Thermoelectric Effect

aThermoelectric effect (Seebeck's Effect)The production of electricity by keeping the junction of two dissimilar metals at different temperature is called the thermoelectric effect. The emf thus set up across the junctions is called thermoelectric emf. The magnitude and direction of thermoelectric emf dependes on the nature of the materials forming the thermocouple and the temperature difference between two junctions. ThermocoupleA couple of wires of different metals formoing a loop which is made for the purpose of producing thermoelectricity is called theromocouple.

Thermoelectric seriesAn arrangement of metals in series in which any two metals can be used to form a thermocouple is called thermoelectric series. Antimony, Iron, Zinc, Silver, Lead, Copper, Platinum, Cobalt, BismuthGreater the distance in series , more thermoemf is producedie, maximum thermoemf is produced from Antimony-Bismuth thermocouple.Variation of Thermoemf with temperature

To study the variation of thermo emf with temperature, an iron - copper thermo couple is taken, which is shown in figure above, in which one junction is immersed in an oil bath and other junction is kept at melting ice. When the temperature of both junction is same then galvanometer shows no deflection i.e. no emf is developed in it. Now the temperature of oil bath is increased gradually by heating it. As we increase the temperature of hot jnction, then galvanometer shows deflection i.e emf is produced and increases and becomes maximum. The temperature of the hot junction at which the thermo emf becomes maximum is called neutral temperature (𝜃n)$({𝜃}_n)$. The neutral temperature depends on the nature of the thermocouple. As the temperature of hot junction is increased beyond neutral temperature, thermo emf starts to decrease and becomes zero. The temperature at which thermo emf becomes zero and changes its polarity is called temperature of inversion (𝜃i)$({𝜃}_i)$ ,If we increase the temperature beyond 𝜃i${𝜃}_i$, the direction of thermo emf reverses. The temperature of inversion depends on temperature of cold junction and nature of metal used in thermo couple. If 𝜃c=00${𝜃}_c=0^0$, the curve is symmetric and parabolic in nature.The varation of thermo emf with temperature 𝜃$𝜃$ is given byE=a𝜃+b𝜃2$$E=a𝜃+b{𝜃}^2$$Here, 𝜃$𝜃$ is temperature difference between cold and hot junction.Where a and b are constants; which depends on the materials of conductor and temperature difference between junctions. If 𝜃c${𝜃}_c$ is the temperature of cold junction then we have, a

𝜃i-𝜃n=𝜃n-𝜃c

𝜃n=𝜃c+𝜃1 2

$$\begin{array}{l}{𝜃}_i-{𝜃}_n={𝜃}_n-{𝜃}_c\\ {𝜃}_n=\frac{{𝜃}_c+{𝜃}_1}{2}\end{array}$$Which shows that neutral temperature lies between temperature of inversion and temperature of cold junction.Thermo Electric PowerThe rate of change of thermo emf with temperature is called thermoelectric power. It is denoted by P and given by P = dE

dT${\displaystyle P=\frac{dE}{dT}}$ Cause of Seebeck EffectDifferent metetal have different free electron density. When two different metals are brought into contact, the free electrons tend to diffuse from the metals with greater electron density to the other with lower electron density. Due to diffusion, a potential difference is set up at the junction of two metals called contact potential. When both junctions are at the same temperature, the contact. potential at the junctions will be equal and opposite. Hence no current flows through the thermocouple. But if one junction is kept at a higher temperature, the rate of diffusion of free electron at the junction will increase. As a result of it, the contact potential at the two junctions will become different and hence there will be an effective emf in the circuit called the thermo emf. Peltier EffectWhen electric current is passed through a thermo couple then heat is either absorbed or released at the junctions depending on the direction of current flow. This effect is called peltier effect. This is reversible effec and is inverse process of see beck effect.

Cause of Peltier EffectIf two dissimilar metals are joined, contact potential is established at the junctions i.e. the potential of one must become above that of the other. For example: in Cu-Fe thermocouple, the potential of Fe is greater than the potential of Cu. At one junction, current flows from lower potential to a higher potential and the energy are required for this purpose, which is absorbed from the junction and hence it is cooled. At another junction, current flows from higher potential to lower potential. The energy is given out at this junction and makes the junction hot. Thomson’s EffectThe phenomenon of evolution or absorption of heat along with the length of a conductor on passing current through it when its two ends are kept at a different temperature is known as Thomson’s effect. If an electric current is passed through a copper wire from its hotter end to the colder end, the heat is evolved and the wire becomes hot. If the current is reversed, heat is absorbed along the conductor. Similarly, is an electric current is passed through an iron wire from its hotter end to the colder end, the heat is absorbed and the wire gets cooled. If the current is reversed, the heat is evolved along the conductor. So, Thomson’s effect is reversible. The substances, which behave like copper, are said to have a positive Thomson’s effect. The substances which behave like iron are said the substances which behave like iron said to have negative Thomson’s effect. Thomson’s effect of lead is nil. So it is used as the standard metal in thermoelectricity.Cause of Thomson’s EffectWhen two ends of a conductor are kept at different temperatures, the number of free electrons in the higher temperature region will have higher than those in the lower region. So there is diffusion of electrons from one region to another and this gives rise to an emf which is called Thomson’s emf.

S.N	Peltier’s Effect	S.N.	Joule’s Effect
1.	It takes place only at the junction.	1.	It takes place throughout the conductor.
2.	Heat is evolved or absorbed.	2.	Heat is always produced.
3.	It is a reversible process.	3.	It is an irreversible process.
4.	It depends on the direction of current	4.	It is independent of the direction of the current.
5.	Heat is evolved or absorbed is proportional to the current.	5.	Heat produced is proportional to the square of the current.
6.	Amount of heat evolved or absorbed depends on the nature of the metals and temperature of the conductor.	6.	Amount of heat produced depends on the resistance of the conductor.

Distinction between Thomson’s Effect and Joule’s Effect

S.N	Thomsons Effect	S.N.	Joule’s Effect
1.	For the evolution of heat, a temperature difference is required.	1.	Temperature difference is not required.
2.	Heat is evolved or absorbed along the wire	2.	Heat is always produced along the length of the conductor.
3.	It is a reversible process.	3.	It is an irreversible process.
4.	It depends on the direction of current	4.	It is independent of the direction of the current.
5.	Heat is evolved or absorbed is proportional to the current.	6.	Heat produced is proportional to the square of the current.
6.	Amount of heat evolved or absorbed depends on a temperature difference between the ends of the conductor.	6.	Amount of heat produced depends on the resistance of the conductor but not on the temperature difference.

Application of thermoelectric EffectThermopileIt is a device used for detection and measurement of heat radiation. Principle It is based on Seebeck effect. It is constructed on the principle that if a number of thermocouples are connected in series, then the thermo emf gets multiplied.Bi-Sb ThermopileIt consists of a number of the Bi-Sb thermocouples connected in series so that the thermo emf produced in all thermocouples are added. One set of junctions is blackened and exposed to heat radiation while the other set of the junctions is protected from heat radiation by an insulating cover. A sensitive galvanometer connected to circuit detects the thermo emf produced by the radiation. The thermopile is also used for the measurement of the high temperature of a furnace. Thermoelectric GeneratorThis generator is based on thermoelectric effect. By heating one junction and keeping other junction at room temperature of a thermocouple, electric current flows through the circuit. The electric power generated in this method can be used to operate the electronic device in remote areas.