Class 12 Physics Thermoelectric Effect Notes

Class 12 Physics Thermoelectric Effect Notes
UNIT 4
CLASS 12 PHYSICS • ELECTRICITY AND MAGNETISM

Thermoelectric Effect

Chapter 15
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Thermoelectric Effect

The phenomenon of conversion of heat energy into electric energy is called thermoelectric effect.

Thermocouple

The device which is made by two dissimilar metal wires with their junction at different temperature is known as thermocouple.
Iron-copper thermocouple with hot and cold junctions G Hot junction Cold junction Fe Cu
Iron-copper thermocouple

If the difference of temperature is maintained at two junctions of thermocouple, a small emf is produced. This emf is called thermoelectric emf or thermo-emf.

The value of thermo-emf depends on:

  1. Temperature difference between the junctions which form thermocouple.
  2. Nature of the metal wires which form thermocouple.

Thermoelectric Series

The number of dissimilar metals joined in series such that any two of them form thermocouple is called thermoelectric series.

Seebeck Effect

If two dissimilar metal wires are joined at their ends to form a closed conducting wire and different temperature is maintained at their junction, then a small emf is produced and small current flows in the circuit. This phenomenon is called Seebeck effect.

The current thus produced without use of cell is known as thermoelectric current and corresponding emf is called thermo-emf.

Thus, the production of thermo-emf in a thermocouple when its junctions are kept at different temperature is called Seebeck effect.

Variation of Thermo-emf with Temperature

Experimental setup for studying variation of thermo-emf with temperature using a copper-iron thermocouple G Hot oil bath Melting ice Copper Iron Thermometer
Copper-iron thermocouple arrangement

To study variation of thermo-emf with temperature, a Fe-Cu thermocouple is taken. One junction of thermocouple is immersed in an oil bath and another junction is kept in melting ice.

When the temperature of both junctions is same (0°C), the emf is zero. As the temperature of oil bath increases, thermo-emf starts increasing and becomes maximum. The temperature of hot junction at which thermo-emf becomes maximum is called neutral temperature (θn).

If the temperature of hot junction increases beyond neutral temperature, the value of thermo-emf decreases and becomes zero at certain temperature. The temperature of hot junction of a thermocouple at which thermo-emf becomes zero is called temperature of inversion (θi).

Graph of thermo-emf against temperature showing cold junction temperature, neutral temperature and inversion temperature Thermo-emf Temperature θc θn θi Emax
Variation of thermo-emf with temperature
If θc is the temperature of cold junction, then:
θn = (θi + θc)/2

The variation of thermo-emf with temperature is given by:

E = αθ + ½βθ²
where α and β are constants.
The values of these constants depend on the materials of conductor and temperature difference of two junctions.

Neutral Temperature

E = αθ + ½βθ²
dE/dθ = α + βθ

At θ = θn, E is maximum. Therefore:

dE/dθ = 0
α + βθn = 0
θn = −α/β

Temperature of Inversion

When θ = θi, thermo-emf becomes zero:

0 = αθi + ½βθi²
θi[α + ½βθi] = 0

Since θi ≠ 0:

α + ½βθi = 0
θi = −2α/β

Thus, temperature of inversion depends on nature of materials from which thermocouple is formed.

Is Seebeck Effect Reversible Effect? Why?

Yes, Seebeck effect is reversible effect because when the hot and cold junctions are interchanged, the direction of both electromotive force and electric current also changes.

Peltier Effect

When a current is passed through a thermocouple whose junctions are at same temperature, heat is absorbed at one junction and heat is evolved (released) at other junction. This effect is called Peltier effect.

This is a reversible effect because when direction of current is reversed, the heat evolved or absorbed is interchanged at the junctions.

Peltier effect showing reversal of current interchanging heat absorbed and heat evolved at the thermocouple junctions G Current in one direction Heat absorbed Heat evolved Fe Cu G Current direction reversed Heat evolved Heat absorbed Fe Cu
Peltier effect

Thomson’s Effect

When a current is passed through two ends of a conductor maintained at different temperatures, heat is evolved or absorbed. This phenomenon of evolution or absorption of heat along the length of conductor on passing current through it, where its two ends are kept at different temperature, is known as Thomson’s effect.

Thermopile

A thermopile is a device to measure the intensity of radiation. Thermopile consists of a number of thermocouples connected in series.
Thermopile represented as multiple thermocouples connected in series Thermocouples in series Hot junctions Cold junctions
Conceptual representation of a thermopile

Solved Numericals

Q.1 — Neutral Temperature from Thermo-emf Equation

The thermo-emf E and the temperature of hot junction θ satisfy a relation E = aθ + bθ², where a = 4.1×10−5 V(°C)−1 and b = −4.1×10−8 V°C−2. If the cold junction temperature is zero °C, find the neutral temperature.

At neutral temperature θn, emf is maximum.

E = aθ + bθ²
dE/dθ = a + 2bθ
At θ = θn:
0 = a + 2bθn
θn = −a/(2b)
= −(4.1×10−5)/[2×(−4.1×10−8)]
= 0.5×103 °C
θn = 500°C

Q.2 — Seebeck EMF of Nickel-Copper Thermocouple

The junctions of a nickel-copper thermocouple are maintained at 0°C and 100°C. Find the Seebeck emf for nickel-copper if α = 16.3×10−6 V(°C)−1 and β = −0.042×10−6 V(°C)−2.

Temperature of cold junction, θc = 0°C
Temperature of hot junction, θh = 100°C
Temperature difference, θ = 100°C

Now:

E = αθ + βθ²/2
= (16.3×10−6×100) + [(-0.042×10−6×100²)/2]
E = 0.00142 V
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