78. The energy distribution of an ideal reflector at higher temperatures is largely in the range of
A.Shorter wavelength
B.Longer wavelength
C.Remains same at all wavelengths
D.Wavelength has nothing to do with it
79. According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
A.Absolute temperature
B.Square of temperature
C.Fourth power of absolute temperature
D.Fourth power of temperature
80. Which of the following property of air does not increase with rise in temperature
A.Thermal conductivity
B.Thermal diffusivity
C.Density
D.Dynamic viscosity
81. Stefan Boltzmann law is applicable for heat transfer by
A.Conduction
B.Convection
C.Radiation
D.Conduction and radiation combined
82. The thermal diffusivities for solids are generally
A.Less than those for gases
B.Jess than those for liquids
C.Inorg than those for liquids and gases
D.More or less same as for liquids and gases
83. Thermal diffusivity of a substance is
A.Directly proportional to thermal conductivity
B.Inversely proportional to density of substance
C.Inversely proportional to specific heat
D.All of the above
84. At thermal equilibrium
A.Absorptivity is greater than emissivity
B.Absorptivity is lesser than emissivity
C.Absorptivity is equal to emissivity
D.Sum of absorptivity and emissivity is unity
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