Physics Part 27

CHAPTER VII

HEAT, ITS PRODUCTION AND TRANSMISSION

(1) SOURCES AND EFFECTS OF HEAT

=138. Importance of the Study of Heat.=--Heat is brought to our attention through the sensations of heat and cold. In winter, we warm our houses and prevent the escape of heat from them as much as possible.

In summer we endeavor to keep our living rooms cool and our bodies from being overheated.

A clear understanding of the several _sources_, _effects_, and _modes of transferring_ heat is of importance to everyone living in our complex civilization, especially when we consider the mult.i.tudes of objects that have as their princ.i.p.al use the _production, transfer or utilization_ of heat.

=139. Princ.i.p.al Sources of Heat.=--_First_ and most important is the _Sun_, which is continually sending to us _radiant energy_ in the form of light and heat waves. These warm the earth, make plants grow, evaporate water, besides producing many other important effects.

_Second_, _chemical energy_ is often transformed into heat. One has but to think of the heat produced by burning coal, wood, oil, and gas, to recognize the importance of this source. Chemical energy is also the source of the heat produced within our bodies. The action of quicklime and water upon each other produces much heat. This action is sometimes employed during balloon trips as a means of warming things.

_Third_, _Electrical Energy_.--In many cities electric cars are heated by the electric current. We have all heard of electric toasters and other devices for heating by electricity. _Electric_ light is produced by the heating of some material to incandescence by an electric current.

The _electric furnace_ has a wide application in the preparation and refining of metals.

[Ill.u.s.tration: FIG. 120.--Boy-scout method of making fire by friction.]

_Fourth_, heat is also produced whenever _mechanical energy_ of motion is overcome, whether it be by _friction_, _concussion_, or _compression_. Friction _always_ results in the production of heat, as when we warm our hands by rubbing them together. When friction is excessive, such as in the case of a heavy bearing not properly oiled, the bearing may get very hot. This is the cause of the "hot box" on a railway car. Friction may produce heat enough to set wood on fire. Some fires in mills are believed to be due to this cause. Every _boy scout_ must learn how to produce fire by friction. (See Fig. 120.) _Concussion_ may be ill.u.s.trated by the heating of a piece of metal by hammering it, while the compression of a gas always makes it warmer, as those who have used a bicycle pump have observed. The production of heat by compressing a gas is ill.u.s.trated by the "fire syringe" (Fig. 121). This consists of a gla.s.s tube with a tightly fitted piston. A sudden compression of the air contained may ignite a trace of carbon bisulfid vapor.

[Ill.u.s.tration: FIG. 121.--A fire syringe.]

The _interior of the earth_ is hot, but its heat seldom gets to the surface except at _hot springs_ and _volcanoes_.

=140. The Effects of Heat.=--There are five important changes produced by heat: (a) change of _size_, (b) change of _temperature_, (c) change of _state_, as the melting of ice or evaporating of water, (d) _chemical_ change, as the charring of sugar when it is overheated, and (e) _electrical_ change. This is ill.u.s.trated by the production of an electric current, by the heating of the junction of two different metals. A thermo-electric generator (see Fig. 122) has been constructed upon this principle and works successfully.

[Ill.u.s.tration: FIG. 122.--A thermo-electric generator.]

Important Topics

1. Importance of a study of heat.

2. Four sources of heat.

3. Five effects of heat.

4. Examples of each.

5. Ill.u.s.trations of transformation of energy which involve heat.

Exercises

1. Write a list of the _sources_ of heat in the order of their importance to you. State why each is important to you.

2. Which _three_ of the _effects_ of heat do _you_ make most use of?

Explain what use you make of each of these effects.

3. Which of the forms of energy can be transformed into heat? How in each case?

4. Into what other forms of energy may heat be transformed? Name the device or process used in each case.

5. What five different commodities are purchased by people in your neighborhood for the production of heat? Which of these costs least for the amount of heat furnished? Which is most expensive? How do you determine these answers?

6. Why do many people buy heat in an expensive form, as in using an electric toaster, when they can obtain it in a cheaper form by burning gas or coal?

7. How many of the five effects of heat have you observed outside of school?

(2) TEMPERATURE AND EXPANSION

=141. Heat and Temperature.=--We should now clearly distinguish between the terms, _heat_ and _temperature_. Heat is _a form of energy consisting of molecular motion_. The temperature of a body is its _degree of hotness_. The _amount of heat_ present in a body and its _temperature_ are very different things. The temperature refers to the intensity of the heat in the body. A quart of water and a red hot iron ball may contain _equal amounts_ of heat, although the ball has a _much higher temperature_ than the water. A cup of boiling water will have the same temperature as a tank full of boiling water, but the tank will contain more heat. Every one knows that it will take longer to boil a kettle full of water than a cupful. A hot-water bag, holding 2 quarts of water will give off heat longer than a 1-quart bag, both being filled with water at the same temperature. To put it in another way, more work is done in heating a large amount of water, than a small amount through the same change of temperature.

=142. Units of Heat and Temperature.=--There are two common units for measuring heat: the _Calorie_ and the _British thermal unit_. The _calorie is the amount of heat required to raise the temperature of a gram of water one centigrade degree_. The British thermal unit is _the amount of heat required to raise the temperature of one pound of water one Fahrenheit degree_. One of the units plainly belongs to the metric system, the other to the English.

An instrument for measuring temperature is called a _thermometer_.

Various scales are placed upon thermometers. The two thermometer scales most commonly used in this country are the _Centigrade_ and the _Fahrenheit_. The _Fahrenheit thermometer scale_ has the temperature of melting ice marked 32. The boiling point or steam temperature of pure water under standard conditions of atmospheric pressure is marked 212 and the s.p.a.ce between these two fixed points is divided into 180 parts.

The centigrade thermometer scale has the same fixed points marked 0 and 100 and the s.p.a.ce between divided into 100 parts. (See Fig. 123.) The centigrade scale is the one used by scientists everywhere.

[Ill.u.s.tration: FIG. 123.--Comparison of centigrade and Fahrenheit scales.]

=143. Comparison of Thermometer Scales.=--It is often necessary to express in centigrade degrees a temperature for which the Fahrenheit reading is given or _vice versa_. Since there are 180 Fahrenheit degrees between the "fixed points" and 100 centigrade degrees, the Fahrenheit degrees are smaller than the centigrade, or 1F. = 5/9C. and 1C. = 9/5F. One must also take into account the fact that the melting point of ice on the Fahrenheit scale is marked 32. Hence the following rule: To change a Fahrenheit reading to centigrade subtract 32 and take 5/9 of the remainder, while to change centigrade to Fahrenheit multiply the centigrade by 9/5 and add 32 to the product. These two rules are expressed by the following formulas.

(F. - 32)5/9 = C., 9C./5 + 32 = F.

Another method of changing from one thermometric scale to another is as follows:

A temperature of -40F. is also _represented_ by -40C., therefore to change a Fahrenheit reading into centigrade, we add 40 to the given reading, then divide by 1.8 after which subtract 40. To change from a centigrade to Fahrenheit reading the only difference in this method is to multiply by 1.8 or

C. = (F. + 40)/1.8 - 40 and F. = 1.8(C. + 40) - 40.

[Ill.u.s.tration: FIG. 124--Comparison of absolute, centigrade and Fahrenheit scales.]

=144. The Absolute Scale of Temperature.=--One often hears the statement "as cold as ice." This expresses the incorrect idea that ice cannot become colder than its freezing temperature. The fact is that ice _may be cooled_ below freezing down to the temperature of its surroundings.

If a piece of ice is placed where the temperature is below the melting point, the ice, like any other solid, cools to the temperature of the surrounding s.p.a.ce. For example, a piece of ice out of doors is at 10F.

when the air is at this temperature. It follows then, that when ice has been cooled below the freezing temperature that heat is required to warm the ice up to its melting point; or in other words that ice at its melting temperature possesses some heat. The temperature at which absolutely no heat exists is called _absolute zero_. There has been devised an _absolute scale of_ temperature. This scale is based upon the centigrade scale, _i.e._, with 100 between the two fixed points; the scale, however, extends down, below the centigrade zero, 273, to what is called _absolute zero_. It follows therefore that upon the absolute scale, the melting point of ice, and the boiling point of water are 273 and 373 respectively. (See Fig. 124.)

The means employed to find the location of absolute zero are of much interest. It has been observed that when heated a gas tends to expand.

If a measured volume of air at 0C. is cooled or heated 1C., it changes its volume 1/273, the pressure remaining the same. If it is cooled 10 it loses 10/273, if cooled 100 it loses 100/273 and so on. No matter how far it is cooled the same rate of reduction continues as long as it remains in the gaseous state. From these facts it is concluded that if the cooling could be carried down 273 that the volume would be reduced 273/273 or that the volume of the gas would be reduced to nothing. This is believed to mean that the molecular motion const.i.tuting heat would cease rather than that the matter composing the gas would disappear.

Scientists have been able to obtain temperatures of extreme cold far down on the absolute scale. Liquid air has a temperature of -292F., or -180C. or 93A. The lowest temperature thus far reported is 1.7A. or -271.3C., obtained in 1911, by evaporating liquid helium.

=145. The Law of Charles.=--The facts given in the last paragraph mean that if 273 ccm. of a gas at 0C. or 273 A. are cooled 100, or to -100C., or 173A., then it will lose 100/273 of its volume or have a volume of 173 ccm. If warmed 100, or up to 100C., or 373A., it will have a volume of 373 ccm. It follows then that in every case the volume will correspond to its absolute temperature, providing the pressure remains unchanged. The expression of this fact in scientific language is called the law of _Charles_. _At a constant pressure the volume of a given ma.s.s of gas is proportional to its absolute temperature._

Expressed mathematically, we have _V_{1}/V_{2} = T_{1}/T_{2}_. Compare the statement and mathematical expression of the laws of Charles and Boyle.