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 multitudes of objects that have as their principal use the _production, transfer or utilization_ of heat.

=139. Principal 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.

_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 illustrated 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 illustrated by the "fire syringe" (Fig. 121). This consists of a glass tube with a tightly fitted piston. A sudden compression of the air contained may ignite a trace of carbon bisulfid vapor.

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 illustrated 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.

Important Topics

1. Importance of a study of heat.

2. Four sources of heat.

3. Five effects of heat.

4. Examples of each.

5. Illustrations 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 space 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 space between divided into 100 parts. (See Fig. 123.) The centigrade scale is the one used by scientists everywhere.

=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 1°F. = 5/9°C. and 1°C. = 9/5°F. 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 -40°F. is also _represented_ by -40°C., 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.

=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 space. For example, a piece of ice out of doors is at 10°F. 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 0°C. is cooled or heated 1°C., 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 constituting 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 -292°F., or -180°C. or 93°A. The lowest temperature thus far reported is 1.7°A. or -271.3°C., 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 0°C. or 273° A. are cooled 100°, or to -100°C., or 173°A., then it will lose 100/273 of its volume or have a volume of 173 ccm. If warmed 100°, or up to 100°C., or 373°A., 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 mass 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.

The formulas for the laws of Boyle and Charles are sometimes combined into one expression as follows:

_PV/T = P´V´/T´_

or the product of the volume and pressure of a constant mass of gas is proportional to its absolute temperature.

Important Topics

1. Heat units; calorie, British thermal unit.

2. Three thermometer scales, fixed points on each.

3. Absolute zero, how determined. Its value on each scale.

4. Law of Charles, its meaning. Combination of laws of Boyle and Charles.

Exercises

1. Does ice melt at the same temperature at which water freezes? Express the temperature of freezing water on the three thermometer scales.

2. A comfortable room temperature is 68°F. What is this temperature on the centigrade and absolute scales?

3. Change a temperature of 15°C. to F.; 15°F. to C.; -4°C. to F.; -20°F. to C.

4. The temperature of the human body is 98.6°F. What is this temperature on the absolute and centigrade scales?

5. The temperature of liquid air is -180°C. What is it on the Fahrenheit scale?

6. Mercury is a solid at -40°F. What is this on the centigrade scale?

7. How much heat will be required to raise the temperature of 8 lbs. of water 32°F.; 5 lbs. 10°F.?

8. How much heat will be required to raise the temperature of 30 g. of water 43°C.; 20 g., 50°C.?

9. Compute the temperature of absolute zero on the Fahrenheit scale.

10. Take three basins of water, one hot, one cold, and one lukewarm. If one hand be placed in the hot water while the other is placed in the cold and after a few minutes both are placed in the lukewarm water, this water will feel cool to one hand and warm to the other. Explain.

11. If 200 ccm. of air at 200° absolute is heated to 300°A. under constant pressure, what volume will the air occupy at the latter temperature?

12. How does one change a reading on the centigrade scale to a corresponding reading on the absolute scale?

(3) EXPANSION OF LIQUIDS AND SOLIDS

=146. Expansion of Gases.=--The law of Charles is found to apply to all gases. That is, all gases change in volume in proportion to the change of temperature provided the pressure remains constant. It is for this reason that we have the _gas thermometer_ (see Fig. 126) which gives in skillful hands more accurate temperature readings than the best mercurial thermometer. Galileo devised and used the first _air thermometer_ which consisted of a hollow bulb blown on a glass tube and inverted in a dish of water. (See Fig. 1.) The _water thermometer_ consists of a glass bulb filled with water which rises into a tube attached to the bulb. One disadvantage of the water thermometer is its limited range since it cannot be used below 0° or above 100°. Why?

=147. Expansion of Liquids.=--The expansion of liquids differs from that of gases in several important respects:

(a) Liquids have a smaller rate of expansion than gases. The _rate_ of expansion per degree is called the _Coefficient of Expansion_. For example, the coefficient of expansion of a gas under constant pressure at 0°C. is {1/273} of its volume per degree centigrade.

(b) Different liquids expand at wholly different rates, that is, their coefficients of expansion differ widely. For example, the coefficient of expansion of mercury is 0.00018 per degree centigrade, of glycerine 0.0005 per degree centigrade, of petroleum 0.0009 per degree centigrade.

(c) The same liquid often has different coefficients of expansion at different temperatures. Water between 5°C. and 6°C. has a coefficient expansion of 0.00002 per degree centigrade, between 8° and 50° of 0.0006, between 99° and 100° of 0.00076. The coefficient of expansion of mercury, however, is constant for a wide range of temperature and, therefore, it is well adapted for use in thermometers.

=148. Peculiarity in the Expansion of Water.=--Water has a peculiar rate of expansion. This is illustrated by the following experiment:

A test-tube filled with cold water is closed by a stopper containing a small glass tube, the water extending up into the small tube. (See Fig. 127.) The test-tube is placed in a freezing mixture of salt and ice contained in a tumbler. As the water cools, the level of the water in the small tube at first _sinks_. But before the water freezes it _rises_ again, showing that after the water cools to a certain temperature that _expansion of the water occurs with further cooling_.

Careful tests show that the water on cooling contracts until it reaches 4°C. On cooling below this temperature it expands. For this reason, when the water of a lake or river freezes, the coldest water is at the surface. On account of this the ice forms at the top instead of at the bottom. If water contracted as it cooled to the freezing temperature the coldest water would be at the bottom. Freezing would begin at the bottom instead of at the surface. Lakes and rivers would freeze solid. In the summer only in shallow waters would all the ice melt. The result would be that fish and other aquatic life would be killed. Climate would be so changed that the earth might become uninhabitable. Since water is densest at 4°C. all the water in a lake or river, when it is covered with ice, is at 4°C. except that near the surface.

=149. The Expansion Of Solids.=--Most solids when heated expand less than liquids and gases. Careful experiments show that expansion is:

(a) Proportional to the change in temperature.

(b) Different in different solids.

Here are a few coefficients of linear (length) expansion.

Brass 0.000018 per degree C. Glass 0.000009 per degree C. Ice 0.000052 per degree C. Iron 0.000012 per degree C. Platinum 0.000009 per degree C. Zinc 0.000027 per degree C.

_The coefficient of linear expansion is the fraction of its length that a body expands when heated one degree._

_The coefficient of cubical expansion is the fraction of its volume that a body expands when heated one degree._

The expansion of solids is used or allowed for in many cases:

a. Joints between the rails on a railroad allow for the expansion of the rails in summer.

b. One end of a steel truss bridge is usually supported on rollers so that it can expand and contract with changing temperatures. (See Fig. 128.)