Planck's law holds good for
Black bodies
Polished bodies
All coloured bodies
All of the above
Correct Answer :
A. Black bodies
Related Questions
Fourier's law of heat conduction gives the heat flow for
Irregular surfaces
Nonuniform temperature surfaces
One dimensional cases only
Two dimensional cases only
Depending on the radiating properties, body will be transparent when
P = 0, x = 0 and a = 1
P=1, x = 0, and a = 0
P = 0, T= 1, and a = 0
X = 0, a + p = 1
The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
Move actually
Do not move actually
Affect the intervening medium
Does not affect the intervening medium
The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number) n, where n is equal to
-1/3
-2/3
1
-1
The amount of radiation mainly depends upon the
Nature of the body
Temperature of the body
Type of surface of the body
All of these
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
Thermal conductivity of water at 20°C is of the order of
0.1
0.23
0.42
0.51
Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.
Cold body to hot body
Hot body to cold body
Smaller body to larger body
Larger body to smaller body
A steam pipe is to be insulated by two insulating materials put over each other. For best results
Better insulation should be put over pipe and better one over it
Inferior insulation should be put over pipe and better one over it
Both may be put in any order
Whether to put inferior OIL over pipe or the better one would depend on steam temperature
Thermal diffusivity of a substance is
Directly proportional to the thermal conductivity
Inversely proportional to density of substance
Inversely proportional to specific heat
All of the above
When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
Conduction
Convection
Radiation
Conduction and convection
40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is
0.45
0.55
0.40
0.75
Thermal diffusivity is a
Function of temperature
Physical property of a substance
Dimensionless parameter
All of these
Which of the following is a case of steady state heat transfer?
I.C. engine
Air preheaters
Heating of building in winter
None of the above
A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,
The better insulation must be put inside
The better insulation must be put outside
One could place either insulation on either side
One should take into account the steam temperature before deciding as to which insulation is put where
When heat is transferred by molecular collision, it is referred to as heat transfer by
Conduction
Convection
Radiation
Scattering
Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
0.002
0.02
0.01
0.1
Fourier's law of heat conduction is valid for
One dimensional cases only
Two dimensional cases only
Three dimensional cases only
Regular surfaces having non-uniform temperature gradients
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)
k. A. (dT/dx)
k. A. (dx/dT)
k. (dT/dx)
k. (dx/dT)
In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in
Parallel flow
Counter flow
Cross flow
All of these
When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by
Conduction
Convection
Radiation
Conduction and convection
In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.
Same
Less
Greater
None of these
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
One
Two
Three
Four
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
25 mm
40 mm
160 mm
800 mm
Kirchhoff's law states that
The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
The wave length corresponding to the maximum energy is proportional to the absolute temperature
The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body
None of the above
Heat transfer by radiation mainly depends upon
Its temperature
Nature of the body
Kind and extent of its surface
All of the above
The heat of sun reaches to us according to
Conduction
Convection
Radiation
None of these
Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in
Electric heater
Steam condenser
Boiler
Refrigerator condenser coils
Which of the following property of air does not increase with rise in temperature?
Thermal conductivity
Thermal diffusivity
Density
Dynamic viscosity