Peltier Effect Definition
When current flows across the junctions of a Thermocouple, heat is generated (evolved) or absorbed i.e the junction gets either heated or cooled. This effect is called the Peltier effect.
The condition of whether the junction shall be heated or cooled is determined by the direction of the current. Thus Peltier effect is the inverse of the Seebeck effect.
We know when the Fe-Cu junction is heated, the electric current flows from Cu to Fe across the hot junction (Seebeck effect).
On the other hand, if both the junctions are at the same temperature and current is sent to flow from Fe to Cu, then heat is produced at the junction where the current enters from Fe to Cu, and heat is absorbed in the junction where the current enters from Cu to Fe. The Peltier effect is reversible.
Explanation of Peltier Effect
When dissimilar metals are joined, electrons are interchanged between the metals. Since the number of electrons per unit volume (electron density) of metal are different, the metal becomes oppositely charged.
This gives rise to an electric field. Now when the external potential difference from a cell or battery is set up as the two ends of the junctions, then work is done in moving a unit charge against or in the direction of the electric field.
Hence Energy is absorbed when a charge moves in overcoming the rise of potential bringing about a fall of temperature. On the other hand, heat is liberated at the junction when the charge moves along the fall of potential.
The amount of heat energy absorbed or evolved at a Junction of two different metals when one coulomb of electricity flows at the junction is called the Peltier Coefficient. it is denoted by π,
Let, Peltier coefficient of a junction = π
Current Flowing through the junction for time t second = i
Energy absorbed or evolved at the junction = π it
if e.m.f (i.e Peltier e.m.f) set up at Junction = e
Then, energy absorbed or evolved the junction = e it.
e it = π it or π= e
Thus, Peltier Coefficient is numerically equal to e.m.f set up at the junction.
Difference between Joule heating effect and Peltier Effect
The Joule effect is always a heating effect and it is proportional to the square of the current flowing through a conductor. It is an irreversible process.
On the other hand, the Peltier effect is directly proportional to the current and is a reversible process i.e, either heat is produced or absorbed depending upon the direction of the current.
Joule heating effect occurs in all parts of the conductor, whereas the Peltier effect is restricted to the junction only.