The phenomenon of conversion of heat energy into electrical energy when the junctions of two dissimilar certain metals are kept at different temperatures is known as thermoelectric effect. The current is produced due to the temperature difference between two junctions. This effect was discovered by German physicist Thomas Johann Seebeck in 1826. So, this effect is also known as Seebeck effect.
The produced current is called thermoelectric current and the pair of metals arranged to produce thermoelectric current is called thermocouple. The electromotive force developed in the circuit is known as thermoelectric emf or thermo emf or Seebeck emf. The thermoelectric emf depends upon the nature of the thermocouple and the temperature difference between the two junctions.
Consider two thick wires of iron and copper are joined together. One junction is at low temperature (say $0°C$) which is called cold junction and other junction is at high temperature (say $100°C$) which is called hot junction.
A galvanometer $G$ is also connected in the circuit. It is observed that there is a deflection in the galvanometer. Seebeck effect is reversible because when the hot and cold junctions are interchanged, the direction of both electromotive force and electric current changes.
Mechanism of Thermoelectric Effect
Different metals have different density of free electrons. When two metals are brought into intimate contact (say by soldering) with other metal, the electrons tend to diffuse from one metal to another, to equalize the electron densities.
When copper is brought into intimate contact with iron, the electrons diffuse from iron to copper. But this diffusion cannot go on continuously due to diffusion, the potential of copper decreases and the potential of iron increases; iron becomes positive with respect to copper.
When the junctions are at the same temperature, the emfs of the junction will be equal in magnitude but opposite in direction. So, the net emf for the whole of the thermocouple will be zero. When the temperature at one junction is increased, it affects the electron density in the two metals differently. The transfer of electrons in the hot junction is much easier than the transfer of electrons in the cold junction. The emf at the two junctions will be different. This produces a net emf in the thermocouple.
[Also See: Electric Potential]
Thermoelectric Series
Thermoelectric series is the set of metals used to make thermocouples. The series is antimony (Sb), iron (Fe), cadmium (Cd), zinc (Zn), silver (Ag), gold (Au), chromium (Cr), tin (Sn), lead (Pb), mercury (Hg), manganese (Mn), copper (Cu), platinum (Pt), cobalt (Co), nickel (Ni), bismuth (Bi).
The current passes through the cold junction from the metal which occurs earlier in the series to that which occurs later in the series. To produce more emf, those metals which are separated widely in the series should be taken.
In the series, the metals Sb and Bi are at the two extreme positions, so this thermocouple gives maximum thermo emf. Lead is taken as reference metal because its thermo emf is very small. Metals to the right of lead are called thermoelectrically positive while those to the left are called thermoelectrically negative.
Variation of Thermo Emf with Temperature
Thermo emf depends upon;
1. Temperature of the cold junction
2. Temperature difference between two junction
Suppose the temperature of the cold junction is kept at $0°C$. When the temperature of the hot junction is increased gradually, the thermo emf increases and reaches a maximum $(E_{\text{max}})$ value at the temperature $θ_n$. This temperature $θ_n$ is called neutral temperature of the couple. Thus, the temperature of the hot junction at which the thermo emf becomes maximum is known as neutral temperature. Its value is different for different thermocouples. For a Cu-Fe thermocouple, $θ_n=270°C$.
When the temperature is further increased, thermo emf decreases and becomes zero at a temperature $θ_i$. This temperature $θ_i$ is known as temperature of inversion. Thus, the temperature of the hot junction at which the thermo emf becomes zero is called temperature of inversion. Its value depends upon the temperature of the cold junction. For Cu-Fe thermocouple, $θ_i$ is about $540°C$.
When the temperature of the hot junction is further increased, thermo emf is reversed and the current is in opposite direction. The graph between thermo emf and the temperature of hot junction is almost parabolic.
The relation between thermo emf $E$ and temperature difference $θ$ between hot and cold junction can be represented by, \[E=αθ+\frac{β}{2}θ^2\] Where, $α$ and $β$ are the thermoelectric coefficient whose value depends upon the pair of metals of the thermocouple.
Relation between θn and θc
Neutral temperature only depends upon the nature of the thermocouple; it does not depend upon the temperature of the cold junction. The temperature of inversion $θ_i$ always exceeds the neutral temperature $θ_n$ almost by the same amount as the neutral temperature exceeds that of the cold junction $θ_c$.
\[\therefore θ_i-θ_n=θ_n-θ_c\] \[θ_n=\frac{θ_i+θ_c}{2}\] Hence, neutral temperature is the average of temperature of inversion and temperature of cold junction.