TRANSFER OF THERMAL ENERGY

CONDUCTION Conduction is the process by which heat is transmitted through a medium from its hotter part to its colder part until they are both at the same temperature. Conduction occurs only in solid. Explanation: Molecules in a solid are arranged very close to each other. When one end of the metal rod is heated, the molecules at this end vibrate faster due to the increased kinetic energy. The molecules collide with the neighboring molecules and cause them to vibrate faster. This process is repeated until the molecules of the rod at the other end vibrate faster. Thus heat has been transferred from one end of the rod to the other end.

Thermal Conductors Materials which allow heat to flow through it well are called thermal conductors. Examples: copper, Aluminum, iron, gold etc….   Thermal Insulators Materials which do not allow heat to flow through it well are called thermal insulators or poor conductors. Examples: Glass, Water, Plastics, Rubber, Wood etc.. Materials containing air (wool, fiberglass, plastic foam, expanded polystyrene)

Comparing Thermal Conductors Diagram below shows one way to compare different metals. The metal rods are all the same size. Each has a blob of wax at one end. They all heated equally at the other end. The best conductor is the metal on which the wax melt first.

To show that water is a bad conductor of heat Procedure   1. Set up the apparatus as shown in the diagram. 2. Heat the water at the top of the test tube gently to boil the water. Observation & conclusion 1. The water at the top of the test tube boils while water at the bottom of the test tube remains cold and the ice does not melt. 2. The experiment shows that water is a bad conductor of heat.

Examples of everyday application: 1. All metals are good conductors of heat. So cooking pans, kettles, electric iron are made of copper, aluminum or steel. 2. Plastic and wood are poor conductors (good insulators). Therefore handles of cooking utensils are made of plastics or wood. 3. Air is a very good insulator. So if a substance contains a layer of trapped air, it can stop the conduction of heat. For example, wool keeps you warm because it traps a lot of air. The air trapped in and between our clothes and blankets keeps us warm. 4. In the same way, the air trapped in fur and feathers keeps animals warm. Birds fluff up their feathers in winter to trap more air.

Convection Convection is the process by which heat is transferred from one place to another by the movements of heated particles of a gas or liquid. Convection can only occurs in fluid (liquid and gas) Convection in liquid In the experiment below, the bottom of the beaker is being gently heated in one place only. As the water above the flame become warmer, it expands and become less dense. It raises upwards as cooler, dense water sinks and displaces the less dense water. The result is a circulating stream, called convection current. Where the water is heated, its particles circulate; they transfer energy to the other parts of the beaker.

Convection in air In the set-up shown in the diagram, smoke is been to move through the box. The candle heats the air above it and causes less dense air to rise, setting up convection currents. The denser colder air is force into the box.

Examples of everyday application: Heated by the sun, warm air rises above the equator as it is displaced by the cooler, dense air sinking to north and south. The result is huge convection current in the Earth’s atmosphere. These cause winds across all oceans and continents.

2. Day breeze (land breeze) and night breeze (sea breeze) During day time, in hot sunshine, the land heats up more quickly than the sea. Warm air rises above the land, as it is displaced by cooler air moving in from the sea. This is called day breeze.

At night the sea heats up. The sea stays warmer than the land At night the sea heats up. The sea stays warmer than the land. Which cools down quickly, warmer air now rises above the sea, as it is displaced by cooler air moving out from the land. This process is called night breeze.

3. Hot water system Cold water in the storage tank sinks down to the boiler, where it is heated. The heated water in the boiler rises to the top of the storage tank. In this way, a supply of hot water collects in the storage tank from the top down. The storage tank is insulated to reduce thermal energy losses by conduction and convection. The header tank provides the pressure to push the water out of the taps.

4. Refrigerator Cold air sinks below the freezer compartment. This sets up a circulating current of air which cools all the food in the refrigerator.

5. Air conditioner Cold air sinks below the air conditioner and hot air at the bottom of the room rises up. This sets up a circulating current of air which cools all air in the room.

6. Room heating Warm air rising above a convector heater or radiator carries thermal energy all around the room-through unfortunately; the coolest air is around your feet.

RADIATION Radiation is the process by which heat is transmitted from a hot object to another in the form of electromagnetic waves (infra red). This process does not require any medium. It can take place in a vacuum. Different objects give out different amounts of radiation, depending on their temperature and their surface. EMITTERS AND ABSORBERS Some surface are better at emitting (sending out) thermal radiation than others. Good emitters of thermal radiation are also good absorbers. Black colour is good absorber and also good emitter of thermal radiation. White and silver colour are bad absorber and emitter of thermal radiation but good reflector of radiation.

  COMPARING ABSORBERS Put water two cans, one is painted with black colour and another is painted with silver colour. Cover the both cans and put thermometer to each cans and kept both cans into direct sunlight for sometime.

It is found that the temperature of black colour can higher than silver colour can. This means black colour is good absorber of heat.

COMPARING EMITTERS Put hot water into two cans, cover both the cans and put thermometer into each can. Keep both cans where there were no sunlight. It is found that the reading of the thermometer in the black can falls more quickly that silver can. This means black colour is good emitter of thermal radiation.

REDUCING HEAT LOST FROM HOMES Energy tends to escape from a hot object, spreading to its cooler surroundings by conduction, convection and radiation. This can be a great problem. We may use a lot of energy (and money) to heat our homes during cold weather, and the energy simply escapes. A well insulated house can avoid a lot of energy wastage during cold weather. The insulation can also help to prevent the house from becoming uncomfortably hot during warm weather.