ThermalConductionThermalconduction is the transfer of energy arising from temperature differencesbetween adjacent parts of a body.Thermal conductivity is recognised as the exchange of energybetween molecules and electrons in the conducting medium. The rate of heat flowin a rod of any material is proportional to the cross-sectional area of the rodand to the temperature difference between the ends and inversely proportionalto the length; that is the rate H equals the ratio of the cross section A ofthe rod to its length l, multiplied by the temperature difference (T2 ? T1) andby the thermal conductivity of the material, designated by the constant k.A substance of large thermal conductivity k is a good heatconductor, whereas one with small thermal conductivity is a poor heat conductoror good thermal insulator. Typical values are 0.093kilocalories/second-metre-°C for copper (a good thermal conductor) and 0.00003kilocalories/second-metre-°C for wood (poor thermal conductor).
ConvectionConvectionis the transfer of internal energy into or out of an object by the physicalmovement of a surrounding fluid that transfers the internal energy along withits mass. Although the heat is initially transferred between the object and thefluid by conduction, the bulk transfer of energy comes from the motion of thefluid. Convection can arise suddenly through the creation of convection cellsor can be forced by propelling the fluid across the object or by the objectthrough the fluid.Spontaneous convection can occur by: – exposed surface area- viscosity- density- conductivity – acceleration due to gravityNatural convection occurs because most fluids have the tendency toexpand when heated—i.e., to become less dense and to rise as a result of theincreased buoyancy.Circulation that occurs this way accounts for the uniformheating of water in a kettle or air in a heated room: the heated moleculesexpand the space they move in through increased speed against one another,rise, and then cool and come closer together again, with increase in density anda resultant sinking.
Forced convection involves the transport of fluid by methods otherthan those that occur from variation of density with temperature. Examples ofconvection are movement of air by a fan or of water by a pump.Atmospheric convection currents can be set up by local heatingeffects such as solar radiation or contact with cold surface masses. Theseconvection currents mainly move vertically and account for many atmospheric existences,such as clouds and thunderstorms. Thermal RadiationThermal radiation is a process by which energy in the form ofelectromagnetic radiation, is emitted by a heated surface in all directions andtravels directly to its point of absorption at the speed of light. Thermalradiation does not require a dominant medium to carry it.
Thermalradiation occurs in wavelengths from the longest infrared rays through thevisible-light spectrum to the shortest ultraviolet rays. The intensity anddistribution of energy within this range of wavelengths depends on thetemperature of the emitting surface. The total radiant heat energy emitted by asurface is proportional to the fourth power of its total temperature (theStefan–Boltzmann law).The nature of the surface links to the rate that which abody radiates or absorbs thermal radiation.
Objects that are good emitters arealso good absorbers (Kirchhoff’s radiation law).A blackened surface is anexcellent emitter and also an excellent absorber. On the other hand, silver isa poor emitter and a poor absorber.
The way the sun heats the earth or heating a room by anopen-hearth fireplace are examples of transfer of energy by radiation. Forheating a room, the flames, coals, and hot bricks radiate heat directly to theobjects in the room with little of this heat being absorbed by the surroundingair.Most of the air that is drawn from the room and heated in the fireplacedoes not re-enter the room in a current of convection but is carried up thechimney together with the products of combustion.