Heat and Mass Transfer
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

800 mm
160 mm
40 mm
25 mm

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Heat and Mass Transfer
According to Prevost theory of heat exchange

Heat transfer by radiation requires no medium
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
All bodies above absolute zero emit radiation
It is impossible to transfer heat from low temperature source to t high temperature source

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