According to Prevost theory of heat exchange
It is impossible to transfer heat from low temperature source to t high temperature source
Heat transfer by radiation requires no medium
All bodies above absolute zero emit radiation
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
Correct Answer :
C. All bodies above absolute zero emit radiation
Related Questions
The automobile radiator is a heat exchanger of
Parallel flow type
Counter flow type
Cross flow type
Regenerator type
The thermal diffusivities for solids are generally
Less than those for gases
Less than those for liquids
More than those for liquids and gases
More or less same as for liquids and gases
Thermal conductivity of air with rise in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
The transfer of heat by molecular collision is known as
Conduction
Convection
Radiation
None of these
The most commonly used method for the design of duct size is the
Velocity reduction method
Equal friction method
Static regains method
Dual or double method
Heat transfer in liquid and gases takes place by
Conduction
Convection
Radiation
Conduction and convection
Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called
Thermal resistance
Thermal coefficient
Temperature gradient
Thermal conductivity
The critical temperature is the temperature
Below which a gas does not obey gas laws
Above which a gas may explode
Below which a gas is always liquefied
Above which a gas will never liquefied
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
In a shell and tube heat exchanger, baffles are provided on the shell side to
Improve heat transfer
Provide support for tubes
Prevent stagnation of shell side fluid
All of these
The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
Temperature
Thickness
Area
Time
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
The total radiation from a black body per second per unit area is ________ fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.
Equal to
Directly proportional to
Inversely proportional to
None of these
The time constant of a thermocouple is
The time taken to attain the final temperature to be measured
The time taken to attain 50% of the value of initial temperature difference
The time taken to attain 63.2% of the value of initial temperature difference
Determined by the time taken to reach 100°C from 0°C
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 case of liquids and gases, the heat transfer takes place according to
Conduction
Convection
Radiation
None of these
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
Free convection flow depends on
Density
Coefficient of viscosity
Gravitational force
All of these
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 thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is
Equal to one
Greater than one
Less than one
Equal to Nusselt number
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 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
Conduction is a process of heat transfer
From one particle of the body to another without the actual motion of the particles
From one particle of the body to another by the actual motion of the heated particles
From a hot body to a cold body, in a straight line, without affecting the intervening medium
None of the above
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
Grey body
Brilliant white polished body
Red hot body
Black body
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
The emissivity for a black body is
0
0.5
0.75
1
Metals are good conductors of heat because
Their atoms collide frequently
Their atoms are relatively far apart
They contain free electrons
They have high density
Stefan Boltzmann law is applicable for heat transfer by
Conduction
Convection
Radiation
Conduction and radiation combined