Energy sources & Transfer of energy, Lecture notes of Physics

Download Energy sources & Transfer of energy and more Lecture notes Physics in PDF only on Docsity! Topic 8: ENERGY SOURCES and TRANSFER OF ENERGY 8.1 Forms of energy a) There are two basic forms of energy: kinetic and potential. b) These 2 forms of energy can appear in many roles: kinetic, potential, heat, electrical, nuclear, sound, light and chemical. (i) Kinetic Energy  Kinetic energy (K.E) is the energy possessed by a moving body.  Eg. A speed boat, a moving car and waterfall have K.E because they are moving.  where K.E = kinetic energy (in J) m = mass of the body (in kg) v = velocity of the body (in m/s) (ii) Potential Energy  Potential energy (P.E) is stored energy.  It is the energy possessed by a body because of its position or condition.  A body which has been raised above the ground has gravitational P.E because of its position. - It will have gravitational potential energy (G.P.E) because when the body is released, it will fall and gain K.E.  A spring which has been stretched or compressed has P.E because of its condition. - A stretched/ compressed spring has elastic P.E because when it is released, it will fly inwards or outwards.  Gravitational potential energy, where P.E = Potential energy (in J) m = mass of the body (in kg) g = gravitational strength (in m/s² or N/kg) h = height above a reference level (iii) Heat Energy  Heat energy makes things hot.  The heat from your body comes from the food you eat.  Heat can cause changes: e.g. a solid into a liquid, liquid into gas and can make a piece of metal to expand. (iv) Electrical Energy  Electrical energy comes from batteries, dynamos and generators.  Electrical energy is the ability of the electric current to do things.  It can heat up electric iron and light a bulb. (v) Nuclear Energy  The two processes where nuclear energy can be obtained are nuclear fusion and nuclear fission. Energy sources and transfer of energy Page 1 KIPOHENU ELECT, SOUNDLIGHT CHEM K.E = ½ mv² P.E = mgh  Large amount of thermal energy is produced in nuclear fusion and nuclear fission.  Nuclear fission is the splitting of atoms of heavy elements such as uranium into light elements. - The thermal energy from the nuclear reaction can be used to heat up water to produce steam. - The steam is then used to drive turbines. - Electricity is then produced by generators connected to these turbines.  Nuclear fusion is the combining of atomic nuclei of light elements to form heavier elements. - The light elements involved in fusion are deuterium and tritium (both isotopes of hydrogen). (vi) Sound Energy  Sound travels in the form of waves (longitudinal waves).  Sound waves carry the energy with them.  Musical notes and noises are all sound energy. (vii)Light Energy  A body which gives out light has light energy.  Light energy travels in waves (transverse waves).  The main source of light energy is the sun. (viii) Chemical Energy  The food we eat possesses chemical energy.  Chemicals stored up chemical energy.  In our body, the chemical energy in the food is changed into heat energy to keep us warm and energy to enable us to work and play. Examples of different forms of energy. Forms of energy Examples in which each form occurs Kinetic Potential Water at the top of waterfall, stretched rubber band, wound up spring Heat Electrical Batteries, lightning, generators, dynamos Nuclear Sound Radios, TV sets, cassettes, musical instruments, CDs, thunder Light Sun, fire, lightning, electric lamps, fluorescent lamps Chemical Conversion and conservation of energy (i) The Principle of Conservation of Energy states that: Energy sources and transfer of energy Page 2Energy can neither be created nor destroyed in any process. It can be converted from one form to another or transferred from one body to another, but the total energy remains constant. a) Wind has kinetic energy. Wind turbines in the USA make electricity. b) A tidal power station makes electricity as the tide moves. Dam traps water at high tide. At low tide, water turns turbines as it flows through dam. c) Underground water flows onto hot rocks and turns into steam. Jet of steam from the ground (called geyser) is used to generate electricity in geothermal power station in New Zealand. d) Energy from the sun is called solar energy. In a solar water heater, water gets heat from the sun. Solar cells are used to provide electricity for satellites. e) Nuclear energy is changed into electrical energy inside a nuclear power station. Water in pipes becomes steam (heated by uranium) which turns the turbines. f) Plant and animal material is called biomass. It can make biogas. The biogas is good for cooking and heating. China has 7 million biogas generators. They give energy for 35 million people. (iii) Energy conversions of energy sources. a) Fossil fuel Mineral oil (petroleum), natural gas and coal are fossil fuels. Mineral oil and natural gas consist mainly of hydrocarbons (compounds of hydrogen and carbon). Coal consists mainly of carbon with some hydrocarbons. Fossil fuels are burnt to give energy. Carbon burns in the presence of oxygen to form carbon dioxide and heat energy is released in the process. Carbon + oxygen carbon dioxide + heat energy Energy sources and transfer of energy Page 5 Hydrocarbons burn in the presence of oxygen to form water and carbon dioxide, and also release heat energy. Hydrocarbon + oxygen water + carbon dioxide + heat energy The combustion of fossil fuels is a chemical process. In this case chemical energy is transformed into heat energy. In thermal power station, heat is used to boil water. (Thermal means caused by heat.) The heat is obtained from burning coal, fuel oil or natural gas. The steam produced in this process is used to drive a turbine. The rotating turbine turns a generator which in, turns, generates electricity. The electricity is then sent through cables to homes, offices and factories. Applications in real life:  The heat energy produced can be used directly for heating homes in winter or for cooking.  It can also be used to generate electricity. b) Hydroelectric generation In mountainous regions where the rainfall is sufficient, water can be used to generate electricity. Dams are built across rivers and water is stored in the lake behind each dam. This stored water has gravitational potential energy. This form of energy can be converted to electric energy. Grand Coulee Dam, North America built in 1942 at close to 7 GW capacities. In the figure shows a cross-section of a hydroelectric power station. The water from the reservoir goes through a tunnel under the dam. The amount of water going through can be controlled by means of a valve. Due to the depth of the reservoir the water comes out of the tunnel at high pressure and a high velocity. This powerful stream of water turns a water turbine which is connected to a generator. The electricity generated by the generator is distributed to cities and towns by cables. The dam stores the water which originally comes from rain. As the reservoir fills, its gravitational potential energy increases. This potential energy changes to kinetic energy when the water comes out of the tunnel at high speed. The water turbine converts this into mechanical energy, which in turn is converted into electric energy by the generator. Applications in real-life:  To produce electricity from hydroelectric power station. c) Solar energy The sun shines everyday, giving energy in the form of heat and light, and this is free! In solar water heater, the heat rays from the sun are concentrated by reflection and then used to heat water. The water circulates either by convection or by means of a pump into the house. Energy sources and transfer of energy Page 6 In figure above shows a calculator powered by solar cells. The solar cells are thin wafers of silicon. They convert light energy directly into electric energy. Each cell is like a tiny battery. However, the current and voltage obtainable from a solar cell are small. A number of cells have to be connected together to produce a sufficient amount of current. Even then they can operate only low-powered devices. Applications in real-life:  Electronic watches.  Calculators.  Solar water heater d) Nuclear energy This form of energy is from the nuclei of atoms. The sun’s energy comes from the fusion of hydrogen nuclei. However, nuclear energy can also be produced from nuclear fission. An example of nuclear fission is when the isotope of uranium, 235U, is bombarded by a neutron. The nucleus splits into two nuclei of lighter mass and three neutrons. The two resulting nuclei are barium and krypton. This splitting up of the nucleus is known as nuclear fission. In the process, a tiny amount of mass is converted into energy, which appears mainly as heat. The three neutrons that are produced can cause fission in other uranium nuclei, which in turn cause fission in more uranium nuclei and so on. Scientists use the term chain reaction to describe this process. If this chain reaction is allowed to happen, the result is an explosion. To make use of the energy that is released, it is necessary to control the nuclear fission and prevent such explosions from occurring. Scientists have built nuclear reactors for this purpose. The heat energy of the nuclear reactor is used to boil water to make steam. The steam drives a turbine which then drives a generator in the power station. Nuclear fusion is another way to get energy. In a hydrogen-bomb and in the Sun, hydrogen nuclei fused together and release energy. This process has not yet been successfully developed for power stations. Differences between fission and fusion Fusion Fission Reason for energy Reduction in mass when light nuclei are Fragments of the explosion are of much Energy sources and transfer of energy Page 7 (2) Find the increase in mass when 4200 J of heat is absorbed by 1 kg of water to cause a temperature rise of 1K. (Take the velocity of light, c = 3 × 108 m/s) Solution: (v) Environmental issues associated with power generation. a) Before building the power generation plans, we have to consider their effects towards our environment. b) There are a few environmental issues relating to these kind of power generations: 1) Pollution  Burning a fossil fuel (coal, oil, natural gas) always produces carbon dioxide gas (CO2). Fuel + oxygen CO2 + H2O + energy CO2 increases the greenhouse effect causing global warming of the Earth. There is no easy way to stop this CO2 emission, except by burning less fuel. Global warming increases the warming of earth. A warmer earth may result in more floods in low-lying areas, more places becoming dry and more droughts in some places.  Sulphur dioxide (SO2) is produced by coal-fired and oil-fired power stations. It causes acid rain which harms forests and buildings. The SO2 gas can be removed, but this makes the electricity 10-20% more expensive. Acid rain is formed when sulphur dioxide or oxides of nitrogen dissolves in rain. Acid rain kills plants and fishes and damages buildings.  Coal-fired stations cause dust and smoke, but these can be removed by electrostatic precipitators.  Nuclear power stations do not emit any CO2 and SO2, and in normal running cause a little pollution providing the nuclear waste is stored carefully. However an accident can affect health over a wide area.  Wind turbines need to be placed on hills or the coast. They can be quite noisy and looks unpleasant.  Hydroelectric schemes flood large areas of land and affect the ecology of the area, as do tidal schemes.  All power stations need cables to distribute the energy, either by unsightly pylons or by very expensive underground cables. 2) Deforestation  When building a large scale of power generator, it also needs a large area of land for its location. Therefore huge amount of trees are needed to be cut down. 3) High cost and needs a lot of materials  Power stations like nuclear power stations, tidal and hydroelectric power stations and also wind turbine need very high cost to build it. Therefore the cost of its electricity is more expensive. Energy sources and transfer of energy Page 10 8.3 Work a) b) Example: A man lifts a brick of mass 5 kg from the floor to a shelf 2 metre high. How much work is done? Solution: 8.4 Efficiency a) We have seen that for the energy sources discussed, some transformation of the energy must take place before it can be used. The final form is usually electric energy, as this is the most convenient form of energy to use today. At each stage a machine or process is necessary to transform one form of energy to another. Is the amount of energy obtained at the end (energy output) the same as the amount of energy put in the beginning (energy input)? According to the Law of Conservation of Energy, no energy is lost. However, these processes of transformation are not completely efficient. For example, in machines there is always friction between moving parts. Some of the energy must Energy sources and transfer of energy Page 11 Work done = force × distance moved in the direction of the force W = F × d, where W = work done by a constant force (in J) F = constant force (in N) d = distance moved in the direction of the force (in m) be used to overcome friction. Again not all the heat used to boil water goes to make steam in a boiler. Some of the heat escapes to the surroundings or heats up the atmosphere. Also, the electric wires in generators have resistance. As the current flows through the wires, they heat up. Thus electric energy is lost as heat. b) Efficiency is the percentage of energy obtained compared with the amount put in. The efficiency of the machine or process is expressed by the following formula: It is given as a percentage. c) The energy input is the amount of energy that goes into the machine or process. d) The energy output is what comes out or is obtained from the machine. e) The table below shows the approximate efficiencies of energy conversion of various machines or processes. Machine / Process Efficiency Solar cell 5 – 15% Water turbine 85% Steam turbine 30% Generator 90% Thermal power station 40% overall Nuclear power station 30% overall It can be seen that the overall efficiency of power stations is low. This is because at each stage as energy is changed from one form to another, some energy is lost. When all these losses are added up, the result is the low efficiency of power stations. The challenge for scientists and engineers is how to design more efficient machines. But however efficient a machine is, it can never be 100% efficient. 8.5 Power a) If two cars of the same weight climb up he same hill, then they do the same amount of work. But if car A climbs the hill in a shorter time than the other car, we say it has a greater power. Energy sources and transfer of energy Page 12 Efficiency = (Energy output / Energy input) × 100%