Thermal conduction

Heat can be transferred from one place to another by three different methods, namely Conduction, Convection and radiation. Conduction usually takes place in solids, Convection in fluids (liquids and gasses), and no medium is required for radiation.

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Conduction is a mode of heat transfer between two neighboring molecule due to the difference in temperature. It occurs in all forms of matter (solids, liquid and gas).Ex. - conduction of heat through metals

Convection is a mode of heat transfer due to the actual movement of molecule and it occurs in fluids only .E g.: - wind is convectional mode of heat transfer

Radiation is a mode of heat transfer which requires no medium for the heat transfer. E.g. sun radiates heat.  

Thermal conduction
Conduction

Conduction is the process in which heat is transmitted from one point to another through the substance without the actual motion of the particles. When one end of a metal bar is heated, the molecules at the hot end vibrate with higher amplitude and transmit the heat energy from one particle to the next particle. However, the particles remain in their mean position of equilibrium. The process of conduction is prominent in the case of solids.

If one end of a metal rod is placed in a stove, the temperature of the other end gradually increases. Here heat is transferred from one end to the other end due to the molecular collisions and the process is called heat (thermal) conduction. Here the average position of a molecule does not change and hence, there is no mass movement of matter.

The ability of a material to conduct heat is measured by thermal conductivity of the material. If ΔQ amount of heat crosses through any cross- section in time Δt, ΔQ/ Δt is called the heat current. It is found that in steady state it is proportional to the area of cross section A, proportional to temperature difference(T1-T2) between the ends and inversely proportional to the length x . Thus,

ΔQ/ Δt ~ A (T1- T2) / x                                                                   ΔQ/ Δt = K A (T1- T2) / x

                                                                                                             Here K is a constant for the material of the slab and is called the thermal conductivity of the material.

In general, solids are better conductors than liquids and liquids are better conductors than gases. Metals are better conductors than non-metals. This is because; in metals we have a large number of 'free electrons' which can move freely any where in the body of the metal. These free electrons help in carrying the thermal energy from one place to another in a metal. 

Convection

Convection

In convection heat is transferred from one place to another by actual motion of heated material. The process of convection is prominent in the case of liquids and gases. Land and sea breezes and trade winds are formed due to convection. Convection plays an important role in ventilation, gas filled electric lamps and heating of buildings by hot water circulation.

In a hot air blower, air is heated by a heating element and is blown by a fan. The air carries the heat wherever it goes. If the material moves due to the difference in density, it is called natural or free convection.

             The main mechanism for heat transfer inside a human body is forced convection.  Heat serves as the pump and blood as the circulating fluid. Heat is lost to the atmosphere through all the three processes conduction, convection and radiation. The rate of loss depend on clothing, the tiredness, air current, humidity  and several other factors. The system, however, transports the just required amount of heat and hence maintains a remarkably constant body temperature.

Radiation

Radiation

The process of radiation doesn’t need a material medium for heat transfer. Energy is emitted by a body and this energy travels in the space just like light. When it falls on a material body, a part is absorbed and the thermal energy of the receiving body is increased .The energy emitted by a body in this way is called radiant energy, thermal radiation or simply radiation .The heat from the sun reaches the earth by this process , traveling millions of kilometers of empty space.