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4

The thermal diffusivities for solids are generally

A. Less than those for gases

B. Less than those for liquids

C. More than those for liquids and gases

D. More or less same as for liquids and gases

Correct Answer :

C. More than those for liquids and gases


Related Questions

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4

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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4

According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the

A. Absolute temperature

B. Square of the absolute temperature

C. Cube of the absolute temperature

D. Fourth power of the absolute temperature

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4

The critical radius is the insulation radius at which the resistance to heat flow is

A. Maximum

B. Minimum

C. Zero

D. None of these

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4

Depending on the radiating properties, body will be transparent when

A. P = 0, x = 0 and a = 1

B. P=1, x = 0, and a = 0

C. P = 0, T= 1, and a = 0

D. X = 0, a + p = 1

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4

Reynolds number is the ratio of

A. Energy transferred by convection to that by conduction

B. Kinematic viscosity to thermal diffusivity

C. Inertia force to viscous force

D. None of the above

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4

Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in

A. Electric heater

B. Steam condenser

C. Boiler

D. Refrigerator condenser coils

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4

The value of the wavelength for maximum emissive power is given by

A. Wien's law

B. Planck's law

C. Stefan's law

D. Fourier's law

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4

Heat transfer in liquid and gases takes place by

A. Conduction

B. Convection

C. Radiation

D. Conduction and convection

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4

Thermal diffusivity is a

A. Function of temperature

B. Physical property of a substance

C. Dimensionless parameter

D. All of these

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4

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

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4

A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,

A. The better insulation must be put inside

B. The better insulation must be put outside

C. One could place either insulation on either side

D. One should take into account the steam temperature before deciding as to which insulation is put where

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4

The product of Reynolds number and Prandtl number is known as

A. Stanton number

B. Biot number

C. Peclet number

D. Grashoff number

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4

Thermal conductivity of wood depends on

A. Moisture

B. Density

C. Temperature

D. All of the above

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4

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

A. Conduction

B. Convection

C. Radiation

D. Conduction and convection

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4

The amount of heat flow through a body by conduction is

A. Directly proportional to the surface area of the body

B. Directly proportional to the temperature difference on the two faces of the body

C. Dependent upon the material of the body

D. All of the above

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4

The most commonly used method for the design of duct size is the

A. Velocity reduction method

B. Equal friction method

C. Static regains method

D. Dual or double method

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4

Thermal conductivity of water at 20°C is of the order of

A. 0.1

B. 0.23

C. 0.42

D. 0.51

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4

Depending on the radiating properties, a body will be black when

A. P = 0, x = 0 and a = 1

B. P= 1, T = 0 and a = 0

C. P = 0, x = 1 and a = 0

D. X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.

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4

The expression Q = ρ AT4 is called

A. Fourier equation

B. Stefan-Boltzmann equation

C. Newton Reichmann equation

D. Joseph-Stefan equation

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4

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

D. α + ρ = 1 and τ = 0

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4

Unit of thermal diffusivity is

A. m²/hr

B. m²/hr °C

C. kcal/m² hr

D. kcal/m. hr °C

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4

Joule sec is the unit of

A. Universal gas constant

B. Kinematic viscosity

C. Thermal conductivity

D. Planck's constant

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4

Sensible heat is the heat required to

A. Change vapour into liquid

B. Change liquid into vapour

C. Increase the temperature of a liquid of vapour

D. Convert water into steam and superheat it

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4

Fourier's law of heat conduction is valid for

A. One dimensional cases only

B. Two dimensional cases only

C. Three dimensional cases only

D. Regular surfaces having non-uniform temperature gradients

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4

In heat exchangers, degree of approach is defined as the difference between temperatures of

A. Cold water inlet and outlet

B. Hot medium inlet and outlet

C. Hot medium outlet and cold water inlet

D. Hot medium outlet and cold water outlet

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4

The value of the wave length for maximum emissive power is given by

A. Kirchhoff's law

B. Stefan's law

C. Wines law

D. Planck's law

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4

The unit of overall coefficient of heat transfer is

A. W/m²K

B. W/m²

C. W/mK

D. W/m

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4

Heat is closely related with

A. Liquids

B. Energy

C. Temperature

D. Entropy

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4

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

A. -1/3

B. -2/3

C. 1

D. -1

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4

Emissivity of a white polished body in comparison to a black body is

A. Higher

B. Lower

C. Same

D. Depends upon the shape of body