Differences and examples of Convection and Radiation

Heat can be transferred from one point of the body to another end of the body by three different methods: conduction, convection, and radiation.

Convection 

The heat transfer method in liquids and gases is convection. In heat transfer by convection, there is a motion of heated molecules that carry the heat with it.

• In liquids and gases, the molecules are free to move. 
• The movement of heated molecules can occur in liquids and gases, which can thus result in the transfer of heat throughout the medium. 
• Hence, the heat transfer method in liquids and gases is by convection. The convection process can be free convection or forced convection.

Free convection is the natural mechanism of convection.

• In the free convection process, when a fluid (liquid/ gas) is heated from the bottom, the bottom surface molecules become less dense. 
• These heated molecules rise above because of buoyancy and are replaced by colder molecules. 
• This process of heat flow continues, and thus the heat transfer through the medium (fluid) is achieved.

In forced convection, the material moves by an external physical means.

Examples of Convection

1. Natural phenomenon of the formation of land and sea breezes is an example of free convection.

• During the day, the land heats up more quickly than the larger water bodies (sea). 
• The air in contact with the land gets heated. 
• It expands and becomes less dense than the cooler air surrounding it.
• Thus, colder air replaces the heated air, which rises. 
• This creates a sea breeze near large water bodies.
• During the daytime, cooler air descends, and a thermal convection cycle manifests.
• It transfers heat away from the land.
• At night, the land loses heat more quickly than the water body. 
• Hence the water body is warmer than the land.
• This sets land breeze as the thermal convection cycle 
• The cycle is reversed, and the heat is transferred away from the water body.

2. The mechanism of heating or cooling a room is based on the method of forced convection.

• The air conditioner uses the principle of convection for cooling a room.
• It releases cold air. 
• The dense cold air sinks while the air conditioner draws the less dense hot air. 

Radiation

Radiation is the method of heat transfer that does not require a material medium like convection which occurs in a fluid medium. Radiation is the heat transfer from one place to another via the emission of electromagnetic energy.

It is a fact that the earth receives heat energy from the sun. Thus it is evident that radiation does not necessarily require a material medium. It can travel through a vacuum. The transfer of heat by radiation has properties the same as EM waves.

• All bodies tend to emit EM radiation. 
• These are thermal radiations. This thermal radiation / radiant energy emitted is dependent on the temperature of the body. 
• When these thermal radiations are incident on a cold body, energy in the form of heat is transferred to the body.
• Thus the transfer of heat by radiation is a twofold process. 
• First is the thermal energy conversion into waves, then the second is the reconversion of waves into thermal energy by the body on which they fall.

Example of Radiation

1) Black Body radiation means that every object tends to emit thermal radiation.

• Example: An iron nail heated to a high temperature becomes red hot. The bulb’s filament is heated because the current flow emits thermal radiation, and we can see the bulb glow.

2) We find that black bodies emit and absorb energy better than lighter colours. 

• Hence, we wear white or light coloured clothes during summer to absorb less heat from the sun, and we do not feel too hot. While in winter we prefer dark coloured clothes as it absorbs more heat so that we may warm.

Differences – convection v/s radiation

1. Convection happens in a fluid medium. Heat is transferred by the movement of molecules in the medium.

Radiation does not require a medium. Heat is transferred using electromagnetic waves.

Conclusion

Convection and radiation are thus two different methods of heat transfer from one point to another. The process of heat transfer by convection happens in the case of fluids. Example – Boiling of water, air conditioner in a room.

Radiation is the process of heat transfer using EM energy. The properties of radiation are similar to that of an EM wave. Radiation does not require a medium. Every object, irrespective of solid, liquid or gas, emits radiation. Radiations are referred to as thermal radiation, which is a twofold process.