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Energy - Energy and Efficiency

This GCSE Physics quiz will challenge you on energy and efficiency. Energy efficiency saves money, energy and the environment. No device can be 100 percent efficient; during any energy transfer, some always escapes into the surroundings and is wasted. Wasted energy is usually in the form of heat but is sometimes sound or light. Filament light bulbs are only about ten percent efficient which means that they waste nearly all the energy that is fed into them. LED lighting is a good way of saving on energy as they are about 75 to 80 percent efficient. Petrol engines are usually between twenty and thirty percent efficient, diesel engines are more efficient than petrol engines but they still waste well over half of the energy supplied to them.

In electrical circuits, as the electrons flow through a conductor, energy is lost as they collide with the atoms of the conductor.

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The conductor therefore heats up and the heat is dissipated (spreads out) into the surroundings. Electrical energy is wasted as sound and light if there are sparks in a circuit. In mechanical systems, the main waste of energy is heat. Even with the best engineered and effectively lubricated moving parts, it is impossible to eliminate friction. This creates heat and some sound which then spreads out into the surroundings. We say the energy is dispersed and this dispersed energy becomes harder and harder to reuse.

For your GCSE exams, you need to be able to carry out calculations about energy transfers, including the efficiency. Outside of your exams, calculating energy transfers and efficiency can be crucial when deciding on ideas to save energy and money. The efficiency of an energy transfer device is the ratio of the useful output energy to the input energy and is expressed as a percentage.

In GCSE exams, the examiners very often present quantitative information (information concerning numbers) about energy transfers and efficiency in the form of a Sankey diagram. This is drawn to scale and you are expected to be able to work out how much of the different energies are transferred as well as to use the numbers in calculations. In some cases, they make it easy and include the values required for any calculations. If you make a mistake with the numbers, don't worry too much - you will still receive credit for using the correct calculation methods so always show your working out if there is space.

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  1. What is the efficiency of a solar cell if 1000 J of solar energy hits the cell and 200 J are converted into electrical energy?
    200 divided by 1000 then multiplied by 100 to convert into a percentage
  2. What is electrical energy converted into when a drill is used?
    It is the kinetic energy that is useful
  3. An old boiler system is replaced with a solar heating system. The old boiler cost £250 per year to run. The new solar heating system costs £0 to run but cost £2,000 to install. How long will it take to start saving money?
    The solar water heating system saves £250 per year so dividing the cost of installation by the savings each year leads you to the correct answer
  4. Electrical energy in a kettle changes to what type(s) of energy?
    Whilst heat is the predominant output of energy transfer, sound and light are given off as wasted energy. It could be argued that the light is actually useful as it lets you see that the electricity to the kettle is switched on but it is generally regarded as wasted since it isn't involved directly in boiling the water
  5. Electrical energy in a TV changes to what type(s) of energy?
    The TV also emits the same types of energy transfers, however they are in a different proportion with light and sound being the dominant transfers
  6. An energy saving bulb replaces a 60 W lamp which cost £4.65 each year to run and 20p to buy. The new low energy bulb costs just £2 per year to run and 65p to buy. How long will it take to start saving money on the running costs due to the change?
    The 60 W lamp costs just under 39p per month to run, the energy saving bulb costs just under 17p per month to run. By the end of 3 months, the 60W bulb will have cost about £1.37 to buy and run but the energy saving bulb has cost £1.26 to buy and run so from late on in the third month, money is saved. The closest answer to this is 4 months, therefore that is the correct choice
  7. What would be a good idea to save energy and money?
    Performing all of the steps above may cost a lot in the short term, however the long term benefits of implementing such a plan would help to reduce your yearly bills and help the environment too!
  8. How could you save money boiling a kettle for one cup of tea?
    Do any members of your family automatically fill up the kettle whenever they make a cup of tea or is everyone 'energy aware' and only uses the amount of water that is needed?
  9. Which appliance uses the most energy?
    You are expected to be aware of appliances that use a lot of energy and those which use less energy either with or without being given any energy transfer or power figures
  10. A heat exchanger can be used to convert which type of wasted energy into a useful source of power?
    It's all in the name! Adding a heat exchanger system to a chimey can recover some of the heat that would have been lost to the outside. This can then go towards heating domestic hot water or heating the house, increasing the efficiency. In eco-houses, heat exchangers are used to warm the air coming in through the ventilation system. This increases the efficiency of the house by keeping heat energy that would otherwise have been lost to the outside, inside the house

Author: Martin Moore

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