Absorptivity of a body will be equal to its emissivity
At all temperatures
At one particular temperature
When system is under thermal equilibrium
At critical temperature
Correct Answer :
C. When system is under thermal equilibrium
Related Questions
The critical radius is the insulation radius at which the resistance to heat flow is
Maximum
Minimum
Zero
None of these
The unit of overall coefficient of heat transfer is
W/m²K
W/m²
W/mK
W/m
When absorptivity (α) = 1, reflectivity (ρ) = 0 and transmissivity (τ) = 0, then the body is said to be a
Black body
Grey body
Opaque body
White body
Film coefficient is defined as the ratio of
Thermal conductivity to the equivalent thickness of the film of fluid
Temperature drop through the films of fluids to the thickness of film of fluids
Thickness of film of fluid to the thermal conductivity
Thickness of film of fluid to the temperature drop through the films of fluids
Which of the following is the case of heat transfer by radiation?
Blast furnace
Heating of building
Cooling of parts in furnace
Heat received by a person from fireplace
When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,
α = 1, ρ = 0 and τ = 0
α = 0, ρ = 1 and τ = 0
α = 0, ρ = 0 and τ = 1
α + ρ = 1 and τ = 0
The emissivity for a black body is
0
0.5
0.75
1
The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as
Emissivity
Transmissivity
Reflectivity
Intensity of radiation
In free convection heat transfer, Nusselt number is function of
Grashoff number and Reynold number
Grashoff number and Prandtl number
Prandtl number and Reynold number
Grashoff number, Prandtl number and Reynold number
A grey body is one whose absorptivity
Varies with temperature
Varies with wavelength of the incident ray
Is equal to its emissivity
Does not vary with temperature and. wavelength of the incident ray
If the energy radiated per second per sq. cm. of the surface for wave lengths lying between λ, and λ + dλ is represented by (eλ.dλ), then eλ is called
Absorptive power
Emissive power
Emissivity
None of these
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
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
A grey body is one whose absorptivity
Varies with temperature
Varies with the wave length of incident ray
Varies with both
Does not vary with temperature and wave length of the incident ray
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
The amount of radiation mainly depends on
Nature of body
Temperature of body
Type of surface of body
All of the above
Depending on the radiating properties, a body will be black when
P = 0, x = 0 and a = 1
P= 1, T = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.
Total emissivity of polished silver compared to black body is
Same
Higher
More or less same
Very much lower
According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
Absolute temperature
Square of the absolute temperature
Cube of the absolute temperature
Fourth power of the absolute temperature
Thermal conductivity of air with rise in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
Unit of thermal conductivity in M.K.S. units is
K cal/kg m² °C
K cal m/hr m² °C
K cal/hr m² °C
K calm/hr °C
Thermal conductivity of water at 20°C is of the order of
0.1
0.23
0.42
0.51
LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is
Higher
Lower
Same
Depends on the area of heat exchanger
When heat is transferred by molecular collision, it is referred to as heat transfer by
Conduction
Convection
Radiation
Scattering
Fourier's law of heat conduction gives the heat flow for
Irregular surfaces
Nonuniform temperature surfaces
One dimensional cases only
Two dimensional cases only
The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
Temperature
Thickness
Area
Time
A perfect black body is one which
Is black in colour
Reflects all heat
Transmits all heat radiations
Absorbs heat radiations of all wave lengths falling on it
The concept of overall coefficient of heat transfer is used in case of heat transfer by
Conduction
Convection
Radiation
Conduction and convection
Thermal conductivity of wood depends on
Moisture
Density
Temperature
All of the above
Depending on the radiating properties, a body will be white when
P = 0, x = 0 and a = 1
P=1, T = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 1 Where a = absorptivity, p = reflectivity, x = transmissivity