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YOUR MICROWAVE OVEN Sometimes technicians will perform this experiment to check that the power level of a microwave oven matches its specifications. We'll do a simplified version, which won't be quite as accurate, but should give you a result close to the value stated on your oven. You'll need a large plastic (microwave-safe) jug, one which measures the capacity in fractions of a litre. You'll get a more accurate result if it measures in ml. It should hold between 1 and 3 litres. You'll also need a reasonably good Celsius thermometer. You'll also need a wooden stirring implement, some water, and of course, a microwave oven. Fill the jug with a measured amout of water, and record this amount. Give it a stir, and use the thermometer to record the water's temperature. Now place the jug of water in the microwave oven and heat it on 'high' for a short length of time. About one minute per litre of water will work. We're just trying to heat the water a little, not make it dangerously hot. After the microwave finishes heating the water, take it out, give it a stir, and measure and record its temperature again. Also record the heating time you chose. The first step in measuring the oven's power is to calculate how much the temperature of the water went up. You can do this by subtracting the initial temperature from the final temperature. Call this number the rise in temperature. Next you need the volume of water in cubic centimetres. (If your container measures in litres, multiply the value by 1000). To calculate the power of the microwave oven, use the formula: Your answer will be in joules per second, or watts. A small microwave will have a power rating of 800 watts or so, with bigger ovens being more powerful. You can find the rating for your oven on a sticker on the back of the oven. If its more than 1000 watts, it may be reported in kilowatts: 1800 watts would be 1.8 kw. How close did you come to the actual value? We had to stir the water to distribute the heat. This is particularly important after heating the water, since as you discovered on the previous page, microwave ovens don't heat things evenly ... there are hot spots. Incidentally, the 4.187 used in the formula above is the energy (in Joules) required to heat 1 cc of water by 1 degree Celsius. This amount of energy is called 1 calorie. (Don't confuse this with 'Calories' in food, which are actually 1000 calories) Your results won't be particularly accurate for a number of reasons. Were you able to find a container to give a volume precise to the nearest cubic centimetre? Probably not. In addition, some of the heat from the water escaped into the jug as the water was being heated, and once you removed the container from the oven, it began to cool. Both of the latter effects will have caused your 'temperature rise' value to be inaccurate. Other related pages on our site you might like to visit include Microwave Boilovers ... why your instant coffee causes the microwaved hot water to 'reboil', and Energy ... what is it? what forms can it take? |