Microwave Radiation
In this lab we will measure properties of electromagnetic microwave radiation using microwave ovens.
Make sure the rotating turntable is removed from the microwave oven before beginning; we will not be using it.
FOR ALL PARTS OF THIS LAB, PLACE A SMALL, OPEN CONTAINER OF COOL WATER SOMEWHERE IN THE MICROWAVE OVEN. If the microwaves have nothing to cook, they will find their way back to the magnetron that made them, and damage or destroy the oven.
PART 1 – Standing waves and wavelengths
1. Microwave ovens use standing waves to cook food. This means that nodes, where the amplitude is zero (where the wave crosses the x-axis), remain at nearly fixed locations in the oven, and cooking won't occur at those locations. (That's why microwave ovens employ turntables, to rotate your food so that it can't stay in a "cold spot.") Your objective for this task is to locate the hot spots and cold spots in the oven.
Use dry cobalt chloride soaked paper (it should be blue), and thoroughly and evenly dampen it with a wet sponge (it should turn pink). If any one spot on the paper is significantly more wet than another, your results probably won't make sense. This paper is a water indicator: when it is cooked with microwaves, it will dry, causing it to turn blue again. Cook the damp, pink paper (make sure there's a separate water bottle in the oven too!) enough that about half of it turns blue again. Measure the typical distance between blue spots and pink spots with a ruler.
CAUTION: Cobalt Chloride is a slightly toxic irritant. Whoever handles the paper should (a) avoid touching his/her eyes and mouth, and (b) wash his/her hands when done with the paper.
2. Repeat this process (re-wet and re-cook the paper) to find a few hot and cold spots throughout the oven cavity. With each time you cook the indicator paper, measure the typical distance between blue and pink spots. Keep track of all these measurements (make a list). NOTE: If this is taking a long time, it may be sufficient to do part of the oven cavity, half maybe, and assume the results are the same for the other half.
To do this, use plastic baskets, tupperware, and bamboo skewers to make raised platforms on which to place damp cobalt chloride paper. Measure the height of the paper each time. Also: are the walls of the oven hot spots or cold spots?
3. Try to figure out the wavelength of the microwaves. To do this, use your measurements of distances between pink and blue spots, and figure out how to convert that distance into the wavelength. (Is that distance half a wavelength, for example?) Try to figure out which sections of cooked indicator paper give you the best measurements for this purpose. You may need to re-measure with the cobalt chloride paper. Typical cooking time here is ~30 seconds.
4. Use rulers to estimate the x-y-z dimensions of the oven cavity. How do your results compare to the wavelength you measured for the microwaves? Can you explain why the oven dimensions are what they are.
5. Use your measured wavelength, and the constant speed of light, to calculate the frequency of the microwaves. Verify that they really are in the microwave band. (Most microwave ovens are 2.45 GHz, although these ovens didn't come with a printed value. Use units to verify that the speed of a wave should equal wavelength x frequency, λf.)
PART 2 – Little holes...
Since a conductor can absorb and reflect microwaves, it can also be used as a shield to block microwaves.
1. Cook a marshmallow in a disposable plastic cup. Stop the oven fast so it doesn't blow up - you don't want to clean goopy marshmallow out of your oven. Time it: how long does it take to cook like this, say, until it's swelling out of the cup?
2. Cook a marshmallow in a plastic cup again, but this time wrap it in aluminum foil, with no holes. Try to smooth out the foil so there won't be much sparking, and put the cup on a small platform such that its foil isn't close to any of the walls, ceiling, or floor of the oven cavity. (And put water in the oven too.) Watch for sparks: they could melt the plastic cup and ruin your experiment, as well as cause some damage the oven floor or walls. Cook for however long you cooked the marshmallow in the previous step. Carefully remove the foil/cup/marshmallow and unwrap to see if the marshmallow cooked or not. CAUTION: For your safety, do not place the foil/cup near to the oven door. Also, the foil is not a perfect conductor, so it has some (small) resistance. Therefore it can get hot, so be careful taking it out of the oven.
3. Repeat the last step with the foil wrapped marshmallow, but this time poke lots of small holes in the foil with a pencil before putting it in the microwave oven. Cook for the time you measured in step 1 (of part 2) above, and see if the marshmallow cooked or not. Try bigger holes. Figure out how big the holes have to be before the marshmallow starts to get cooked. Smooth out the foil in the vicinity of the holes as you make them.
4. Figure out what's going on with the holes. (a) How might their size relate to the wavelength? (b) What is the nature of the little holes in the microwave oven door, that you can see your food through? (c) Why are some radio dishes, satellite TV receivers, and radio telescopes made from a wire mesh, rather than a solid metal dish? Could a curved mirror in an optical telescope have pencil sized holes in it without significantly degrading it's functionality? (d) Why do x-rays (whose wavelengths range down to about 10-12 meters) go through your body? How big are the "holes" in your skin, say?
PART 3 – present
your results
Your instructor will choose people from each team to present their team's findings to the class for argument, discussion, etc.
|