Energy - Transfers and Efficiency

This GCSE Physics quiz on energy looks at transfers and efficiency. The principle of conservation of energy states that 'energy can neither be created nor destroyed, only changed from one form into another'. Energy changes make the world work. Plants transfer the light energy from the Sun into chemical energy of food by photosynthesis. Animals, including humans, change the chemical energy of food into heat and kinetic energy. We transfer the chemical energy stored in coal, oil and gas into electrical energy to run our electronic appliances and machines; into kinetic energy to run our vehicles; into heat energy to cook our food and more.

The electrical energy generated at power stations is transferred into other forms of energy by our machines and electronic devices.

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Once transferred into other forms, energy then becomes more difficult for us to use, it is then called wasted energy. An example of this is a car engine running on petrol. An engine converts the chemical energy of its fuel into heat energy by burning. The gases created by burning the fuel have a larger volume than the fuel itself, this fact is used to create kinetic energy as these gases cause the pistons to move. This kinetic energy is then used to drive the wheels round. Only about 30 percent of the energy contained in the original petrol is actually transferred into kinetic energy that moves the car, the rest is wasted as heat, sound and light energy and simply disperses into the air. Diesel engines are slightly better at transferring energy, up to 40 percent ends up as useful kinetic energy.

The amount of energy that is transferred by something is called the efficiency. It can be calculated as a percentage by dividing the amount of useful energy transferred by the original amount of energy available. It is then converted into a percentage by multiplying by one hundred. This efficiency can be calculated using either energy or power. Using more efficient devices will reduce the amount of energy wasted which will benefit the whole world.

We talk of energy being renewable or non-renewable. As science has progressed, humans have begun to realise that our supplies of chemical energy won't last forever. Once they are gone, we will have to wait for tens of millions of years for them to be re-formed so we need to regard these as being non-renewable. Scientists and engineers are therefore trying to find ways of using other sources of energy ~ for example, the wind, waves and tides, the heat of the earth and energy from the Sun. We call these renewable energies because they will always be there, or at least for as long as the Earth exists.

The big problem is that these alternative forms of energy are much less concentrated than the chemical energy of the fossil fuels. This makes them more difficult to use ~ for example, it is said that it could take about 6,000 wind turbines spread out over 250,000 acres to replace one power station. Some of these could be built out at sea but this then brings other problems.

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  1. Calculate the efficiency of a heater which can convert 700 joules of chemical energy into 175 joules of heat energy.
    Remember, efficiency is normally given as a percentage
  2. What happens to wasted energy?
    Any energy wasted by a process will always dissipate into its surroundings. Although it is still there, because it is so spread out, it makes it much more difficult to use
  3. What can be done to energy?
    Not all forms of energy can be stored
  4. Which formula can be used to calculate the percentage of energy transferred by a device?
    This is called the efficiency of an energy transfer
  5. An LED converts around 80% of the energy supplied to it into light. How much energy is converted into light if a total of 50 J is supplied to it?
    LEDs are a much more efficient way of transferring electrical energy into light energy than the conventional filament bulb and fluorescent lamps ~ they last longer too
  6. A wind turbine generates 100 joules of electricity from a gust of wind which has 300 joules of energy. How efficient is the turbine?
    Wind turbines are an example of renewable energy
  7. During an energy transfer, only part of it may be usefully transferred. What happens to the remainder of the unused energy?
    In every energy transfer, some energy is wasted
  8. A solar panel is 20% efficient at converting light energy into electricity. If it generates 200 joules of electrical power, how much power is wasted?
    This is an alternative way for the examiners to check your understanding of efficiency
  9. What cannot be done to energy?
    It is impossible to create or destroy energy. In a science lesson, you may have watched a balloon being popped - all of the elastic energy within the balloon is transferred into sound, heat and kinetic energy
  10. Which formula can be used to calculate the percentage of power transferred by a device?
    More efficient devices waste less energy

Author: Martin Moore

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