Thermal Radiation-

 

Before you go through this article, make sure that you have gone through the previous article on Thermal Radiation.

 

We have learnt that-

• Thermal radiation are electromagnetic waves emitted by a body by virtue of its temperature.
• They are produced due to thermal motion of particles & cause heating effect.

 

In this article, we will learn about Prevost’s Theory of Heat Exchange.

 

Prevost’s Theory of Heat Exchange-

 

According to this theory,

• All bodies, regardless of their temperature (as long as its above absolute zero) continuously emit and absorb thermal radiation.
• The rate at which a body emits / radiates heat depends on the nature of its surface and temperature. The hotter the body, the more it radiates.
• The rate at which a body absorbs heat depends on the nature of its surface and the temperature of its surroundings.
• The rise or fall in temperature of a body is the net result of heat exchange between the body and its surroundings.

 

Let-

T_b = Temperature of the body

T_s = Temperature of the surrounding

 

Now, following three cases are possible-

 

Case-01: If T_b > T_s

• In this case, the rate of emission > rate of absorption.
• So, the body loses heat and thus its temperature will decrease.
• This results in cooling of the body.

 

Example- Hot tea kept in a room

 

Case-02: If T_b < T_s

• In this case, the rate of absorption > rate of emission.
• So, the body gains heat and thus its temperature will increase.
• This results in heating of the body.

 

Example- Ice kept in a room

 

Case-03: If T_b = T_s

• In this case, the rate of emission = rate of absorption.
• So, there is no net heat exchange and thus there is no change in temperature.
• A dynamic thermal equilibrium is established between the body and its surroundings.

 

Example- A book lying in a room

 

Important Note In this state, if a body absorbs a large fraction of the heat falling upon it, it must also radiate the same amount of heat back to the surroundings otherwise its temperature will change. This shows that a body which is a good absorber is also a good radiator of heat and vice-versa.

 

Important Conclusions of Prevost’s Theory-

 

• Heat radiation is a continuous process.
• No body stops emitting heat, even at low temperature (> 0K).
• Heat exchange depends on temperature difference, not on emission alone.
• It explains cooling and heating of bodies.
• It supports Stefan-Boltzmann law (emission depends on temperature).