Prepared By: Shakil Raiman.  Sum of the kinetic energy of the molecules of an object is called thermal energy or heat energy.  All substance above “absolute.

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Prepared By: Shakil Raiman

 Sum of the kinetic energy of the molecules of an object is called thermal energy or heat energy.  All substance above “absolute zero” temperature has some thermal energy.  Thermal Energy or Heat Energy is transferred from a higher temperature region to a lower temperature region.

 Two bodies A and B are in thermal equilibrium when there is no net transfer of thermal energy.  In thermal equilibrium, the rate of thermal energy transferred from A to B is equal to the rate of thermal energy transferred from B to A.  If two bodies are in thermal equilibrium, they have the same temperature.

 Average kinetic energy of the molecules of an object is called temperature.  The temperature of a substance is a number which express its degree of hotness or coldness. The hotter an object is the higher is its temperature.  It is measured in K (kelvin) or  C.  Absolute zero is the lowest possible temperature which is 0 K or -273  C.  At absolute zero temperature particles does not vibrate or move. All particles are in rest or no motion situation.

 In general we use two temperature scale.  They are degree Celsius (°C) or Kelvin (K).  The change in Celsius scale is equal to the change in Kelvin scale.  The ice point of water in Celsius is 0 °C and in kelvin is 273 K.  The steam point of water in Celsius is 100 °C and in kelvin is 373 K.

 Celsius to Kelvin : Add 273  Kelvin to Celsius : Subtract 273

 Conduction is the process by which heat energy is transferred through a medium without the movement of the material medium.  Explanation:  At the hotter part of the object, the molecules vibrate vigorously. This vibration is gradually passed along to the other atoms as they collide with adjacent molecules. Metal has many free electrons. These free electrons move through the metal, transferring the heat energy more quickly.

 All metals are good conductor of heat as they have free electron to carry the heat energy faster.  Water is a poor conductor of heat. Most liquid (except Mercury, Hg) are poor conductor of heat. That is why ice kept in the bottom of a test tube does not melt faster if the tube is heated from top.  Air is a very poor conductor of heat.

 Metal are good conductor of heat as they have huge number of free electrons. So cooking pans, kettles, electric iron are made of copper, aluminum or steel.  Plastic and wood are poor conductors (good insulators). Therefore handles of cooking utensils are made of plastic or wood.  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 us warm because it traps a lot of air. The air trapped in and between our clothes are blankets keeps us warm.  In the same way, the air trapped in fur and feather keeps animals warm. Birds fluff up their feather in winter to trap more air.  A refrigerator has insulation material around itself. The insulation reduces the amount of heat conducted into the refrigerator from the warmer room.

 Convection is the process by which heat energy is transferred from one place to another by the movement of heated particles of a gas or liquid.  Explanation: When the fluid at the bottom of a container is heated, it expands. The fluid is now less dense than the surrounding fluid and therefore starts to rise. So colder fluid takes this place from the adjacent place. The movement of the fluid due to a difference is density sets up a convection current and continues as long as all the fluid gets hot.

 All solid are poor convector of heat as they cannot flow.  Fluid (liquids and gases) are good convector as the particles are free to move.

 Car engines are cooled by convection currents in the water pipes. Water is a very good substance to carry the unwanted heat away from the engine to the radiator. The radiator is a heat exchanger where the hot water gives up its energy to the air.  In domestic hot-water system, convection currents drive the hot water up from the boiler to the storage tank while cold water flows down to the boiler.  The sun can cause very large convection currents of air. This flow of air is wind. In daytime, the land has a higher temperature than the sea. The warm air rises over the land and the cool air falls over the sea. So we feel a breeze from the sea.  Rising air convection currents over the land are used by glider pilots to keep their gliders in the sky.

 Radiation is a method of heat transfer whereby heat energy is transmitted from a hot object to another in the form of infra-red. This process does not require any medium. It can take place in vacuum.  Explanations: Hot objects emits infra-red radiation which is a electromagnetic wave. When the emitted radiation hits other objects, some of it is absorbed. This makes the molecules of the objects vibrate faster – so the objects become hotter.

 Different objects give out different amounts of radiation, depending on their temperature and their surface. The following table summarizes the factors which affect the rate of energy transfer by radiation. FactorsRate of Energy transfer FastSlow Colour and texture of the surface Dull black surfaceBright Shiny surface TemperatureHigh TemperatureLow Temperature Surface AreaLarge AreaSmall Area

 The cooling fins on the back of a refrigerator, in a car radiator and on a motorcycle engine should be dull black so that they will radiate away energy at a faster rate.  Brightly polished kettles and teapots do not lose much energy by radiation. They can keep the water inside warm for a longer time.  The solar panel has a dull black surface to absorb more heat energy from the sun.  A fire-fighting suit is bright and shiny so that it does not absorb much energy and burn the fire-fighter. Similarly, people living in hot weather wear white clothes and paint their houses white or silvery to reduce absorption of energy from the sun.

Black surfaces absorb more radiation than white surfaces:  Two dimensionally identical vertical plane surfaces made up with same material is placed equal distances away from a cubical hot water tank as shown in the diagram below.  One of the plane facing tank is painted black while the other surface is painted white.  On both sides of the plane surfaces, facing away from the tank, a cork is attached to the center with some wax.  It will be found that cork attached to the back of the surface painted black will drop first.

Black surfaces emit more radiation than white surfaces at the same temperature:  Two dimensionally identical vertical plane surfaces made up with same material is placed equal distances away from a cubical hot water tank as shown in the diagram below.  One of the surface of the tank facing one vertical plane is painted black while the other surface of the tank is painted white facing the other plane.  On both sides of the plane surfaces, facing away from the tank, a cork is attached to the centre with some wax.  It will be found that cork attached to the back of the vertical surface facing the black cubical surface of the tank will drop off first.

 Wish you all very good luck and excellent result.

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