This Physics quiz is called 'Electricity - The Usefulness of Electrical Appliances' and it has been written by teachers to help you if you are studying the subject at senior high school. Playing educational quizzes is one of the most efficienct ways to learn if you are in the 11th or 12th grade - aged 16 to 18.
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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 cell 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!
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 senior high school exams! With some very basic knowledge of senior high school 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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