Question

Explain the different modes of ‘Heat Transfer’.

Hint:

Name the three modes: conduction, convection and radiation. Give a short explanation, the governing law and one example for each.

Answer 1

Step 1: Understanding the Concept:
Heat always flows from a hotter body to a colder body. This flow is called heat transfer. It takes place in three modes: conduction, convection and radiation.

Step 2: Conduction.
Conduction is the flow of heat through a solid, or between solids in contact, with no visible movement of the material.
The vibrating molecules pass their energy to the neighbouring molecules.
Fourier's law gives the rate of heat flow:
\[ Q = -kA\frac{dT}{dx} \]
Here \(k\) is the thermal conductivity, \(A\) is the area and \(\frac{dT}{dx}\) is the temperature gradient.
Example: heat moves along a copper pipe of the evaporator, or through a wall of the cold room.

Step 3: Convection.
Convection is the flow of heat by the movement of a fluid (a liquid or a gas).
The fluid carries heat from a hot surface to its cooler surroundings.
In natural convection, the density difference moves the fluid. In forced convection, a fan or pump moves it.
Newton's law of cooling gives the rate: \( Q = hA(T_s - T_f) \), where \(h\) is the heat transfer coefficient.
Example: air moving over the condenser coil of a refrigerator.

Step 4: Radiation.
Radiation is the flow of heat as electromagnetic waves. It needs no medium and can pass through a vacuum.
Every hot body radiates heat. The Stefan-Boltzmann law gives the rate:
\[ Q = \varepsilon \sigma A T^{4} \]
Here \(\varepsilon\) is the emissivity, \(\sigma\) is the Stefan-Boltzmann constant and \(T\) is the absolute temperature.
Example: heat from the Sun entering a room through a glass window.

Final Answer:
The three modes of heat transfer are conduction (through solids), convection (through moving fluids) and radiation (through waves, no medium needed).