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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
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Heat transfer takes place as per
A. Zeroth law of thermodynamics
B. First law of thermodynamic
C. Second law of the thermodynamics
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Planck's law holds good for
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The unit of overall coefficient of heat transfer is
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When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,
A. α = 1, ρ = 0 and τ = 0
B. α = 0, ρ = 1 and τ = 0
C. α = 0, ρ = 0 and τ = 1
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4
The critical thickness of insulation for a sphere is
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The amount of radiation mainly depends upon the
B. Temperature of the body
C. Type of surface of the body
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When heat is transferred by molecular collision, it is referred to as heat transfer by
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According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
B. Brilliant white polished body
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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)
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A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is
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According to Stefan's law, the total radiation from a black body per second per unit area is proportional to
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Which of the following has maximum value of thermal conductivity?
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The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
C. Affect the intervening medium
D. Does not affect the intervening medium
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Fourier's law of heat conduction gives the heat flow for
B. Nonuniform temperature surfaces
C. One dimensional cases only
D. Two dimensional cases only
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Which of the following statement is wrong?
A. The heat transfer in liquid and gases takes place according to convection.
B. The amount of heat flow through a body is dependent upon the material of the body.
C. The thermal conductivity of solid metals increases with rise in temperature
D. Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.
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Thermal conductivity of air with rise in temperature
D. May increase or decrease depending on temperature
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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
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The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
A. Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
B. Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
C. Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
D. Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
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Which of the following is a case of steady state heat transfer?
C. Heating of building in winter
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Heat is closely related with
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Fouling factor is used
A. In heat exchanger design as a safety factor
B. In case of Newtonian fluids
C. When a liquid exchanges heat with a gas
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Total emissivity of polished silver compared to black body is
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Thermal conductivity of non-metallic amorphous solids with decrease in temperature
D. May increase or decrease depending on temperature
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Moisture would find its way into insulation by vapour pressure unless it is prevented by
A. High thickness of insulation
C. Less thermal conductivity insulator
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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
D. Conduction and convection
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The heat of sun reaches to us according to
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Thermal conductivity of water in general with rise in temperature
D. May increase or decrease depending on temperature
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Thermal conductivity of solid metals with rise in temperature normally
D. May increase or decrease depending on temperature
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According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
B. Square of the absolute temperature
C. Cube of the absolute temperature
D. Fourth power of the absolute temperature