P1.2 Using Energy

  • Created by: StephBea
  • Created on: 07-04-15 12:55

2.1 Forms of Energy

  • Energy exists in many different forms and can be transferred from one form to another form.
  • Forms of energy include; light, sound, kinetic, nuclear, electrical, gravitational potential, elastic potential and chemical.
  • Some forms of energy are known as stored energy and these are gravitational potential, eastic potential and chemical.
  • Any object above the ground has gravitational potential energy.
  • When an object falls and gains speed, its gravitational potential energy decreases and its kinetic energy increases.
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2.2 Conservation of Energy

  • Energy can be transferred from one form to another or one place to another but can never be created or destroyed. Conservation of energy applies to all energy changes.
  • This means that the total energy is always the same, this is called the conservation of energy and applies to all energy transfers.
  • For example; a falling object transfers gravitational potential energy to kinetic energy, a stretching elastic band transfers chemical energy to elastic potential energy and in a solar cell, light energy is transferred to electrical energy.
  • A swinging pendulum transfers energy from gravitational potential energy to kinetic energy and back again as it swings.
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2.3 Useful Energy

  • Useful energy refers to energy in the place we want it and the form we need it. Wasted energy is energy that is not useful energy. Both useful energy and wasted energy end up transferred to the surroundings, making it warmer. As energy spreads out, it gets more and more difficult to use it for energy transfers.
  • A machine is something that transfers energy from one place to another or from one form to another, the energy we get from a machine will consist of both useful and wasted energy.
  • Energy is often wasted because of friction between the moving parts of a machine, the energy warms up the machine and surroundings and is not useful.
  • Friction can, however, be useful in some instances for example in the brakes of a bicycle or a car. Some of the kinetic energy of the vehicle is transferred to energy heating the brakes.
  • Sometimes wasted energy is transferred as sound, but the amount of energy is usually very small. This energy will also eventually be transferred to the surroundings, making them warmer.
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2.4 Energy and Efficiency

  • Energy is measured in joules (J), and this is the unit used for all forms of energy.
  • Energy supplied to a machine is often called the input energy, from the conservation of energy we know that the the total input energy will always equal the useful energy transferred and the energy wasted added together.
  • A machine's efficiency is determined by the percentage of supplied energy that is usefully transferred, so the more energy wasted by the machine, the less efficient it is. The efficiency equation is;
  • Efficiency = useful energy transferred by the applicance (J)/total input energy(J) x100% 
  • In a light bulb where for every 25J of supplied energy, 5J are usefully transferred into light energy, the efficiency would be: 5/25 x100% = 1/5 or 20% efficiency. Efficiency is a ratio, so has no unit.
  • No applicance can be 100% efficient, with the exeption of an electrical heater, which transfers all of the supplied electrical energy to heating the surroundings.
  • If a machine is adjusted so that less energy is wasted becuase of friction, the efficiency of the applicance will increase as a higher percentage of the input energy will be transferred into useful energy.
  • The efficiency of an applicance is often represented by a Sankey diagram showing input, useful and wasted energy.
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Sankey Diagrams

  • Sankey diagrams are used to represent the energy transfers through an applicance.
  • If the input energy to an applicance was 100J and it had an efficiency of 75%, this would mean 75J of energy were usefully transferred while 25J were wasted.
  • The Sankey diagram for this would look like this, where the useful energy is represented by a straight arrow and the wasted energy, a curved arrow out to the side:


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