Work, Power & Energy Energy What is energy? Energy refers to the capacity for doing work. It exists in several different forms and types, some of which are described in the table below: Chemical Energy Food and fuels (e.g. oil, gas, coal, wood) store chemical energy. Energy from food is released by chemical processes in our body, aiding us in doing jobs. Fuels cause energy transfers when burnt in engine or boiler. Stored in chemical bonds. Gravitational Potential Energy Energy a body has because of its position. Due to gravitational attraction of the Earth for the object. Elastic Potential / Strain Energy Work has to be done to compress or stretch a spring or elastic material and energy is transferred to e.p.e.; the p.e. is stored in the form of strain energy (or elastic potential energy). Kinetic Energy Energy a body possesses when it is in motion e.g. a car moving, a boy sledding/skiing, a ball bouncing Electric Energy Derived as a result of the movement of electrically charged particles. Stored when repelling charges have been moved closer together and attracting charges pulled further apart Heat/Internal Energy Total energy in a closed thermodynamic system. Sum of potential energy and internal kinetic energy of the atoms within the body. Conservation of Energy The principle of conservation of energy states that “Energy cannot be created or destroyed – it is always conserved”. This means that even if a system loses energy in one form, that energy does not get lost completely – it gets transferred to another form of energy. All energy transfers result in surroundings being heated e.g. when a brick falls its potential energy becomes kinetic energy; as it hits the ground, its temperature rises and heat and sound are produced. Some energy is also converted into other non-useful forms of heat. Stores of Energy & Energy Transfers Different forms of energy can be transferred from one form to another, such as in the example described above. We use energy transfer diagrams showing each form of energy (whether stored or not) to display energy transfers. For example, in this energy transfer diagram for a power station, we first see chemical energy stored in fuels, being burnt. This combustion in the boiler results in heat energy which is then converted into kinetic energy as the movement of steam molecules that move across to the turbine. This kinetic energy is also seen in the movement of the turbine and the generator; the latter produces electrical energy as a result which is transferred to the grid. There can be several types of energy transfer: 1. Mechanical work: a force is pushing the boat across the water/through a distance 2. Electrical work: when charge flows 3. Heating: when energy is transferred between hotter and colder regions 4. Radiation: when energy is transferred as a wave e.g. as light or sound For example, in a toy electric train, the following energy transfer diagram can be used via electrical work: When a girl places a book from a table onto an upper shelf, mechanical work occurs: Energy is transferred as a wave (infrared radiation) from a heater that's heating food e.g. toasting bread Heat travels from a region of hotter temperature to a region of colder temperature e.g. from a handwarmer to our hands: Efficiency What is efficiency? The efficiency of a device is the percentage of the energy supplied to it that is usefully transferred. It is calculated with the following expression: Efficiency = ( useful energy/power output / total energy/power input ) x 100 As energy is usually dissipated as other forms of heat and non-useful energy, efficiency is rarely ever 100%. Q. An electric motor is required to produce 120 W of mechanical output power. The efficiency of the motor is 80%. What is the electrical power input to motor (W) and the waste heat output from motor (W)? 80/100 = 120 / (power input) 0.8 = 120 / power input Power input = 120 / 0.8 = 150 Power input - useful power = waste heat 150 - 120 = 30 Calculations Kinetic Energy Kinetic energy of an object in motion can be calculated with the following expression (given mass in kg and velocity in m/s) Kinetic Energy = ½ (mass x velocity2) Ek = ½ (mv2) A football of mass 0.4kg moving with velocity 20m/s has Kinetic Energy = ½ (0.4 x 202) = 80Nm or 80J Gravitational Potential Energy The change in G.p.e. can be calculated with the following expression, given mass in kg, and height in metres: Gravitational Potential Energy = (mass x gravitational field strength x vertical height) ∆Ep = mgh This is as to lift a body of mass m through a vertical height h at a place where the Earth’s gravitational field strength is g, the body needs a force equal and opposite to the weight mg of the body. The Potential Energy gained by an object of 0.1kg when raised vertically by 1m is 0.1 x 10 x 1 = 1Nm or 1J Relationship between GPE and KE A mass m at height h above the ground has potential energy = mgh. When it falls, its velocity increases and it gains kinetic energy at the expense of its potential energy. Assuming air resistance is negligible, its kinetic energy on reaching the ground equals the potential energy lost by the mass. This is an example of conservation of energy Another example can be taken with the case of the pendulum: KE and GPE are interchanged continually. The energy of the bob at either end of its swing is all GPE and all KE when it passes through its central position; in other positions, it has both GPE and KE. The system slowly loses energy as heat until it stabilizes. Work Done Work is done when a force moves in a particular direction. E.g. NO work is done in a scientific sense by someone who is static and holding a heavy pile of books – though an upward force is being exerted on the books, there is no motion. If more force has to be exerted in a particular direction, more work is also done. We calculate work done with the following equation: Work Done = force x distance W.d. (J) = Fd Joules is the unit of work – it is the work done when a force of 1N moves through 1m. 1Nm = 1J. The Work Done when a mass of 3kg is lifted through 2m is calculated by first calculating the weight of the mass (w = mg; 3 x 10 = 30N). As a force equal to the weight has to be exerted vertically upwards to be able to lift the mass, 30N is the force used in this equation as well. Thus, 30 x 2 = 60J which is the work done. Power The power of a device is the work it does per second or the rate at which it does work or the rate at which energy is transferred from one form to another. The unit of power is Watt (W); it is the rate of working of 1 Joule per second; 1W = 1J/s. We may also use megawatts (1 000 000W = 1 MW) or kilowatts (1000 W = 1 kW) to represent power. It is calculated by: Power = Work done / time taken P = w.d. / t or Power = Energy transferred / time taken P = ∆E / t If a machine does 500 J of work in 10 s, its Power is 500/10 = 50 J/s = 50 W. Energy Resources Resource How is useful energy obtained? Renewable or not? How readily available is it? Impact on the environment? Fossil fuels Coal, oil, natural gas, formed from remains of organisms millions of years ago, storing chemical energy. Burnt to produce heat to boil water to create steam which moves turbine and drives generator. Non-renewable – will eventually run out Relatively easy and cheap to obtain for now. Currently being used the most. Coil and oil release sulfur dioxide which causes breathing problems and acid rain. Generated from biomass (organic material from plants and animals). Burned as fuel to generate electricity as heat is generated which boils water to create steam that moves a turbine and drives a generator Renewable - more plants can be grown to replace old dead ones; human and animal waste also continuous Biofuels (vegetable fuels) Also release CO2 which adds to greenhouse effect and increases global warming Very portable - can be transported by road, rail or boat, or even pipelines Cheap and readily available Not sustainable because growing crops for biofuels takes up land that could be used for food production. Reduces biodiversity. Remove nutrients from soil. Releases CO2 in the atmosphere Hydroelectric Solar HEP stations use KE from moving water; often water comes from dams built across river valley. Water high up behind the dam contains GPE which is transferred to KE as water rushes down to the turbine (which is then connected to the generator) Renewable Released by nuclear fusion in the sun. Solar cells convert light energy directly into electrical energy. E.g calculators use solar energy Renewable HEP stations can be easily switched on Flood farmland and push people from their homes Rotting vegetation underwater releases methane (greenhouse gas) - Expensive and inefficient; cost of electricity is high - Do not work at night - Low energy density means it requires large collecting devices - Useful for small-scale power generation in remote areas with no electricity supply No harmful polluting gases Nuclear Nuclear fuels undergo a controlled chain reaction in the reactor to produce heat. Heat changes water to steam which drives a turbine and then the generator Non-renewable 1kg of nuclear fuel produces millions of times more energy than 1kg of coal (high energy density) Ready availability Does not produce CO2 Large amounts of radioactive material could be released into the atmosphere; this is radioactive and hazardous to health Geothermal Hot water and steam from deep underground can be used to drive turbines; in volcanic areas, rock heats water so that it rises to the surface naturally as hot steam which drives turbines. In places where it doesn’t naturally rise, deep wells are drilled down to the hot rocks where cold water is pumped down to be heated and return to the surface as steam Renewable Most parts of the world do not have suitable areas where geothermal energy can be exploited; mainly in places such as Iceland and New Zealand No fuel costs Wind Produced as a result of giant convection currents driven by heat energy from the sun. Wind turbines use wind to drive turbines directly; turns a generator which produces energy Renewable - as long as Sun exists, wind will too If no wind, no electricity. Dependent on higher amount of windy days as well as large open spaces to build windmills No harmful polluting gases Tidal barrages built over river estuary contain electricity generators which are driven by water tides rushing through tubes to the barrage Renewable Tidal barrages very reliable and easily switched on No harmful polluting gases Water in the sea rises and falls because of waves on the surface. Wave machines use the KE in this movement to drive electricity generators. Renewable Tides Waves No fuel costs Wind farms are noisy and an eyesore Destroy the habitat of estuary species, including wading birds Difficult to scale up the designs for wave machines to produce large amounts of electricity. No harmful polluting gases EXAM TIPS ● ● ● ● ● When calculating work done, remember to only take the direction in which the force is acting e.g. if an object is being moved up diagonally, you will take the diagonal distance rather than the vertical or horizontal distance in the equation w.d. = force x distance Similarly, when calculating gravitational potential energy, remember to only take the vertical direction instead of any other direction e.g. if an object is being moved up diagonally, you’ll take the vertical distance it has moved up instead of the diagonal distance or horizontal distance it has traveled. This is as change in g.p.e. Is directly linked only to the vertical height. Efficiency questions can be about both energy and power output, but that doesn’t mean power and energy are the same at all – they are different entities. When asked about useful energy change in a certain scenario, you’re meant to take note of what the initial energy is stored as and what it is eventually finally converted into, disregarding any of the intermediary energy changes. E.g. in a fossil fuel power station, the chemical energy stored in the fossil fuels is to be converted into electrical energy which is our final energy change, even though it does get converted into heat energy and kinetic energy in the process. Also take note of the fact that efficiency decreases in a system i.e. in a system where energy is continuously being converted from one form to another such as in a power station, at each phase of energy change, some energy is dissipated to the surroundings, and energy in the system decreases, making its total output lesser than the initial input. The more stages there are, the more energy is dissipated.
