The heat transfer by conduction through a thick sphere is given by
Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)
Correct Answer :
B. Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
Related Questions
The product of Reynolds number and Prandtl number is known as
Stanton number
Biot number
Peclet number
Grashoff number
The expression Q = ρ AT4 is called
Fourier equation
Stefan-Boltzmann equation
Newton Reichmann equation
Joseph-Stefan equation
Which of the following would lead to a reduction in thermal resistance?
In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
In convection, stirring of the fluid and cleaning the heating surface.
In radiation, increasing the temperature and reducing the emissivity.
All of the above
Heat transfer takes place as per
Zeroth law of thermodynamics
First law of thermodynamic
Second law of the thermodynamics
Kirchoff's law
Total heat is the heat required to
Change vapour into liquid
Change liquid into vapour
Increase the temperature of a liquid or vapour
Convert water into steam and superheat it
Thermal conductivity of air with rise 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)
Joule sec is the unit of
Universal gas constant
Kinematic viscosity
Thermal conductivity
Planck's constant
Which of the following statement is wrong?
The heat transfer in liquid and gases takes place according to convection.
The amount of heat flow through a body is dependent upon the material of the body.
The thermal conductivity of solid metals increases with rise in temperature
Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.
The concept of overall coefficient of heat transfer is used in case of heat transfer by
Conduction
Convection
Radiation
Conduction and convection
The value of the wavelength for maximum emissive power is given by
Wien's law
Planck's law
Stefan's law
Fourier's law
Emissivity of a white polished body in comparison to a black body is
Higher
Lower
Same
Depends upon the shape of body
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
Unit of thermal conductivity in S.I. units is
J/m² sec
J/m °K sec
W/m °K
Option (B) and (C) above
Stefan Boltzmann law is applicable for heat transfer by
Conduction
Convection
Radiation
Conduction and radiation combined
Thermal conductivity of glass wool varies from sample to sample because of variation in
Composition
Density
Porosity
All of the above
In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by
Convection
Radiation
Conduction
Both convection and conduction
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
Planck's law holds good for
Black bodies
Polished bodies
All coloured bodies
All of the above
The process of heat transfer from one particle of the body to another by the actual motion of the heated particles, is called
Conduction
Convection
Radiation
None of these
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
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
The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called
First law of thermodynamics
Newton's law of cooling
Newton's law of heating
Stefan's law
The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as
Conduction
Free convection
Forced convection
Radiation
The critical radius is the insulation radius at which the resistance to heat flow is
Maximum
Minimum
Zero
None of these
The transfer of heat by molecular collision is known as
Conduction
Convection
Radiation
None of these
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 rate of energy transferred by convection to that by conduction is called
Stanton number
Nusselt number
Biot number
Peclet number
The natural convection air cooled condensers are used in
Domestic refrigerators
Water coolers
Room air conditioners
All of these
According to Newton's law of cooling, the heat transfer from a hot body to a cold body is
Directly proportional to the surface area
Directly proportional to the difference of temperatures between the two bodies
Either (A) or (B)
Both (A) and (B)