Transfer Of Thermal Energy
The video includes heat radiation, convection and conduction.
*Thermal energy flows from a region of higher temperature to a region of lower temperature.
Thermal energy is transferred by any of these 3 processes..
CONDUCTION
The main process by which thermal energy is transferred from one end of a solid to another.
USES OF GOOD CONDUCTORS OF HEAT
Cooking utensils are usually made of aluminium or stainless steel where direct heating is involved
Soldering iron rods are made of iron with the tip made of copper as it is a much better conductor of heat than iron
Heat exchangers such as those used in laundry enable precious fuel to be saved.
USES OF BAD CONDUCTORS OF HEAT
Handles of utensils are made of wood or plastics. In this way, the hot utensil can be picked up without scalding our hands.
Table mats are usually made of cork so that hot kitchenware can be placed on them without damaging the table-top.
Sawdust is used to cover ice blocks because of its good insulating property.
Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked
Woollen blankets or clothes are used to keep people warm on cold days
Fibreglass, felt and expanded polystyrene foam which trap large amounts of air are employed as insulators in the walls of houses, ice boxes and refrigerators. Double glazing is also commonly used as air is a very poor conductor of heat.
CONVECTION
Transfer of heat through liquid or gas
SOME EVERYDAY APPLICATIONS OF CONVECTION
Household hot-water systems
Water is heated in the boiler by gas burners. The hot water expands and becomes less dense. Hence, it rises and flows into the upper half of the cylinder. To replace the hot water, cold water from the cistern falls into the lower half of the cylinder and then into the boiler due to the pressure difference. The overflow pipe is attached to the cylinder just in case the temperature of the water becomes too high and causes a large expansion of the hot water. The hot water tap is led from the overflow pipe and must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap.
Electric kettles
The heating coil is always placed at the bottom of the kettle. When the power is switched on, the water near the heating coil gets heated up, expands and becomes less dense. The heated water therefore rises while the cooler regions in the upper part of the body of water descend to replace the heated water.
Air-conditioners
The rotary fan inside an air conditioner forces cool dry air out into the room. The cool air, being denser, sinks while the warm air below, being less dense, rises and is drawn into the air conditioner where it is cooled. In this way, the air is recirculated and the temperature of the air falls to the value preset on the thermostat.
Refrigerators
Refrigerators work in very much the same way as air conditioners. The freezing unit is placed at the top to cool the air so that the cold air, being denser, sinks while the warm air at the bottom rises. This sets up convection currents inside the cabinet which help to cool the contents inside.
RADIATION
SOME EVERYDAY APPLICATIONS OF RADIATION
Vacuum flasks
It is designed to keep liquids hot by minimising heat losses in 4 possible ways-Conduction, Convection, Radiation and Evaporation.
Conduction and covection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flask. Conduction through the trapped air above the liquid is mionimal since air is a very poor conductor of heat. The stopper is made of plastic which is also a poor conductor of heat.
Convection and evaporation can only occur when the plastic stopper is removed during use. Heat loss by radiation is harder to stop as radiant heat can pass through vacuum.
The minimise heat loss through radiation, the walls of the glass are silvered to reflect the radiant heat back into the hot liquid.
Teapots
Since shiny surfaces are bad emitters of radiation, shiny teapots can keep tea warmer than a black teapot for a longer time. In addition, since shiny surfaces are bad absorbers of radiation, shiny containers can keep a cold liquid cooler for a longer time than black containers.
The greenhouse
The greenhouse traps heat to help plants grow better. On a warm day, infra-red radiation from the Sun passes through the glass of the greenhouse and is absorbed by the soil and plants inside. The soil and the plants therefore become warmer and they emit infra-red radiation of longer wavelengths. These longer wavelengths of radiation are unable to pass through the glass and so heat is trapped inside the greenhouse. This causes the temperature inside the greenhouse to rise and this is good for plant growth.
