Introduction

Same as the heat energy produced from electrical energy, electricity is produced from heat energy. Such process represents thermoelectricity and the phenomenon in which electrical energy produced from thermal energy is called thermoelectric effect. Such effects are mainly divided into 3 types. These are:

a) Seebeck’s effect

b) Peltier’s effect

c) Thomson’s effect

a) Seebeck’s effect:

If 2 different metallic wires are used to form closed circuit and two junctions are kept at a different temperature, a small amount of emf is set up in the circuit and small amount of electric current is flowing in the defined direction. Such process represents Seebeck’s effect. The small magnitude of emf produced in it represent thermoemf and small amount of electric current produced in it represent thermoelectric current.

i) Thermocouple:

The couple of dissimilar metal are used to form a loop and produce thermoelectricity representing thermocouple. The magnitude of the thermoemf and direction of electric current are found due to the help of metals of the thermocouples and temperature of the function.

ii) Thermoelectric series:

An arrangement of the metals from which any two metals are selected to form a thermocouple is called thermoelectric series. The thermoelectric series of metals are antimony, iron, zinc, silver, lead, copper, platinum, bismuth.

Variation of Thermo-emf with temperature

An arrangement shown in the figure represent iron copper thermocouple which is used to show the variation between thermo emf and temperature. One junction of the thermocouple is dipped in the hot oil bath and next junction of the thermocouple is kept on a melting ice.

Initially, temperature of both hot junction and cold junction is same i.e., 0^oC. So, galvanometer shows null point deflection which means that thermo-emf becomes zero. When certain amount of heat energy is supplied to the hot oil bath, the temperature of hot junction increases which means that thermo-emf of the junction gradually increases. At neutral temperature, thermo-emf of the junction becomes maximum. Hence, neutral temperature is defined as temperature of hot junction in which thermo-emf becomes maximum.

When temperature increases beyond neutral temperature, the thermo-emf start to decrease and become zero at a temperature of inversion.

Hence, temperature of inversion is defined as temperature of hot junction in which the value of thermo-emf becomes zero.

Temperature of inversion depend upon temperature of cold junction and nature of the material used to form thermocouple.

From the graph between thermo-emf and temperature, we get, the relation,

E = [latex]\alpha \theta + \frac{1}{2}\beta \theta^2[/latex]

Where, α = β = thermoelectric constant.

From figure (graph), we can write,

[latex]\theta_n – \theta_c = \theta_i – \theta_n[/latex]

Or, [latex]2\theta_n = \theta_c + \theta_i[/latex]

Or, [latex]\theta_n = \frac{\theta_c + \theta_i}{2}[/latex]

Hence, neutral temperature is also defined as the average value of temperature of cold junction and temperature of inversion for iron copper thermocouple, the value of neutral temperature is 250^oC and value of temperature of inversion is 500^oC.

Relationship between thermoelectric constant (α, β, [latex]\theta_n[/latex] and [latex]\theta_i[/latex])

Let temperature of cold junction is 0^oC and temperature of hot junction is [latex]\theta^oC[/latex]. Due to the variation between thermoemf with temperature, we get,

E = [latex]\alpha \theta + \frac{1}{2}\beta \theta^2[/latex]

Differentiating above eq^n. with respect to [latex]\theta[/latex], we get,

[latex]\frac{dE}{d\theta} = \alpha + \beta \theta[/latex]

For maximum thermoemf, the slopes equal to zero.

i.e. [latex]\frac{dE}{d\theta} = 0[/latex]

For [latex]\theta = \theta_n[/latex]

Or, [latex]\alpha + \beta \theta_n = 0[/latex]

[latex]\therefore \theta_n = – \frac{\alpha}{\beta}[/latex]

Similarly, for [latex]\theta = \theta_i[/latex], thermo-emf becomes 0.

i.e. E = 0 at [latex]\theta = \theta_i[/latex].

i.e. [latex]\alpha \theta_i + \frac{1}{2}\beta\theta_i^2 = 0[/latex]

Or, [latex]\theta_i(\alpha + \frac{1}{2}\beta \theta_i)[/latex]

[latex]\theta_i[/latex] ≠ 0.

[latex]\therefore \alpha + \frac{1}{2}\beta \theta_i = 0[/latex]

[latex]\therefore \theta_i = \frac{-2\alpha}{\beta} = 2\theta_n[/latex]

Thermoelectric Power (P)

It is defined as the rate of change of thermo emf with absolute temperature. i.e.

P = [latex]\frac{dE}{dT}[/latex]

Peltier’s effect:

When an electric current is passed through a thermocouple, heat energy absorbed or evolved at a junction of a thermocouple which depends upon the direction of current flow. This process represents Peltier’s effect. It is a reversible effect when direction of current flow is altered, the junction of heat absorbed or evolve is interchanged. Peltier’s effect is inverse process of Seebeck effect.

Peltier’s Coefficient:

It is defined as the product of absolute temperature and thermoelectric power which is denoted by π.

π = [latex]T\frac{dE}{dT}[/latex]

[latex]\therefore \pi = TP[/latex]

Thomson’s effect:

[latex]
\begin{aligned}
\text{Hot} &\xrightarrow{\text{Heat evolved, Cu}} \text{Cold} \\
\text{Hot} &\xrightarrow{\text{Heat absorbed, Cu}} \text{Cold} \\
\text{Hot} &\xrightarrow{\text{Heat absorbed, Fe}} \text{Cold} \\
\text{Hot} &\xrightarrow{\text{Heat evolved, Fe}} \text{Cold}
\end{aligned}
[/latex]

When an electric current is passed through a non-uniform heated conductor, heat energy is absorbed or evolved at junction such process of evolution or absorption of the heat energy represent Thomson’s effect.

When an electric current is passed through hotter end to colder end of the copper wire heat energy is evolved across it. When an electric current is passed from colder end to hotter end of the copper wire, heat energy is absorbed across it such process represents positive Thomson’s effect. Other examples are silver, zinc, etc.

Similarly, when an electric current is passed through hotter end to colder end of an iron wire, heat energy is absorbed across it. When an electric current is passed through colder end to hotter end of an iron wire, heat energy is evolved across it. Such process represents negative Thomson’s effect. Other examples are Platinum, cobalt, etc.

In case of lead, Thomson effect is nil. Therefore, it is used as best resistor in thermoelectricity.

Applications of Thermoelectric Effect

Thermopile

The device used for the detection and measurement of heat radiation is called thermopile. Based on Seebeck effect, it consists of a number of Bi-Sb thermocouples connected in series so that the thermo emf produced are added. One of the junctions is exposed to heat while other junctions are protected from heat radiation by an insulating cover. A galvanometer is connected to the circuit that detects the thermo emf. It is also used for measurement of high temperature of a furnace.

Thermoelectric Generator

By heating one junction and keeping other junction of a thermocouple at room temperature, electric current flows through the circuit. The electric power generated can be used to operate electronic devices in remote areas.