Question

The process in which rate of transfer of heat is maximum is :
A. Conduction
B. Convection
C. Radiation
D. In all these, heat is transferred with same speed

Answer 1

Hint: Heat transfer works on the basic principle of temperature and flow of heat. Temperature determines the amount of thermal energy available and the movement of thermal energy is called heat flow. Use the formulas of heat energy radiated by material by conduction, convection and radiation to compare the values of heat energy transferred.

Complete step by step answer:
The exchange of thermal energy between two bodies/systems by dissipating heat is called heat transfer. Heat transfer works on the basic principle of temperature and flow of heat. Temperature determines the amount of thermal energy available and the movement of thermal energy is called heat flow.
On the microscopic level, the thermal energy of molecules is in direct relation with their kinetic energy. As the temperature increases the thermal energy of molecules increases due to which their frequency of vibration increases (they tend to vibrate fast), as a result their kinetic energy also increases.
Rate of Conduction $Q = \dfrac{{[k \cdot A \cdot ({T_{hot}} - {T_{cold}})]}}{d}$ , where Q= heat transferred per unit time; k=thermal conductivity of the barrier; A= heat-transfer area; d=thickness of the barrier; ${T_{hot}} = $temperature of hot region and ${T_{cold = }}$ temperature of the cold region.
Rate of Convection, $Q = {h_c} \cdot A \cdot ({T_s} - {T_f})$ where Q= heat transferred per unit time; ${h_c} = $ convective heat transfer coefficient; A= heat-transfer area of the surface; ${T_s} = $ temperature of the surface and ${T_f} = $ temperature of the fluid.
Rate of Radiation, $P = e \cdot \sigma \cdot A \cdot (T_r^4 - T_c^4)$ where P= net radiated power; A= radiating area; ${T_r} = $ temperature of the radiator; ${T_c} = $ temperature of surroundings; e= emissivity and $\sigma = $ Stefan’s constant.
Now on comparing the above formulas, we see that the rate of radiation involves the fourth power of temperature which makes the answer quite big compared to the other two.
Thus, we can conclude that the rate of transfer of heat is maximum in the process of radiation.

So, the correct answer is “Option C”.

Additional Information:
Heat transfer is grouped into three categories:
Conduction:
In conduction heat is transferred by direct molecular contact. The higher speed particles collide with slower speed particles and thus increase their kinetic energy. Thus their thermal energy also increases. As a result, heat is transferred from a region of higher thermal energy to the region of lower thermal energy.
Convection:
When a liquid or gas is heated, the fluid above the hot surface expands, becomes less dense and rises. As the fluid rises, the upper fluid comes down and gets heated. This is how heat transfer takes place via convection.
Radiation:
Radiation is the process of transferring heat energy from one body to another without heating the intervening medium. This transfer of energy takes place by electromagnetic waves.

Note:
The process of heat conduction depends on the temperature gradient, area of cross-section of the material, length of the path traveled and physical properties of the material.
In convection, when the molecules are heated, they expand and their molecular volume increases while the molecular mass remains the same. Density is inversely proportional to the volume when mass is fixed. Thus the density of the fluid decreases.
In radiation, electromagnetic radiation is the result of the movement of charged protons and electrons.