength times change in vertical height, 00:01:56.773 --> 00:02:02.222 change in gravitational potential energy equals M times G times delta H. 00:02:02.372 --> 00:02:06.453 Use kilograms, newtons per kilogram and metres to obtain joules. 00:02:06.603 --> 00:02:10.052 Use the vertical height change, not the length of a slope. 00:02:14.733 --> 00:02:21.089 A 2 kilograms object raised 3 metres in G equals 10 newtons per kilogram gains 60 joules. 00:02:21.239 --> 00:02:26.401 The reference level is chosen for the calculation; energy depends on a change in height. 00:02:29.067 --> 00:02:35.998 Kinetic energy equals one half times M times V squared, with mass in kilograms and speed in metres per second. 00:02:36.148 --> 00:02:41.576 Squaring speed means doubling speed quadruples kinetic energy at unchanged mass. 00:02:44.233 --> 00:02:53.127 A 4 kilograms object moving at 5 metres per second has kinetic energy one half times 4 times 25 equals 50 joules. 00:02:55.800 --> 00:03:00.891 Efficiency equals useful energy transferred divided by total energy supplied. 00:03:01.041 --> 00:03:04.396 It has no unit and is a fraction between 0 and 1. 00:03:04.546 --> 00:03:08.305 Multiply by 100 to express it as a percentage. 00:03:10.967 --> 00:03:20.171 A lamp supplied 100 joules and transferring 25 joules by useful light has efficiency 0.25, or 25 percent. 00:03:20.321 --> 00:03:24.305 The remaining 75 joules is transferred in unwanted ways. 00:03:26.967 --> 00:03:31.533 Lubrication reduces friction and unwanted heating in moving machinery. 00:03:31.683 --> 00:03:35.189 Insulation reduces unwanted thermal transfers. 00:03:35.339 --> 00:03:38.355 Both can improve efficiency for the intended task. 00:03:41.033 --> 00:03:45.440 Heating normally transfers energy from a hotter region to a cooler region. 00:03:45.590 --> 00:03:51.701 Temperature is not the same quantity as energy; heating can raise temperature or cause a change of state. 00:03:54.367 --> 00:04:01.672 In conduction, energy passes between neighbouring particles; free electrons also transfer energy efficiently in metals. 00:04:01.822 --> 00:04:04.957 Material does not flow along the conductor as a whole. 00:04:07.633 --> 00:04:10.788 Convection happens in fluids (liquids and gases). 00:04:10.938 --> 00:04:14.369 A warmer region usually becomes less dense and rises. 00:04:14.519 --> 00:04:19.179 Cooler, denser fluid sinks to take its place, setting up a convection current. 00:04:19.329 --> 00:04:22.771 Convection transfers energy by bulk movement of fluid. 00:04:27.433 --> 00:04:33.331 Infrared radiation transfers energy without needing a material to travel through (a medium). 00:04:33.481 --> 00:04:37.462 It can travel through a vacuum, unlike conduction and convection. 00:04:40.133 --> 00:04:44.765 A low thermal conductivity makes a material a good thermal insulator. 00:04:44.915 --> 00:04:49.693 For otherwise identical walls, greater thickness reduces the rate of conduction. 00:04:52.367 --> 00:05:00.898 A larger temperature difference across a wall increases the rate of energy transfer; a higher thermal conductivity also increases that rate. 00:05:03.567 --> 00:05:10.211 Trapped air in cavity-wall insulation and double glazing reduces conduction; small pockets limit convection. 00:05:10.361 --> 00:05:13.739 Unfilled large air spaces can still allow convection. 00:05:13.889 --> 00:05:17.096 Small air pockets inhibit large convection currents. 00:05:21.767 --> 00:05:26.833 Loft insulation, thick low-conductivity walls and double glazing reduce cooling. 00:05:26.983 --> 00:05:30.043 Draught proofing reduces transfers by moving air. 00:05:30.193 --> 00:05:33.879 Shiny surfaces can reduce infrared absorption and emission. 00:05:36.533 --> 00:05:39.633 Fossil fuels are coal, oil and natural gas. 00:05:39.783 --> 00:05:45.306 They form over very long times and are used faster than they are replaced, making them non-renewable. 00:05:47.967 --> 00:05:56.946 Combustion transfers energy from chemical stores; electricity generation often uses heating to make steam that drives turbines and generators. 00:05:59.600 --> 00:06:04.815 Burning fossil fuels releases carbon dioxide, contributing to climate change. 00:06:04.965 --> 00:06:09.556 Some fuels also cause air pollution and habitat damage during extraction. 00:06:12.233 --> 00:06:15.473 Nuclear fuels such as uranium are non-renewable. 00:06:15.623 --> 00:06:23.152 Nuclear reactions release energy for heating and electricity generation with low operational carbon dioxide emissions. 00:06:25.833 --> 00:06:35.346 Nuclear generation produces radioactive waste requiring secure management; accidents, decommissioning costs and lifecycle emissions must be considered. 00:06:35.496 --> 00:06:39.731 Low operational emissions do not mean no environmental impacts. 00:06:42.400 --> 00:06:46.576 Renewable resources are replaced naturally quickly enough for continued use. 00:06:46.726 --> 00:06:54.915 Examples include wind, solar, tides, hydroelectricity, geothermal energy and biofuels grown and replaced sustainably. 00:06:57.567 --> 00:07:06.899 Wind turbines transfer kinetic energy of air; hydroelectricity uses water dropping from a higher level; tidal systems use tidal water movements. 00:07:07.049 --> 00:07:09.182 These can drive generators. 00:07:11.833 --> 00:07:17.017 Solar cells transfer energy electrically from sunlight; solar heating warms water. 00:07:17.167 --> 00:07:19.791 Their output varies with sunlight and weather. 00:07:22.467 --> 00:07:25.708 Geothermal heating uses energy from hot rocks. 00:07:25.858 --> 00:07:36.126 Biofuels release energy by combustion; plant growth can take up carbon dioxide, but farming, processing, transport and land-use changes affect net emissions. 00:07:38.800 --> 00:07:42.597 Resource reliability differs: wind and solar are variable, 00:07:42.747 --> 00:07:44.433 tides are predictable, 00:07:44.583 --> 00:07:47.868 stored-water hydroelectricity can respond quickly, 00:07:48.018 --> 00:07:52.045 and geothermal can provide steadier output in suitable locations. 00:07:54.700 --> 00:08:01.375 Compare cost, start-up time, availability, land use, habitat impacts, pollution, waste and reliability. 00:08:01.525 --> 00:08:04.691 No single resource is best in every location. 00:08:04.841 --> 00:08:08.165 Use several criteria rather than a single advantage. 00:08:12.833 --> 00:08:18.166 Energy use changes with demand, price, technology and environmental priorities. 00:08:18.316 --> 00:08:24.683 Use the data in a question to describe trends without inventing causes or treating a correlation as proof. 00:08:27.333 --> 00:08:29.808 That completes Conservation of energy. 00:08:29.958 --> 00:08:33.303 Revisit the notes and test yourself on the revision website.