SM0486
(a) When you increase the level of energy applied to the molecules of a
material (in this case R1 and R2), there is a proportional increase in the kinetic
energy in the material.
(b) The increase in the kinetic energy in
a material represents an
increase in the heat generated in the body.
(4) The wattage values for R1 and R2 in figure 2 were chosen arbitrarily.
From the energy developed in R1 and R2 as the applied voltage is increased, you can
imagine how the heat generated in each of these increases. From you experience in
testing electronic equipment, you know how hot some resistors get when they draw
too much current.
If you compare the wattage values of R1 with the wattage
developed when the switch is in the 5 position, you can see that R1 will probably
"burn out" and cause an open circuit.
(5) Electrical energy is just one of the basic forms of energy which can be
transformed into heat.
We used electrical energy as an example because we know
that you are familiar with many transformations of electrical energy to heat energy
which occur in electronic stoves, blankets, irons, and many other heating devices.
f. Temperature.
(1) The word "temperature" becomes important when you need to know the
intensity of heat in a body. You have watched water boil when fire from a stove
heats the water, or you have felt heat if you touched an object which has been
exposed to the sun. You have probably been in a room where the heat was so intense
that you could feel it on parts of your body. Our point is this: Regardless of how
heat is generated (sun, fire, friction, or other means), its generation causes an
increase in the motion of molecules in the material to which the heat is
transferred.
What you feel as "heat", however, depends on the intensity of the
heat at a particular spot, not on the total amount of heat. An all-metal poker,
for instance, may be too hot to touch at the tip, but perfectly comfortable to hold
by the handle.
(2) When you measure the temperature of a body, you are measuring the
intensity of heat rather than the amount of heat. The amount of heat possessed by
a body at a given temperature depends on its weight and its specific heat.
The
specific heat of a given material is the amount of heat necessary to raise the
temperature of a specific number of grams of that material 1. Specific heat can be
expressed in calories-per-pound-per-degree Celsius or in BTU's-per-pound-per-degree
Fahrenheit. The relationship between the quantity of heat (in BTU's) and specific
heat is involved in the problem which follows: PROBLEM: How many BTU's are
necessary to heat 5 pounds of iron from 80 to 100F.? Solution: The specific heat
of iron is 0.11 BTU/1 F. Therefore,
5 x 0.11 x (100 - 80)
= 0.55 x 20
= 11.00 BTU's
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