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P1.5-P1.9 (Energy and Efficiency (From the principle of conservation of…
P1.5-P1.9
Energy and Efficiency
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The less energy that is wasted by a machine, the more efficient the machine is.
The efficiency of a device can be calculated using the equation
efficiency =(useful energy transferred ÷ energy supplied) × 100
Efficiency is a ratio, so doesn't have a unit.
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Different machines waste energy in different ways. These include friction between moving parts, air resistance, and electrical resistance.
Energy and Power
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The unit of power is the watt, W. 1 watt is equal to the rate of transferring 1 joule of energy in 1 second (i.e., 1W=1J/s).
Often a watt is too small a unit to be useful, so power is given in kilowatts (kW). 1kW=1000W.
Power can be calculated using the equation P=E/t, where:
P is the power in watts, W.
E is the energy transferred in joules, J.
t is the time taken for the transfer in seconds, s.
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Energy Dissipation
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Useful energy is energy transferred to where it is wanted in the form that is wanted. Wasted energy is energy that is not usefully transferred.
Both the useful energy and the wasted energy will eventually be transferred to the surroundings, which will warm up. The energy is dissipated. As the energy spreads out it becomes more difficult to use for further energy transfers.
Energy is often wasted because of friction between the moving parts of a machine. Sometimes friction may be useful - for example, in the brakes of a bicycle or a car. The kinetic energy store of the vehicle decreases and the thermal energy stores of the brakes and the surroundings increase.
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Electrical Energy
Energy In The Home
Energy in the home is mostly supplied by electricity, gas, and oil. Electrical appliances are extremely useful because they transfer energy at the flick of a switch.
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Many electrical appliances transfer energy by heating. This may be a useful transfer, for example in a kettle, but for many other appliances, energy is wasted by heating.
Appliances should be designed to waste as little energy as possible in order to make them as efficient as possible.