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 Heater
_Materials you will need_:
1 dry cell battery 1 foot iron picture wire Pliers
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" Heat
Conduction 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 Air
Convection 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 Sun
Radiation 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 Ways
How do you make popcorn? Did you know that you can do this kind of a heating job three different ways?
_Materials Needed_
Popcorn Cooking oil or shortening Salt and b.u.t.ter 4-qt. saucepan, with cover. (A gla.s.s cover is preferred.) Potholder Electric range 2 250-watt heatlamps 2 spring clamp type lampholders Wire mesh corn popping basket or wire mesh kitchen strainer (improvise a screen wire cover)
_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, b.u.t.ter 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.
LESSON NO. B-13
Credit Points 2
MYSTERIOUS MAGNETISM
In ancient times, people found certain rocks that clung together in bunches. These rocks were very mysterious. People didn"t understand them and many superst.i.tions grew up about lodestones, as these rocks were called. Lodestone (sometimes spelled loadstone) means leading stone.
People even told Columbus not to sail out of sight of land because a giant lodestone was just over the horizon waiting to pull all the nails out of his ships.
The Chinese were the first to use magnets. They found that if you hung a lodestone by a string, one end of the stone would always point in the direction of the North Star. They had the first magnetic compa.s.ses.
An artificial magnet can be made by stroking or gently rubbing a piece of steel with a lodestone. This piece of steel then can be used to magnetize another piece of steel. This can be continued on and on.
Lodestones are not always available but you can get the same results with an electric current. So, magnetism and electricity are very closely related.
What to Do
Learn about magnetism by doing the experiments that follow.
Seeing is believing!
Materials You Will Need