Unit THE NATURE OF HEAT

Transcription

1 Unit THE NATURE OF HEAT Heat is a form of energy, in the form of infrared radiation. Heat from the sun travels through space at the speed of 300,000,000 m/s. Upon arriving on earth, much of the radiant heat is absorbed by different kinds of matter and is converted into heat that we can feel (sensible heat). HEAT AND THE MOTION OF MOLECULES According to the kinetic theory, heat energy acquired by a body is transformed into increased kinetic energy of the molecules. We observe this increased kinetic energy whenever a solid, a liquid, or a gas expands on heating. A further increase in kinetic energy will eventually cause the particles of a solid or liquid to become a gas. When an ice cube (a solid) is heated, it melts and becomes liquid water. When the water is heated, it vaporizes and becomes gaseous water. When an object is heated it grows bigger. We say it expands. When an object cools down, it gets smaller. We say it contracts. Page 1 of 12

2 EXPANSION OF SOLIDS The metal ball shown in the diagram will just slip through the metal ring when they are both cold. When the ball is heated the ball will no longer pass through the ring. iron iron brass brass materials expand more than others. A bi-metallic strip is made of two different metal strips, often brass and iron. On heating the bimetallic strip bends because brass expands more than iron. This shows that some The increase in size of the ball or in the length of the strip is not due to an increase in the size of the particles, but rather to an increase in the average distance between the particles. When an object is heated, its particles vibrate faster, collide more violently, and consequently move farther apart, thereby increasing the volume of the object. This increase in volume is called expansion. When the object is cooled, the opposite change occurs and the volume of the object decreases. This decrease in volume is called contraction. Different solids and liquids expand at different rates when heated. Page 2 of 12

3 EXPANSION OF LIQUIDS When a fluid (liquid or gas) is heated, it expands. Liquids expand more than solids. Gases expand more than liquids. Liquids, like solids, expand when heated. When water is heated, it expands. When the same water is cooled to its original temperature, the water contracts to its original volume. At lower temperatures, however, the behaviour of water is an exception to this rule. When water is cooled below 4 C, the water expands-unlike other liquids. Since the volume of ice is greater than the volume of water from which the ice is formed, the density of ice is less than the density of water. This is why ice floats on water. EXPANSION OF GASES Gases confined in an elastic container expand when they are heated and contract when they are cooled. Gases, generally expand at the same rate when heated to the same temperature, at a given pressure. When a gas is heated, the atoms start moving more and so they take up more space. The substance expands. Page 3 of 12

4 TEMPERATURE Heat and temperature are two terms that are often confused. We know that the temperature of a small sample of molten iron is considerably higher that the temperature of the water in the ocean. However, the total heat in a sample of molten iron is much less than the total heat of the water in the ocean. Heat is related to the motions of particles in matter. Heat depends on the total kinetic energy of the particles in a body. Because the water in the ocean is colder than the sample of molten iron, the velocity of the particles in the water is less than the velocity of the particles in the molten iron. However, the much larger quantity of water compensates for the smaller velocity of the particles and thus the particles of water in the ocean possess greater kinetic energy. This means that there is more heat in the water in the ocean than in a small sample of molten iron. Temperature depends on the average kinetic energy of the particles, that is, the kinetic energy per particle. So the large mass of ocean water has a smaller average kinetic energy per particle and consequently has a lower temperature than a small sample of molten iron. MEASURING TEMPERATURE Instruments designed to measure temperature are called thermometers. Most thermometers are based on the principle that matter, on heating, expands and, on cooling, contracts. In general, matter expands and contacts regularly. This means that the amount of expansion or contraction in length is generally equal for the same increase or decrease in temperature. Page 4 of 12

5 LIQUID THERMOMETERS Thermometers containing liquids such as mercury and alcohol are useful and accurate because these liquids usually expand and contract uniformly (regularly). THE FAHRENHEIT AND CELSIUS TEMPERATURE SCALES Temperature markings on thermometers are indicated in Fahrenheit degrees or Celsius degrees. Both Fahrenheit and Celsius scales are calibrated by using the boiling and freezing points of water. The temperature of our bodies is 37 o C. This is equivalent to 98.6 o on the Fahrenheit scale. THE KELVIN SCALE When an object cools down, the molecules vibrate more slowly. At a certain temperature, the molecules loose all their energy and stop vibrating. This is the lowest temperature we can reach and is known as ABSOLUTE ZERO. Absolute zero, -273 C, is also called 0 Kelvin TRANSFER of HEAT When objects are at different temperatures, heat is transferred from the warmer object to the cooler object until both objects are at the same temperature. Heat transfer can occur through one of three methods: Conduction, Convection, or Radiation. Page 5 of 12

6 CONDUCTION When one end of a metal rod is held in a flame, the entire rod will become hot enough to burn the hand. The heat from the flame reaches the hand by travelling through the rod. Substances that allow heat to travel through them are called conductors. In general metals are good conductors. However, some metals conduct heat more readily than others. This can be demonstrated by heating rods of aluminium, copper, iron and glass into a brass sphere or disk and then attaching a small ball of wax to the end of each rod. When the end of the rods is heated, the wax at the tip of each metal melts in the order in which the different metals conduct heat. The wax at the tip of the copper melts first and the wax at the tip of the glass melts last. When one end of a rod is heated, the molecules in that end of the rod vibrate faster and strike other nearby molecules, causing them to vibrate faster. In this manner, the increased molecular motion is transferred from one end of the rod to the other, permitting the heat to travel through the rod. Substances that do not readily allow heat to pass through them are called insulators. Gases and liquids are generally poor conductors of heat because their molecules are farther apart than are the molecules in solids. Therefore, neighbouring molecules in a gas or in a liquid are less affected by the increased motions of heated molecules, and consequently heat is not conducted rapidly. Page 6 of 12

7 Substances like wood or plastic are poor conductors of heat, so they are used to make handles for metallic objects that are to be heated. The clothing we wear is also a poor conductor of heat, enabling us to retain body warmth. Porous material is generally non-conducting because it contains layers of trapped air which do not permit heat transfer. CONVECTION Although gases and liquids are poor conductors of heat, heat is transferred through them by the process of convection. Convection is the transfer of heat due to the motion of the liquid or gas itself. For example, when a beaker of water is heated the water layer closest to the heat source is warmed slowly by conduction. As the water becomes warmer, it expands, becomes less dense, and rises. This brings heat to the upper layer. At the same time, cooler water from the upper portion of the beaker moves down, takes the place of the rising water, and becomes heated itself. Page 7 of 12

8 When warm enough, this water rises and carries heat upward. As these processes continue, heat that enters the bottom of the beaker is distributed throughout the beaker until all the water is at the same temperature. The moving water in such a case is said to have set up convection current. Heat is also transferred through gases by convection. It is by this means, in part, that a stove or a radiator heats a room. Heat from the radiator warms the air above it, causing the air to expand, become less dense, and rise. The cooler air that moves in to take the place of the warmed air is also soon warmed. As this air rises, a convection current is established. The formation of a convection current in air is demonstrated with a convection box apparatus. First the candle is lighted, then smoking touch paper is placed over the chimney, opposite the candle. The smoke can be seen to move down this chimney, across the box, and out through the other chimney. SEA BREEZE During the day, the land warms up more than the sea. The warm air over the land rises up. The hot air is replaced by cooler and denser air which comes from above the sea. So during the day we feel a sea breeze - wind blowing from the sea onto the land. LAND BREEZE During the night the sea has more heat to lose and cools more slowly. The air above the sea is now warmer than that above the land. Thus hot air above the sea rises upwards, and is replaced by cooler air from above the land. During the night we feel a land breeze - wind blowing from the land onto the sea Page 8 of 12

9 RADIATION We know that light energy and heat energy travel from the sun to the earth through space, which is an almost perfect vacuum. These forms of energy, are transferred from the sun to the earth by radiation, that is, by means of rays, or waves. You can understand this method of heat transfer by standing a short distance from an open fire. Since no source of heat is being touched, you cannot receive heat by conduction. Since warm air rises vertically from the heat source, the heat cannot reach you by convection. The heat that is transferred to you from the fire by radiation. The heat radiated by one body ( the sun, for example) is most rapidly absorbed by other bodies that are black in color and rough in texture. In warm climates, white clothing which reflects the radiant heat of the sun is cooler than dark clothing which quickly absorbs the radiant heat. Similarly, bodies that are rough and dark tend to radiate heat better than shiny smooth bodies. This is why steam radiators are often dark and have a roughened surface. It is for the same reason that coal burning stoves are black. Bodies that are shiny and smooth do not absorb heat readily. Instead, these bodies reflect heat. Page 9 of 12

10 Thus, aluminium used for roofing keeps homes cool in the summer and warm in the winter. This principle is utilized in the thermos (vacuum) bottle, which is so constructed as to permit liquids to retain their temperatures for a long time. A thermos bottle is double walled, with a partial vacuum between the walls to prevent heat transfer by conduction or convection. A cork stopper also prevents heat transfer by conduction. The inner glass walls are silvered to reflect radiant heat back into the liquid, thereby minimizing heat loss by radiation. Thus, a hot liquid remains hot because heat is lost very slowly. A cold liquid remains cold in thermos bottles because outside heat enters very slowly by conduction, convection, or radiation. THE GREENHOUSE EFFECT. The inside of a greenhouse is warmer than the outside. This is due to the fact that the rays coming from the sun have a short wavelength which can get through the glass. These rays are absorbed by the plants which get warmer. The plant does not get very hot, and the radiation it emits is of a longer wavelength which cannot pass through the glass. Thus energy is radiated in, but cannot radiate out again. Page 10 of 12

11 Evaporation: When we put some deodorant or some perfume on our hands, it feels cold. This is because the liquid uses heat from our bodies to change into a gas. The deodorant/perfume has evaporated. Also when we run our body sweats to keep us cool. Similarly, when we wash our clothes, we hang them outside to dry up. The sun warms the clothes up and the water molecules get enough energy to escape into the air. In this case the water evaporates and becomes a gas called water vapour. MEASURING HEAT We measure the quantity of heat by a unit called the Joule. The Joule is the amount of heat needed to raise the temperature of 1 kilogram of water 1 degree Celsius. CALORIES AND FOOD Your body requires energy in order to perform its daily tasks. Most of this energy comes from energy-rich foods such as carbohydrates and fats. This energy is released when the body utilizes these foods. Using special calorimeters, scientists have measured the energy content, or the number of calories present, in fixed quantities of certain foods. For example, a slice of white bread contains about calories; a typical chocolate bar may contain about calories. Page 11 of 12

12 Specific Heat Capacity Different materials have different specific heat capacities. We can find the specific heat capacity of a substance using: Q = mc Q = heat energy received/given out [ in Joules ] m = mass of substance [ in kg ] c = specific heat capacity [ in J/kg o C or J/kgK ] = change in temperature [ in o C ] The specific heat capacity of a substance is the amount of energy (in Joules) that is needed to raise the temperature of 1kg of a substance by 1 o C. Different substances require different amounts of heat to cause the same temperature change in the same mass. The thirst of a substance for heat is measured by its specific heat capacity (symbol c). The ability of water to stabilize temperature depends on its relatively high specific heat. The specific heat of water is 4200 J/kg C.Compared with most other substances; water has an unusually high specific heat. Because of its high specific heat, the water that covers most of planet Earth keeps temperature fluctuations within limits that permit life. Also, because organisms are made primarily of water, they are more able to resist changes in their own temperatures. Page 12 of 12