Form 4: Integrated Science Notes TOPIC: TEMPERATURE CONTROL AND VENTILATION TEMPERATURE CONTROL: HEAT TRANSFER Heat energy can be transferred from a place where it is very hot to a place where it is not so hot. There are three methods of heat transfer: conduction, convection and radiation. Conduction: Conduction is the transfer of heat energy through a material without movement of the material itself. This is the usual method of heat transfer in solids. Some materials are good conductors, for example metals are good conductors of heat. Some materials are poor conductors (insulators), for example air, glass and wood. Convection: Convection is the transfer of heat energy by the circulation of fluids from one place to another due to differences in temperature. If you drop a large crystal of potassium permanganate into a beaker of water and then heat the beaker gently, you will observe an upward current of coloured water which spreads out at the surface and then moves down the sides of the beaker. This movement is called convection currents. Convection currents are responsible for land and sea breezes. During the day the land is hotter than the sea. Hot air from the land rises and cold air from the sea moves in to take its place. During the night the sea is hotter than the land. Hot air rising from the sea is replaced by cold air from the land. Radiation: Radiation is the transfer of heat energy by means of electromagnetic waves. Unlike convection and conduction, radiation takes place without a material medium. Heat travels from the Sun to the Earth by radiation.
THERMOSTATIC CONTROL OF HOUSEHOLD APPLIANCES The Electric Iron: The electric iron contains a bimetallic strip. This consists of two metal strips bonded to each other. When heated on strip expands more than the other. In the diagram below metal X expands more than metal Y. A bimetallic strip is contained in the thermostat. The current going to the heating element passes through the bimetallic strip, heating it. When the strip becomes too hot, it bends away and the circuit is broken. When the strip cools, the strip straightens and contact is made so that the circuit is reformed. Bimetallic strips are also used in ovens and electric water heaters. Diagram showing a bimetallic strip being used in a thermostat. THERMOMETERS The Liquid in Glass Thermometer: The most common type of thermometer is the liquid in glass thermometer. It consists of a bulb filled with a liquid (usually mercury or alcohol) and a capillary tube attached to the bulb. The bulb is enclosed in a glass tube. The liquid in glass thermometer works on the principle of expansion: liquids expand when heated. When the liquid in the bulb of the thermometer is heated, it expands and rises up the narrow tube. This type of thermometer is used in the lab.
The Clinical Thermometer: The clinical thermometer differs from the laboratory thermometer in three ways: There is a constriction in the capillary tube above the bulb. This constriction is necessary for the recording of accurate temperatures; the position of the mercury remains constant as it reaches the temperature of the body. The constriction causes a break in the mercury when the bulb cools down so that, when the thermometer is removed from the patient s mouth, the thread above the constriction remains intact and an accurate reading can be made. The range of the temperature is very small (35 to 45 ) because there is little variation in body temperature. The base is usually smaller so that slight changes in temperature can show up as longer lengths on the mercury thread. The Maximum and Minimum Thermometer: This thermometer is used to measure the highest and lowest temperatures that occur during a day. This thermometer contains: A bulb C, which contains alcohol A second bulb, which is nearly full with alcohol A U-shaped tube containing mercury, which connects the two bulbs. When the temperature rises: The alcohol in bulb C expands and pushes the mercury thread up the right-hand side of the thermometer. A steel index X marks the highest point at which the mercury thread stops. When the temperature drops: The alcohol in bulb C contracts and the mercury thread moves up the left-hand side pushing the Y index (index X remains in position). The index Y remains in position, corresponding to the lowest temperature recorded. The thermometer is reset by dragging the metal indices back to their original positions with a magnet.
VENTILATION Ventilation is the movement of air in such a way that fresh air replaces stale air in an enclosed space. Ventilation removes excessive heat from enclosed spaces and replenishes the oxygen supply. Lack of Ventilation: Causes dampness Causes heat Causes lack of oxygen Encourages odours and rapid growth of bacteria Cooling Effects of Evaporation Evaporation is the conversion of liquid to vapour, and energy is needed for this conversion. When we sweat, excess heat energy of the body causes evaporation and hence the heat is lost as the vapour moves away from the skin. Keeping cool Relative humidity is the amount of water vapour in the air expressed as a percentage of the maximum amount of water that the air can hold at the same temperature. When both temperature and humidity are high, the rate of evaporation of sweat from the skin is slowed down. As a result the body does not regulate temperature efficiently. We feel hot and sticky and get tired very easily. When it is windy, evaporation takes place very easily. Temperature, humidity and air movements can also affect such processes as drying clothes. Quick drying occurs in warm, dry, sunny and windy conditions because these conditions enhance the evaporation of water.
Comfort in an enclosed area can only be achieved by a combination of the correct conditions of temperature, humidity and wind. Houses can be built with roofs that make use of the natural movement of air. Such homes are designed with the windows and doors positioned to allow free movement of air. The Air Conditioner The air conditioner is a mechanical ventilator, it makes enclosed spaces more comfortable. The warm air from the room is drawn into the unit by a fan; the air passes over cooling coils, which cools it. This cool air is then sent back into the room at lower temperatures. In some air conditioning units, push air into the room at lower temperatures. In some air conditioning units, only the air from the room is cooled and recirculated. In other air conditioning units fresh air can be drawn from outside and the stale air expelled.