Physics Part 30

[I] Radiant heat is really _radiant energy_ and becomes heat when it is absorbed by a body.

When radiant energy falls upon any object it may be (a) _reflected_ at the surface of the object, (b) _transmitted_ through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. _Well-polished surfaces are good reflectors._ Rough and blackened surfaces are _good absorbers_.

Transparent objects are those which transmit light well, but even they absorb some of the energy.

=154. The Radiometer.=--Radiant heat may be detected by means of the radiometer (Fig. 135). This consists of a gla.s.s bulb from which the air has been nearly exhausted. Within it is a wheel with four vanes of mica or of aluminum mounted on a vertical axis. One side of each vane is covered with lampblack, the other being highly polished. when exposed to radiant heat from any source the vanes revolve with the bright side in advance.

The bulb is so nearly exhausted of air that a single molecule remaining may travel from the walls of the bulb to the vanes without coming in contact with another molecule.

The blackened sides absorb more heat than the highly polished sides. The air molecules striking these blackened sides receive more heat and so rebound with greater velocity than from the other side, thus exerting greater pressure. The blackened sides therefore are driven backward. If the air were not so rarified the air molecules would hit each other so frequently as to equalize the pressure and there would be no motion.

[Ill.u.s.tration: FIG. 135.--A radiometer.]

_Sun's Radiation._--Accurate tests of the amount of the sun's radiation received upon a square centimeter of the earth's surface perpendicular to the sun's rays were made at Mt. Wilson in 1913. The average of 690 observations gave a value of 1.933 calories per minute. These results indicate that the sun's radiation per square centimeter is sufficient to warm 1 g. of water 1.933C. each minute. Although the _nature_ of _radiation_ is not discussed until Art. 408-411 in light, it should be said here that all bodies are radiating heat waves at all temperatures, the heat waves from cool bodies being much longer than those from hot bodies. Gla.s.s allows the short luminous waves to pa.s.s through freely but the longer heat waves from objects at the room temperature pa.s.s through with difficulty. This is the reason why gla.s.s is used in the covering of greenhouses and hot beds. Water also absorbs many of the longer heat waves. It is therefore used in stereopticons to prevent delicate lantern slides from being injured by overheating.

Important Topics

1. Conduction in solids, liquids, gases.

2. Non-conductors; uses, best non-conductors.

3. Radiation, three characteristics.

4. The sun's radiation, amount. The radiometer.

Exercises

1. Does clothing ever afford us heat in winter? How then does it keep us warm?

2. Why are plants often covered with paper on a night when frost is expected?

3. Will frost form in the fall of the year sooner on a wooden or a cement sidewalk? Why? On which does ice remain longer? Why?

4. Why in freezing ice-cream do we put the ice in a wooden pail and the cream in a tin one?

5. Is iron better than brick or porcelain as a material for stoves?

Explain.

6. Which is better, a good or a poor conductor for keeping a body warm?

for keeping a body cool?

7. Should the bottom of a teakettle be polished? Explain.

8. How are safes made fireproof?

9. Explain the principle of the Thermos bottle.

10. Explain why the coiled wire handles of some objects as stove-lid lifters, oven doors, etc., do not get hot.

(5) TRANSMISSION OF HEAT IN FLUIDS. HEATING AND VENTILATION

=155. Convection.=--While fluids are poor conductors, they may transmit heat more effectively than solids by the mode called _convection_. To ill.u.s.trate: if heat is applied at the _top_ of a test-tube of water, the hot water being lighter is found at the top, while at the bottom the water remains cold. On the other hand, if heat is applied at the _bottom_ of the vessel, as soon as the water at the bottom is warmed (above 4C.) it expands, becomes lighter and is pushed up to the top by the colder, denser water about it. This circulation of water continues as long as heat is applied below, until all of the water is brought to the boiling temperature. (See Fig. 136.)

When a liquid or a gas is heated in the manner just described, the heat is said to be transferred by _convection_. Thus the air in the lower part of a room may receive heat by conduction from a stove or radiator.

As it expands on being warmed, it is pushed up by the colder denser air about it, which takes its place, thus creating a circulation of the air in the room. (See Fig. 137.) The heated currents of air give up their heat to the objects in the room as the circulation continues. These air currents may be observed readily by using the smoke from burning "touch paper" (unglazed paper that has been dipped into a solution of pota.s.sium nitrate ["saltpeter"] and dried).

[Ill.u.s.tration: FIG. 136.--Convection in a liquid.]

=156. Draft of a Chimney.=--When a fire is started in a stove or a furnace the air above the fire becomes heated, expands, and therefore is less dense than it was before. This warm air and the heated gases which are the products of the combustion of the fuel weigh less than an equal volume of the colder air outside. Therefore they are pushed upward by a force equal to the difference between their weight and the weight of an equal volume of the colder air.

The chimney soon becomes filled with these heated gases. (See Fig. 138.) These are pushed upward by the pressure of the colder, denser air, because this colder air is pulled downward more strongly by the force of gravity than are the heated gases in the chimney.

Other things being equal, the taller the chimney, the greater the draft, because there is a greater difference between the weight of the gases inside and the weight of an equal volume of outside air.

[Ill.u.s.tration: FIG. 137.--Convection currents in a room.]

[Ill.u.s.tration: FIG. 138.--Fire place showing draft of a chimney.]

=157. Convection Currents in Nature.=--Winds are produced by differences in the _pressure_ or _density_ of the air, the movement being from places of high toward places of low pressure. One of the causes of a difference in density of the air is a difference in temperature. This is ill.u.s.trated by what are called the _land_ and _sea breezes_ along the sea sh.o.r.e or large lakes. During the day, the temperature of the land becomes higher than that of the sea. The air over the land expands and being lighter is moved back and upward by the colder, denser air from the sea or lake. This const.i.tutes the _sea breezes_ (Fig. 139). At night the land becomes cooler much sooner than the sea and the current is reversed causing the _land breeze_. (See Fig. 140.)

[Ill.u.s.tration: FIG. 139.--Sea breeze.]

[Ill.u.s.tration: FIG. 140.--Land breeze.]

The _trade winds_ are convection currents moving toward the hot equatorial belt from both the north and the south. In the hot belt the air rises and the upper air flows back to the north and the south. This region of ascending currents of air is a region of heavy rainfall, since the saturated air rises to cool alt.i.tudes where its moisture is condensed. The _ocean currents_ are also convection currents. Their motion is due to prevailing winds, differences in density due to evaporation and freezing, and to the rotation of the earth, as well as to changes in temperature.

=158. The heating and ventilation of buildings= and the problems connected therewith are matters of serious concern to all who live in winter in the temperate zone. Not only should the air in living rooms be comfortably heated, but it should be continually changed especially in the crowded rooms of public buildings, as those of schools, churches, and a.s.sembly halls, so that each person may be supplied with 30 or more cubic feet of fresh air per minute. In the colonial days, the _open fire place_ afforded the ordinary means for heating rooms. This heated the room mainly by _radiation_. It was wasteful as most of the heat pa.s.sed up the chimney. This mode of heating secured ample _ventilation_. Fire places are sometimes built in modern homes as an aid to ventilation.

Benjamin Franklin seeing the waste of heat in the open fire places devised an iron box to contain the fire. This was placed in the room and provided heat by conduction, convection, and radiation. It was called _Franklin's stove_ and in many forms is still commonly used. It saves a large part of the heat produced by burning the fuel and some ventilation is provided by its draft.

[Ill.u.s.tration: FIG. 141.--Heating and ventilating by means of a hot-air furnace.]

=159. Heating by Hot Air.=--The presence of stoves in living rooms of homes is accompanied by the annoyance of scattered fuel, dust, ashes, smoke, etc. One attempt to remove this inconvenience led to placing a large stove or fire box in the bas.e.m.e.nt or cellar, surrounding this with a jacket to provide a s.p.a.ce for heating air which is then conducted by pipes to the rooms above. This device is called the hot-air furnace.

(See Fig. 141.) The heated air rises because it is pushed up by colder, denser air which enters through the cold-air pipes. The _hot-air furnace_ provides a good circulation of warm air and also ventilation, provided some cold air is admitted to the furnace from the outside. One objection to its use is that it may not heat a building evenly, one part being very hot while another may be cool. To provide even and sufficient heat throughout a large building, use is made of _hot water_ or _steam heating_.

[Ill.u.s.tration: FIG. 142.--A hot-water system of heating.]

[Ill.u.s.tration: FIG. 143.--One-pipe system of steam heating.]

=160. Hot-water Heating.=--In hot-water heating a furnace arranged for heating water is placed in the bas.e.m.e.nt. (See Fig. 142.) Attached to the top of the heater are pipes leading to the radiators in the various rooms; other pipes connect the radiators to the bottom of the boiler.

The heater, pipes, and radiators are all filled with water before the fire is started. When the water is warmed, it expands and is pushed up through the pipes by the colder water in the return pipe. The circulation continuing brings hot water to the radiator while the cooled water returns to the heater, the hot radiators heating the several rooms.

=161. Steam Heating.=--In _steam heating_ a steam boiler is connected to radiators by pipes. (See Fig. 143.) The steam drives the air out of the pipes and radiators and serves as an efficient source of heat. Heating by steam is _quicker_ than heating with hot water. It is therefore preferred where quick, efficient heating is required. Hot water is less intense and more economical in mild weather and is often used in private homes.

[Ill.u.s.tration: FIG. 144.--Heating by an indirect radiator with side-wall register.]

=162. Direct and Indirect Heating.=--In heating by _direct radiation_ (Figs. 142, 143), the steam or hot-water radiators are placed in the rooms to be heated. With direct radiation, ventilation must be provided by special means, such as opening windows, doors, and ventilators.