  Credit Points 3 HOW ELECTRICITY HEATSIn ancient times, people thought that heat was a material just as air is. They called it "caloric". When something got warm, they said, caloric flowed into it. When something cooled off, caloric flowed out of it. It did not bother them that they could not see caloric. They could not see air either! Now we know that heat is not a material. It does not take up space. It does not weigh anything. Instead, it is a form of energy. And when we say that heat is a form of energy, we mean that it can be used to do work. What to Do1. Make a simple resistance heater. 2. Make some popcorn by: (a) conduction (b) convection (c) radiation "Resistance" Makes HeatThere are at least four ways that electricity can make heat. The one that we'll cover here is resistance heating. (The others are: dielectric heating, where the lines of force of an electrostatic field pass through a non-conductive material and heat it; the heat pump, which is a refrigerator in reverse; and electronic heating, which uses high frequency waves similar to radio waves to create high speed movement of the molecules or tiny particles which rub together to make heat.) Resistance heating occurs because every conductor of electricity opposes the flow of current through it. Some conductors resist more than others. When they do, a certain amount of warming takes place. The more resistance that is offered, the more heating there is. Some materials, like silver, copper, and aluminum, offer little resistance. We say they are good conductors. Other materials, like iron, offer more resistance. They are still conductors, but not as good as the others mentioned. The size of the conductor, and its length are the other two things that affect its resistance. The smaller it is, the greater its resistance. Also, the longer it is, the greater its resistance. Therefore, when we only want to move electricity from place to place, we want relatively large, "good" conductors. Here, we do not want to make heat. In fact, we want to avoid it, because too much heat in the wrong place can cause a fire. But when we want heat, we choose relatively small, "poor" conductors, and the more heat we want, the longer they must be. If you will think of the filament inside a lamp bulb; you may recall that it is a very fine wire, coiled so as to get a maximum length, and made of tungsten which has a high resistance. Because of all these factors, this filament glows at a white heat, and is a source of both light and heat. Make a Simple Resistance HeaterMaterials you will need: Use a short strand of iron picture wire and hook the ends to the terminals of a dry cell battery. Use pliers so that you do not burn your fingers. Disconnect the wires as soon as they become hot. Tell why the wires heat. Conduction is "Touching" HeatConduction occurs when you set a pan containing food right on a heating element. An egg cooking in a hot frying pan is a good example of conduction at work. This method is the most efficient single way of using electric heat for cooking. Convection Depends on AirConvection warms food in pans that are not actually touching the heating element. It uses the hot air around the element to carry heat to the pan. Your oven in your range works by convection. Most houses are warmed in winter in the same way. The heat produced in a furnace warms the air as it circulates through. This air in turn keeps your body warm. Radiation is Like the SunRadiation heating is more difficult to explain. It results when heat or energy waves strike an object and are converted into heat. The energy we receive from the sun is a good example. When you are wearing dark clothes on a chilly day, you may become uncomfortably hot. The sunshine warms you even though the air around you has not been heated. Radiant energy has a way of being absorbed by dark objects and reflected by light colored or shiny surfaces. Did you ever notice how snow melts faster on a black top road than it does on a concrete road? The electric heat lamp is one of the most familiar sources of radiant heat. Other examples are panels and cables that are built into the walls and ceilings of homes to provide heat. Make Popcorn 3 WaysHow do you make popcorn? Did you know that you can do this kind of a heating job three different ways? Materials Needed First, make popcorn the way you usually do. Set a front surface unit control on the range at "medium high". Pour enough oil to very lightly cover the bottom of the pan. When the pan is hot, pour in enough popcorn to cover the bottom with one layer of kernels. Use the potholder in one hand to hold the cover on, and with the other move the pan back and forth across the unit. When the popping stops, remove from the heat. How did the heat get to the popcorn? Second, make popcorn in the oven. Add the oil to the pan, cover it and put it in the oven. Turn the oven on, with the automatic control set at 400°. When the oven indicator light goes off, this means that the proper temperature has been reached. With the potholder, remove the pan and add one layer of popcorn kernels. Replace the pan in the oven. When the popping stops (listen for it) remove the pan. What kind of heating took place here? Third, make popcorn with the heat lamps. Clamp the lampholders to the back of a chair or other vertical support. They should be 6 to 8 inches apart and pointed directly at each other. Put about 2 tablespoonfuls of popcorn in the Wire basket or strainer. Do not add oil. Hold the basket midway between the two lamps. When the popping stops, turn off the lamps. What kind of heating was this? Now, butter and salt the popcorn you have made and share it with others. What Did You Learn?1. How is heat transferred from one body to another? 2. Could chicks or pigs receive warmth from a heat lamp without the air in the pens becoming warm? Explain. 3. How does a broiler unit in a range cook meat? 4. How does an oven bake food? 5. Tell why iron picture wire was used instead of copper wire for your heating demonstration. |
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