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HEAT. Heat is a form of energy that is transferred from one body to another because of a difference in temperature. Temperature is the property that gives physical meaning to the concept of heat.
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HEAT • Heat is a form of energy that is transferred from one body to another because of a difference in temperature. • Temperature is the property that gives physical meaning to the concept of heat. • The temperature of a substance is a measure of the average speed of all atoms and molecules in the substance.
WHAT IS HEAT ENERGY? • All matter is made of atoms and molecules in constant motion. When energy is absorbed by matter, the random internal energy and the motion of these atoms and molecules is increased. • This increase makes itself felt in the form of heat, and when it occurs, the temperature of the matter rises.
TEMPERATURE • Temperature is the property that gives physical meaning to the concept of heat. • It can be observed that if a hot poker is plunged into cold water, the poker becomes cooler and the water becomes warmer. This means that the hot body gives up some of its heat to the cold body. The exchange of heat will continue until the water and the poker have the same temperature. • Thus, the temperature of a substance will determine whether heat flows from it or to it when the substance is in contact with another body at a different temperature.
TEMPERATURE • In 1802, it was found that all gases, when heated one degree, expand by 1/273 of the volume that they occupy at the freezing point of water. It was reasoned that if the gas were cooled, its volume would decrease by the same amount as the temperature decreased. • Further study has supported the idea of an absolute zero. It is now defined as the temperature at which all molecular and atomic motion stops completely. • The temperature of a substance is a measure of the average speed of all atoms and molecules in the substance. Absolute zero is also the temperature below which it is impossible to go.
In the top diagram, two identical gases at different temperatures are separated by a barrier. The hotter gas consists of molecules with a larger average kinetic energy than the molecules of the cooler gas. When the gases are combined, the temperature of the mixture settles at an equilibrium temperature that is between the temperature of the hot gas and the temperature of the cold gas. The heat flows from the warmer gas to the cooler gas until the average kinetic energy of the two gases are the same.
THERMAL EXPANSION • All matter increases in volume when there is an increase in temperature. In the case of gases, the increase is a large one. If the pressure and the weight of gas remain the same, the increase in volume will be in direct proportion to the increase in temperature. • The application of heat to a solid causes it to expand, but to a much smaller degree than a gas. • Liquids in general behave like solids and expand slightly when the temperature is raised.
THERMAL EXPANSION • The expansion of solids as they grow warmer has practical consequences. • Engineers must make sure there are gaps in the metal of bridges so that there is room for the bridge to expand in warm weather. Otherwise, the structure would buckle or crack.
HEAT and WORK • There is a direct relationship between heat and work. For example, a body does work to bring a moving body to rest. The kinetic energy of the moving body is changed into energy of motion within the molecules of both bodies. This kind of work is called friction. • The increase in molecular motion results in an increase in the amount of heat contained in these bodies, and their temperatures rise.
HEAT and WORK A steam engine is an example of a machine that converts heat energy into mechanical energy.
CONDUCTION • Conduction is a point-by-point process of heat transfer. If one part of a body is heated by direct contact with a source of heat, the neighboring parts become heated successively. • Thus, as shown in the diagram, if a metal rod is placed in a burner, heat travels along the rod by conduction. • This may be explained by the kinetic theory of matter. The molecules of the rod increase their energy of motion. This violent motion is passed along the rod from molecule to molecule.
CONVECTION • Convection depends upon the movement of the material which is heated. It applies to free-moving substances (i.e. liquids and gases). The motion is a result of changes of density that accompany the heating process. • When a liquid or gas is heated, its density (mass per unit volume) decreases; that is, it becomes lighter in weight. A warmer volume of gas will rise while a colder, and thus heavier, volume of gas will descend. • This process is described as natural convection. A familiar example of natural convection is the circulation of air from a hot-air furnace. • When a liquid or gas is moved from one place to another by some mechanical force, the process is known as forced convection. The circulation of air by an electric fan is an example of forced convection.
CONVECTION • On our globe, areas nearer to the equator receive more heat energy per unit area. • As a result, convection occurs on a global scale on our planet. • Convection makes weather.
CONVECTION • Tropical areas receive more heat from the sun than do polar regions. The tropical atmosphere is hotter than the atmosphere in other areas. • As a result, the warm tropical air moves toward the poles. This starts a global movement, or circulation, of air. The global air movement has a great deal to do with what man does on Earth. • Oceans also receive differing amounts of heat. This results in the flow of water masses which move continuously through the Earth's oceans
CONVECTION Differential heating occurs near bodies of water because the heat capacity of water is higher than dirt and rock. During the day, the land heats up faster than water. This makes air rise faster over land than water, and causes a convection cell. At night, the land cools more quickly, and then the air over water is warmer so the air over the water rises. The same phenomenon occurs seasonally, making colder weather come later near bodies of water and slowing the arrival of warm weather.
RADIATION • Radiation begins when the internal energy of a system is converted into radiant energy at a source, such as a heater. • This energy is transmitted by waves through space, just as the sun radiates heat outwards through the solar system. • The radiant energy strikes a body where it is absorbed and converted to internal energy. It then appears as heat. • An electric heater produces radiant energy in this way. It may be absorbed, reflected, or transmitted by a body in its path. When the radiant energy is absorbed, the internal energy of the body increases and its temperature rises.
RADIATION and EQUILIBRIUM • All bodies, whether hot or cold, radiate energy. The hotter a body is, the more energy it radiates. • All bodies receive radiation from other bodies. The exchange of radiant energy goes on continuously. • A body at constant temperature has not stopped radiating. It is receiving energy at the same rate that it is radiating energy. There is no change in internal energy or temperature.
SPECIFIC HEAT • Specific heat measures the amount of heat energy needed to raise the temperature of one gram of a substance by one degree Celsius. • Water has a particularly high specific heat, which means that more heat must be added to water than to most other substances to raise its temperature. It takes very little heat to raise the temperature of gold.
SPECIFIC HEAT • The specific heat, or heat capacity, of a substance is the amount of heat that is required to raise the temperature of the substance by one degree. • The value for specific heat varies widely for different substances. In the metric system, the unit of specific heat is the calorie. It is defined as the amount of heat that is required to raise the temperature of one gram of water by one degree Celsius.
