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Electricity - The Usefulness of Electrical Appliances

This GCSE Physics Electricity quiz will challenge you on the usefulness of electrical appliances. It is almost impossible to spend a single day at home without using several different electrical appliances - they have completely changed the way people live since the end of the second world war. For example, television sets and radios were rare, only the most well-off people could afford them. Nowadays, most homes have several TVs and radios (the latter are often built into mobile phones and smartphones). At the same time, another electrical device that is common did not even exist - home computers. Electrical appliances are great when they are working, but it's really annoying when there is a power cut or the battery is flat!

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Since electricity first made its way into homes (the first electrical appliance for the home was an incandescent (filament) light bulb), more and more of the monthly household expenditure has gone on the cost of electricity. Manufacturers have found better and better ways for making sure that the appliances they sell use the minimum possible amounts of electricity - cost savings are always a good marketing tool they can use to sell more of their products.

Knowing how electricity costs are worked out is useful, not just to help you to pass your GCSE! With some very basic knowledge of GCSE physics, anyone can work out how expensive it will be to use a particular electrical or electronic device. An example of this is to compare the cost of running a filament light bulb to the costs of the equivalent fluorescent or LED lamp. Filament lamps create their light by transferring electrical energy into heat and light energies by making a thin wire (the filament) glow red hot. The heat is wasted but the light is useful, unfortunately most of the energy transferred ends up as heat which is a waste of money and electricity. Fluorescent and LED lamps do not rely on the heat energy to produce light (some heat is produced but much less than with filament bulbs), so they cost less to produce the same amount of light. If you know the cost of electricity, the power rating of each bulb and the time each one is switched on, you can calculate which is the most economical.

Electricity is sold in units called kilowatt hours (kWh) which is about equal to the transfer of 3.6 megajoules of energy. If you leave an appliance with a power rating of one kW running for one hour, it will consume one kWh; if you switch it off after half an hour, it will only have used 0.5 kWh. A two kW appliance running for 30 minutes will also use one kWh of electricity and so on. In other words, the units of electricity used is calculated by multiplying the power in kilowatts by the length of time the electrical appliance was switched on. Something that you need to be careful of is the confusion between kilowatts which is the power of an appliance and kilowatt hours which is a measure of the energy used by an appliance.

If you need only to compare two devices that have been running for just a few minutes (or seconds), you can take a short cut by multiplying the power by the time in minutes (or seconds). The one that gives the bigger number will have used more energy, just remember that for the correct values in kWh, you need to convert the minutes to a fraction of an hour first.

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  1. Which of the following items use electricity in the home?
    Hang your head in shame if you got that one wrong!!
  2. Which does not normally rely on electricity?
    OK, that's the end of the warm up questions!
  3. If a 3 kW kettle is switched on for 3 minutes, when compared to a 2 kW toaster switched on for 5 minutes, which uses more energy?
    Using simple ratios often can help to work the answer out - three times three for the kettle as opposed to two times five for the toaster. Working it out correctly, the kettle uses 0.15 kWh whilst the toaster uses 0.17 kWh of power
  4. Another kettle has a power rating of 3 kW of power and transfers 2 kW of that into heat energy to boil water. How efficient is the kettle?
    Efficiency is calculated by dividing the output by the input and converting the resulting fraction to a percentage by multiplying by one hundred
  5. What is an advantage of using a kettle instead of a pan on a gas cooker to boil water?
    Even with a lid on the pan, more of the energy will end up in the water when using the kettle
  6. Which would be safest and most efficient for toasting bread?
    Minimizing risk and maximizing efficiency is always a major factor in the design of appliances for specific purposes. Not all exam questions will make it as obvious as this, so be on the lookout for this type of question in your GCSE exams and tests
  7. Which appliance would work if the electricity was not working in a house?
    Whilst many appliances run off mains electricity, it is possible to rely on other sources of energy to power appliances. Finding a balance between appliances could not only save you money but also save harmful gases from being released into the atmosphere as a result of how electricity is generated by power companies. In this case, an electric lawn mower would be the compromise - you can't cut the lawn during a power cut but does that really matter?
  8. What is a disadvantage of an oven?
    As ovens have large capacities, it is sometimes inefficient to cook small items in it. Instead, see if it is possible to use a microwave or a slow cooker as both have smaller capacities and are more efficient for smaller items
  9. How much power does a kettle use if it transfers 1,000 joules of energy in 10 seconds?
    power = energy transferred divided by the time taken to transfer the energy. One watt is a rate of transfer of energy of one joule per second
  10. If an electric hob uses 75 W of power in 300 seconds to boil water in a pan, whilst a kettle uses 100 W to boil the same mass of water in 150 seconds, which is the more energy efficient appliance to use for this process?
    This is a quantitative variation of question 5. From the equation linking energy, time and power you can calculate that the kettle uses only 15,000 J whereas the hob uses 22,500 J

Author: Martin Moore

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