Heat flows from a body at higher temperature to a body at lower, temperature.
There are three ways of heat transfer. These are conduction, convection and radiation.
The mode of transfer of heat by vibrating atoms and free electrons in solids from hotter to colder part of a body is called conduction of heat.
The amount of heat that flows in unit time is called the rate of flow of heat.
The rate at which heat flows through solids depends on the cross sectional area of the solid, length between hot and cold ends temperature difference between hot and cold ends and nature of the material.
The rate of flow of heat across the opposite faces of a metre cube maintained at a difference of 1 K is called the thermal conductivity of the material of the cube.
Good conductors are used for quick transfer of heat. Thus cookers, cooking plate, boiler, radiators and condensers of refrigerators etc. are made of metals.
Water is a poor conductor of heat.
Materials which trap air are also bad conductors such as wool, felt, fur, feathers, polystyrenes and fibre glass.
Transfer of heat by actual movement of molecules from hot place to a cold place is known as convection.
Land and sea breezes are also the examples of convection.
Gliders use upward movement of hot air currents due to convection of heat. Air currents help them to stay in air for a long period.
Birds are able to fly for hours without flapping their wings due to the upward movement of air currents.
The term radiation means the continual emission of energy from the surface of a body in the form of electromagnetic waves.
Radiations are emitted by all bodies. The rate at which radiations are emitted depends on various factors such as colour and texture of the surface, temperature and surface area.
A dull black surface is a good absorber of heat as its temperature rises rapidly.
A polished surface is poor absorber of heat as its temperature rises very slowly.
Radiations from the Sun pass easily through glass/polythene and warm up the materials inside a greenhouse. The radiations given out by them are of much longer wavelengths. Glass/polythene does not allow them to escape out and thus maintains the inside temperature of the greenhouse.
Earth’s atmosphere contains carbon dioxide and water vapours. It causes greenhouse effect and thus retains the temperature of the Earth.
The bottoms of cooking pots are made black to increase the absorption of heat from fire.
White surfaces reflect more heat than coloured or black surfaces. Similarly, polished surfaces are good reflectors than rough surfaces and reflection of heat radiations is greater from polished surfaces. Therefore, We wear white or light coloured clothes in summer
We polish the interior of the cooking pots for reflecting back most of the heat radiation inside the hot pots.
A thermos flask consists of a double-walled glass vessel. It reduces the transfer of heat by conduction, convection and radiation.