Thermoelectric Effect Note | Class 12 Physics | NEB | NEPALI EDUCATE

Syllabus: Content to study

1. Seebeck effect; Thermocouples
2. Peltier effect: Variation of thermoelectric e.m.f. with temperature; Thermopile

Complete Note

Introduction:

The phenomenon in which electricity is generated by the means of thermal energy is called thermoelectric effect.
The thermal electric effects are:
1. Seebeck effect
2. Peltier effect
3. Thomson effect

Seebeck effect:

The phenomenon of conversion of heat energy into electrical energy when two junctions of thermocouples are kept at different temperatures is called Seebeck effects.
Thermocouples:
The pairs of wire arranged to produce thermal electric current are called thermocouples.
Example: Fe-Cu Tube

Diagram: Fe-Cu Thermocouples

Thermoelectric Series:

The arrangement of metals in series in which two metals can be used to form a thermocouple is called thermoelectric series the greater the difference in the order in the series the higher is the value of EMF produced.

Thermoelectric series is given below:

Antimony Iron Zinc Silver Lead Copper Platinum Cobalt Bismuth

Variation of thermos-EMF with temperature:

To study the variation of thermo-emf with temperature, an iron copper thermocouple is taken as shown in the figure. One junction has emerged in an oil bath and the other junction is kept at melting ice most temperature is kept constant. The temperature of the oil bath is increased gradually by heating it.

When the temperature of the hot junction is increased and the cold junction is kept at 0 degree Celsius the deflection on the galvanometer increases that is EMF also increases till it becomes maximum at θ_n neutral temperature.
As the temperature of the hot junction is increased beyond neutral temperature thermos-EMF starts to decrease and ultimately becomes zero at θ_i temperature called as temperature of inversion.
If the temperature is increased beyond θ_i the direction of thermos-emf is reversed. The inversion temperature depends upon the temperature of cold junction and nature of metal used in thermocouple.
The variation of thermos-emf with temperature is shown in the figure given below

Diagram: Variation of EMF with Temperature

The variation of thermos-emf with temperature is given by the expression:
\[E = \alpha \theta  + \frac{1}{2}\beta {\theta ^2}\]

Where α and β are constant and their value depend upon the material of the conductor and the temperature difference between the junctions.

If θ_c be the temperature of the cold junction. Then we have
\[{\theta _{i\;}}--{\rm{ }}{\theta _{{\bf{n}}\;}} = \;{\theta _{\bf{n}}}\; - {\rm{ }}{\theta _{\bf{c}}}\]

\[{\theta _{n\;}} = {\rm{ }}\frac{{({\theta _i} + {\rm{ }}{\theta _c})}}{2}\]

Thermoelectric power:

The rate of change of thermos-emf with temperature is called thermoelectric power.
It is denoted by P and given by:
\[P = \frac{{dE}}{{dT}}\]

Peltier Effect:

The phenomenon of generation or absorption of heat at the two junctions of thermocouples due to the flow of current through it is called peltier effect.

Diagram: Peltier Effect

This is the reverse effect. It is the inverse process of Seebeck effect.

Thompson Effect:

The process of evolution is the absorption of heat along the length of a conductor on passing current through it when it's too and are kept at different temperature is known as Thomson effect. If an electric current is passed through a copper wire from its hotter end to the colder and the heat is evolved or released and wire become hot.

Diagram: Thompson Effect in Iron

If the current is reversed i.e. (cold to the hot end) it is absorbed and the wire becomes cold.

Application of thermoelectric effect:

Thermopile:

àIt is a device used for detection and measurement of heat radiation.
àIt is based on the Seebeck effect.
àFor construction of thermophile multiple thermocouples are connected in series then thermo EMF gets multiplied.

Construction of thermopiles:

It consists of a number of Bi-Sb thermocouples connected in series that show that thermos-EMF produced in all thermocouples are added. One set of junctions is blackened and exposed to heat radiation while other end is protected from heat radiation by an insulating cover. A galvanometer is connected to the circuit to detect thermos-EMF which is produced from heat radiation.

Difference between Joule's Effect and Peltier Effect and Seebeck Effect and Thomson Effect:

Joule's Effect	Peltier Effect	Seebeck Effect	Thomson effect
Heat produced is directly proportional to square of the current passing through the conductor.	Heat produced or absorbed at the junction is proportional to the current through the junction.	The temperature difference of the junction is used to produce thermos-EMF and vice versa.	Thomson's heat is proportional to the current passing through the conductor.
It is not reversible.	It is the reversible effect.	It is the reversible effect.	It is the reversible effect.
It has a heating effect.	It is heating as well as a cooling effect.	One is cooler and the other junction is hotter.	It has a heating as well as a cooling effect.
The effect is due to the collision of free electrons with atoms or ions of conductors.	The effect is due to the flow of current through the junction.	The effect is due to variation of temperature at the junction of the thermocouple.	This is due to the variation in temperature at different part of the conductor.
Heat produce depend upon resistance of conductor.	Heat exchange depends upon the nature of the conductor at the temperature of the junction.	This effect depends upon the nature of material used from the junction.	This effect depends upon nature of conductor and temperature difference of different parts of the conductor.