This unit deals with conduction and convection, while radiation is covered in the following unit . For a good conductor, such as a metal, K can be measured by Searle's method , measuring the temperature gradient along a lagged bar for a known rate of heat flow. For a poor conductor, a thin sample is used to maximise the heat flow and reduce heat losses, in apparatus known as Lee's disk .
Natural convection occurs due to the expansion of fluid in contact with a hot body.
The reduced density causes it to rise under the influence of gravity. Forced convection occurs when the motion of the fluid is maintained by some external agency such as a fan or pump. A convection coefficient, h , can again be defined such that where A is the area of the body and T the temperature difference between body and fluid. The glass of the window of a room is 5 mm thick and has an area of 4 m 2 . Repeat the calculation for a double glazed window consisting of two 5 mm thick glass sheets separated by non-convecting air 2.5 mm thick.