Heat And Mass Transfer - Study Mode
[#76] Planck’s law holds good for
Correct Answer
(A) Black bodies
Explanation
Solution: Planck's law is a formula for the spectral radiance of an object at a given temperature as a function of frequency (Lf) or wavelength (Lλ). Planck’s law holds good for black bodies. It has dimensions of power per solid angle per area per frequency or power per solid angle per area per wavelength.
[#77] Thermal conductivity of a material may be defined as the
Correct Answer
(D) All of the above
Explanation
Solution: Thermal conductivity of a material may be defined as the: - Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C. - Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C. - Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces.
[#78] A perfect black body is one which
Correct Answer
(D) Absorbs heat radiations of all wave lengths falling on it
Explanation
Solution: A perfect blackbody is one that absorbs all incoming light and does not reflect any.
At room temperature, such an object would appear to be perfectly black .
However, if heated to a high temperature, a blackbody will begin to glow with thermal radiation
[#79] The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
Correct Answer
(B) Do not move actually
Explanation
Solution: The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body Do not move actually
[#80] The thermal diffusivities for solids are generally
Correct Answer
(C) More than those for liquids and gases
Explanation
Solution: Thermal diffusivity is defined as the ratio of thermal conductivity to the product of density and specific heat capacity at constant pressure. It indicates how quickly a material can conduct thermal energy relative to its ability to store it . Solids generally have much higher thermal conductivity compared to liquids and gases , and although their density and specific heat may also be higher, the conductivity dominates, resulting in higher thermal diffusivity. Liquids and gases typically have lower thermal conductivity and, in many cases, similar or lower specific heat, which makes their thermal diffusivity values lower than that of solids. Option A: Less than those for gases Incorrect. Gases usually have very low thermal diffusivity. Option B: Less than those for liquids Incorrect. Liquids have lower thermal diffusivity than most solids. Option C: More than those for liquids and gases Correct. Solids, particularly metals, typically have higher thermal diffusivity than both liquids and gases. Option D: More or less same as for liquids and gases Incorrect. Thermal diffusivity varies significantly among solids, liquids, and gases. Correct Answer: Option C: More than those for liquids and gases