Thermal Conduction, Convection and Radiation

Thermal Conduction

Thermal conduction is the transfer of energy arising from temperature differences between adjacent parts of a body. Thermal conductivity is recognised as the exchange of energy between molecules and electrons in the conducting medium. The rate of heat flow in a rod of any material is proportional to the cross-sectional area of the rod and to the temperature difference between the ends and inversely proportional to the length; that is the rate H equals the ratio of the cross section A of the rod to its length l, multiplied by the temperature difference (T2 - T1) and by the thermal conductivity of the material, designated by the constant k.

A substance of large thermal conductivity k is a good heat conductor, whereas one with small thermal conductivity is a poor heat conductor or good thermal insulator. Typical values are 0.093 kilocalories/second-metre-°C for copper (a good thermal conductor) and 0.00003 kilocalories/second-metre-°C for wood (poor thermal conductor).

Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid that transfers the internal energy along with its mass. Although the heat is initially transferred between the object and the fluid by conduction, the bulk transfer of energy comes from the motion of the fluid. Convection can arise suddenly through the creation of convection cells or can be forced by propelling the fluid across the object or by the object through the fluid.

Spontaneous convection can occur by:

• exposed surface area
• viscosity
• density
• conductivity
• acceleration due to gravity

Natural convection occurs because most fluids have the tendency to expand when heated—i.e., to become less dense and to rise as a result of the increased buoyancy.Circulation that occurs this way accounts for the uniform heating of water in a kettle or air in a heated room: the heated molecules expand the space they move in through increased speed against one another, rise, and then cool and come closer together again, with increase in density and a resultant sinking. Forced convection involves the transport of fluid by methods other than those that occur from variation of density with temperature. Examples of convection are movement of air by a fan or of water by a pump. Atmospheric convection currents can be set up by local heating effects such as solar radiation or contact with cold surface masses. These convection currents mainly move vertically and account for many atmospheric existences, such as clouds and thunderstorms.

Thermal Radiation

Thermal radiation is a process by which energy in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light. Thermal radiation does not require a dominant medium to carry it. Thermal radiation occurs in wavelengths from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays. The intensity and distribution of energy within this range of wavelengths depends on the temperature of the emitting surface. The total radiant heat energy emitted by a surface is proportional to the fourth power of its total temperature (the Stefan–Boltzmann law).

The nature of the surface links to the rate that which a body radiates or absorbs thermal radiation. Objects that are good emitters are also good absorbers (Kirchhoff’s radiation law).A blackened surface is an excellent emitter and also an excellent absorber. On the other hand, silver is a poor emitter and a poor absorber. The way the sun heats the earth or heating a room by an open-hearth fireplace are examples of transfer of energy by radiation. For heating a room, the flames, coals, and hot bricks radiate heat directly to the objects in the room with little of this heat being absorbed by the surrounding air.Most of the air that is drawn from the room and heated in the fireplace does not re-enter the room in a current of convection but is carried up the chimney together with the products of combustion.