Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

The correct answer is (C) Gases.

Let's break down why:

Heat transfer by molecular collision refers to how heat energy moves through a substance because its molecules bump into each other.

* Solids: In solids, molecules are packed very closely together. This means collisions happen frequently and efficiently. Think of it like a tightly packed crowd where people can easily bump into each other and pass something along. So, solids are usually good conductors of heat via collision.

* Liquids: Liquids have molecules that are closer than gases, but not as close as solids. Collisions happen more often than in gases, making them better conductors than gases but not as good as solids.

* Gases: In gases, the molecules are very far apart and move randomly. This means collisions are infrequent. Because of the large spaces between molecules, energy transfer by collision is the least effective in gases.

In Summary:
The smallest heat transfer by molecular collision occurs in gases because their molecules are the most spread out.

Stefan-Boltzmann law, statement that the total radiant heat power emitted from a surface is proportional to the fourth power of its absolute temperature.

Cork is a good insulator because it has porus body

Heat flowing from one side to other depends directly on , Face area , Time ,Temperature difference

Thermal conductivity of solid metals with rise in temperature normally Decreases

The heat transfer by conduction through a thick sphere is given by $${\text{Q}} = \frac{{4\pi {\text{k}}{{\text{r}}_1}{{\text{r}}_2}\left( {{{\text{T}}_1} - {{\text{T}}_2}} \right)}}{{{{\text{r}}_2} - {{\text{r}}_1}}}.$$

When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

If the Prandtl number (Pr) is 1, the two boundary layers (hydrodynamic and thermal) are the same thickness.
If the Pr > 1, the thermal boundary layer is thinner than the velocity boundary layer.

Real materials emit energy at a fraction called the emissivity of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.