How Heat Works For Us
We have learned that there are three methods for transferring heat energy, namely: conduction, which takes place mainly in solids; convection, which takes place mainly in fluids, either gases or liquids; and radiation, which applies to the transfer of energy through space.
Conduction
The knowledge of these principles and the application of them are very important to our everyday life. In heating and cooling our homes, offices and factories, and in cooking food, we are interested in methods of transferring heat from one place to another. In other cases, such as in the use of refrigerators, we are mainly interested in preventing a transfer of heat. In other words, we control the transfer of heat so that we can get it to a place where it is needed or keep it out of places where it is not needed.
You already know that if you hold one end of a poker while the other end is placed in a bed of live coals, within a few minutes the entire poker becomes hot. The molecules of the hot coals are in very rapid vibration. The molecules of the iron poker which are near the coals receive some of this energy of vibration and in turn transmit this vibration to their less active neighbors. These transmit energy to the next molecules and so on, until the whole poker is heated. In no case does one molecule from the live bed of coals move along the poker to your hand; the molecules of a solid are held in the same positions with respect to one another. Only the vibrations are communicated along the poker.
Silver is the best conductor of heat known, copper is next, and gold and aluminum are not very far behind. Metals are much better conductors than other substances.
Feathers, fur, straw, wool and cork are poor conductors of heat. Liquids and gases in general are very poor conductors. A very poor conductor is called an insulator. The poor conductivity of such things as wool, fur and so on, is chiefly due to the fact that they contain such large air spaces. Substances which contain a large number of small air spaces are in general poor conductors.
We see therefore that substances differ widely in their ability to conduct heat. The following simple experiment will prove this point. Twist the ends of two thick wires of iron and copper together. Place some wax on the end of each wire and heat the twisted part in a flame. In a few minutes you will notice that the wax at the end of the copper wire will melt first. This proves the superior conducting power of copper.
If you have ever stood barefooted, on a tile floor, you know that your feet feel much colder than when you stand on a rug in the same room. Heat from your feet is quickly conducted to the tile. This proves that the tile is a better conductor of heat than the rug. You probably know that most modern cooking utensils are made of copper or aluminum; and now you know the reason why. They conduct well the heat from the stove to the food that is to be cooked.
Insulators
Many materials are useful, not because they are good conductors, but because they are poor conductors. Our woolen winter clothing, for instance. Air is a much poorer conductor of heat than wool and since there are many air spaces in wool, this material is one of the best heat insulators known. Wool clothing does not actually give us any warmth in winter; it prevents the heat of the body from escaping. Clothing made from the poorest conductors is “warmest.” Several light sweaters are warmer than one heavy sweater because there is a layer of non-conducting air between each two. The warmth of a fur coat is much appreciated by women, but its beauty is apparently appreciated more. If it were not so, fur coats would be worn with the fur on the inside instead of on the outside. Linen and cotton conduct heat twice as fast as wool and are therefore more suitable for summer clothing. On cold winter nights fowls on the roost spread their feathers to increase the size of the air spaces. A pad of flannel is good for lifting hot pans, and a wooden handle is put on a soldering iron because flannel and wood are poor conductors. Glass is also a poor conductor of heat. When hot tea is poured into a glass it is liable to crack because the inside of the glass gets heated first and expands, while the outside has not yet been heated—unless a good conductor, such as a metal spoon, is put into the glass to conduct the heat away.
The walls and doors of your refrigerator contain materials which are poor conductors of heat, such as sawdust and cork. They keep the heat of the room from being conducted into the refrigerator. While we are on the subject of refrigerators, it may be interesting to point out that they were an important part of the equipment of polar explorers. Can you tell why?
Furnaces and hot-water pipes are covered with asbestos or magnesia prepared in a form so as to contain a great number of air spaces. These substances will withstand high temperatures and are poor conductors of heat; so the heat is not wasted by leaking out through the walls of the furnace, or the pipes. Houses are built with double walls and sometimes with double roofs and double windows. The air spaces between the walls keep the heat from escaping in winter and the outside heat from coming in during the hot weather. Thus the house is warmer in winter and cooler in summer. We say such a house is well insulated. A well-insulated house needs less fuel than one that is not insulated.
Advertisements in newspapers and magazines now call your attention to many kinds of insulating materials that are used in the construction of houses.
Convection
We often warm our hands by holding them over a radiator or stove. Heat is carried from the stove or radiator to the hands by a stream of air. Thus we see that warm air is streaming upward from the source of heat to some colder place. The reason for this is that substances expand when heated, and their density is correspondingly decreased. This means that air over a heated surface is less dense than the surrounding air. The colder, heavier air will displace this lighter air and push it upward. Such convection currents may be produced in either liquids or gases. Ordinary ventilation depends upon convection. Air which is exhaled (breathed out) from your lungs is warmer and lighter than the cold air in a room. If the window is open at the top, this warm, used air will escape out the window, pushed up by the colder air which comes in from nearer the floor.
The hot gases in a ‘chimney are lighter than the air outside and the effect we call the draft is due to the greater pressure exerted by colder air. The speed with which the air is forced up the chimney depends in part on the difference in weight between the column of hot gases in the chimney and a column of outside air of the same height and cross section. The hotter the gases and the taller the chimney, the greater the draft.
This accounts for the tall chimneys constructed for factories. A stack built for smelting copper ores in Montana is 580 feet high.
A cheap and convenient heating system for a house is found in the hot-air furnace. This system consists of a stove with a jacket about it from the top of which pipes lead to the rooms to be heated. Through the pipes air is pushed up by convection currents.
Cold air is led into the base of the jacket where it is heated, in turn, and pushed up into the pipes by the colder air behind it. The cold air in each room is forced out through openings near the floors. In many of the more modern homes convection alone is not relied upon for the circulation of warm air. An electrically operated fan or blower circulates the air by pushing it through. In such cases the air is made to pass through a pad of loosely woven felt or other fibrous material to take out dust and smoke. Such systems are commonly referred to as air conditioning.
Land and sea breezes are also caused by convection. The land has a lower specific heat than the water. In other words, the land heats up more quickly than the water but it also loses its heat more quickly. Therefore during the daytime the land has a higher temperature than the water. The air over the land is pushed upward by the cooler air from the sea. About noon a cool sea breeze begins to blow toward the land. At night, the reverse is true; that is, the land cools more quickly, going below the temperature of the water. The warmer air over the water is forced upward and the winds consequently blow offshore, from the land toward the water. This is commonly known as a land breeze. For these reasons fishermen along the coast go to sea at night with the land breeze and return in the forenoon with the sea breeze.
In steam-heating systems, water is heated to boiling and the steam, which occupies about i,óoo times as much volume as the water had occupied, expands through the pipes and into the radiators. It is distributed by its own pressure throughout the system. When the steam reaches a radiator in a room, the cooler air outside the radiator causes the steam to condense, because heat must flow from a higher heat-level to a lower heat-level—from a hotter thing to a cooler thing. As enough heat leaves the steam, the steam becomes water; it condenses. As the steam condenses in the radiator, each gram sets free 540 calories of heat; this much heat was added to the gram of boiling water in order to convert it to steam. The heat from the radiators is distributed to the room by convection and by radiation. After condensation the water at a temperature below xoo degrees Centigrade returns to the boiler, usually through the same pipe. This process is repeated as long as the boiler produces steam.
Radiation
We have already spoken briefly about the process of transmission of energy without the aid of intervening molecules—radiation. If you stand before an open fire you are heated. Since the air is a non-conductor, you do not receive this heat by conduction. Since convection carries the heated air upward, you do not get the heat by convection. The energy must be transmitted to you by some other method. Heat comes to us from the sun across millions of miles of space where there is no material in which conduction or convection can take place. In such cases the heat is called radiant heat. Radiant heat may pass through objects without heating them. Energy, or radiant heat, from the sun passes through the upper layers of the earth’s atmosphere without heating them.
Glass permits short waves of radiant energy from the sun to penetrate, but not longer waves like those of a flame. If a pane of glass be held before a gas flame, it will transmit only a little of the heat and will become very hot because it has absorbed much of this heat. The reason is that the flame emits long waves. The sun’s heat, however, passes readily through a glass-enclosed greenhouse; yet the heat from inside the greenhouse can not escape through the glass. Heat comes from the sun through the atmosphere without heating it. The short waves from the sun can penetrate the atmosphere, but when they strike the earth they are absorbed and warm it up.
The earth radiates longer waves which are mostly absorbed by the surrounding atmosphere. If the atmosphere were not present, we would burn to death during the day and freeze to death at night. This is one of the reasons why life can not be maintained on the moon, which does not have a thick blanket of atmosphere. Orange-growers in Florida and California protect their crop from sudden frost by burning smudge pots. These smoky fires are built for the purpose of providing a layer of smoke which absorbs radiation from the earth, and thus provides a sort of extra blanket.
Absorption and Reflection of Heat
Have you ever wondered why light-colored clothes are worn in summer, or why the Arabian horses are white?
Surfaces differ in their ability to absorb radiant heat. Polished materials are good reflectors of heat; hence they are poor absorbers. Clean snow is a good reflector; hence it will not absorb much heat. This accounts for the fact that the snow in the country does not melt so rapidly as the snow in the city. In the city the snow gets dirty more rapidly and it melts faster. All black substances are found to be good absorbers of heat. Lay a black cloth and a white cloth in the sun on a cold day. In a short time you will find that more snow has melted under the black cloth than under the white cloth. The black cloth has absorbed more heat, and ha in turn, radiated more heat, and so melted the snow more quickly. Can you now see why light-colored clothes are worn in the summer? The Arabians use white horses because in that hot country dark-colored horses would more easily be exhausted from heat. A good absorber of heat is also a good radiator of heat; and a poor absorber of heat is a poor radiator of heat.
Refrigeration
In many homes a gas flame is used directly to produce ice in a refrigerator while in others an electrical motor is used. In either type of refrigerator we have one of the most interesting examples of repeated transmissions of energy. In the second type, the electric refrigerator, the energy of burning fuel is transformed into electrical energy at the power house; this energy is changed by means of a motor to mechanical energy y operating a pump which in turn compresses a gas until it liquefies. The heat produced when this gas is compressed is carried away by running water or by the circulation of air. We choose a gas which liquefies easily such as sulphur dioxide or the new commercial preparation “freon.” The cooled liquid then evaporates through a valve with a small opening into coils of pipe in the compartment of the refrigerator where the ice cubes are kept. The pressure in these pipes is kept very low by the pump which acts both as an exhaust pump and as a compressor. In order to evaporate in the coils the liquid must have heat energy supplied to it. The only place heat energy can come from is the food and if the food gives up this heat energy, it will be cooled. Thus we see that to evaporate, the liquid must take heat from the food.
Most of us are familiar with the cooling effect of evaporation. You have often heard swimmers say that it is warmer in the water than out of it. This should not be surprising to us if we understand the principles of evaporation. When you come out of the water, your body is wet and water evaporates from it. The heat necessary to vaporize the water is taken from the body, leaving the body cool. Some liquids evaporate even faster than water. If a little alcohol or ether is poured on the hand and allowed to evaporate, your hand will become cooled. Every molecule that evaporates from your hand must take enough heat away from it to give it sufficient energy to leave your hand. Only the fast-moving alcohol molecules will escape, leaving the slower ones behind. As you already know, slow-moving molecules in a liquid mean low temperature. In the summer-
time people use electric fans for the sole purpose of evaporating the moisture from their bodies at a faster rate. This evaporation takes heat from their bodies.
Converting Heat to Work
Heat energy can be converted into mechan a1 energy by means of a machine called a heat engine. For example, if we boil water in a covered pot, we may notice the cover moving up and down. When sufficient heat energy is added to the water molecules they are converted into a gas—steam—and the pressure of the steam against the cover is sufficient to raise it. This is a crude but simple example of how we convert heat energy into mechanical energy. With this principle in mind let us devise a simple ideal steam engine just for the sake of understanding the principle of operation. Watt, the inventor of the steam engine, probably went through the same reasoning process. If we allow steam from the boiler to enter inlet i, it will enter the cylinder and push the piston to the right as shown in diagram i. Now if we close inlet i and open inlet number 2, the expanding steam will drive the piston to the left provided that outlet r is open for the spent steam to escape. If now inlet 2 is closed and inlet i is open, the steam will expand against the piston and drive it to the right provided that outlet 2 is open. Outlet x must, of course, be closed, otherwise a pressure will not be built up against the piston. In this ideal model of steam engine everything would work fine if all the inlets and outlets were opened and closed at the right time. In a real steam engine the opening and closing of the inlets and outlets, called valves, is entirely automatic.
The principle of the modern steam engine is based on the ideal engine we have just described. The steam chest, contains an ingenious device called a slide valve, that slides from one end of the box to the other. Its purpose is to uncover the inlets, or ports, which allow steam to pass either to the right-hand or left-hand side of the piston, P. (The piston slides in the cylinder) Since the slide valve, must move left and right, it is connected to an eccentric on the shaft of the flywheel through a rod. Steam flows from the boiler through the pipe, into the cylinder, and exerts a force, pushing the piston, to the left. As the piston moves, it turns the shaft by means of the driving rod and a crank. This in turn moves the eccentric rod which causes the slide valve to move to the right. When the piston has moved about one-third of its stroke, the slide valve closes the port. The steam is now trapped in the cylinder and continues to expand, driving the piston forward. When the piston reaches the left end, the slide valve has moved far enough to the right to admit fresh steam through the port and to open the right end of the cylinder through to the exhaust port.
The piston is then pushed back toward the right, which in turn forces the cool steam in the right end out of the exhaust port. As the piston moves back and forth, the slide valve also moves back and forth. First it admits steam into one side of the cylinder and then into tile other, at the same time opening one exhaust port, and then the other. This back-and-forth motion of the piston, known as reciprocating motion, is changed to a rotary motion of the shaft by a connecting rod and crank. Actually the inlet ports opening to the steam chest are shut off before the piston reaches the end of the stroke, and the piston is driven the remainder of the way by the expansion of the steam trapped in the cylinder. The inertia of the heavy flywheel steadies the motion of the crankshaft and insures constant speed of rotation.
