• SLC Science
  • Physics
    • Gravitational force
      • Law of gravitation
      • Acceleration(g)
      • Free fall
      • Numerical
    • Pressure
      • Def.Pressure
      • Principles
      • Upthrust
      • Numerical
    • Heat
      • Heat Energy
      • Anomalous expansion of water
      • Specific heat capacity
      • Numerical
    • Electricity
      • Electricity
      • Fuse
      • Faraday'sPrinciple
      • Transformer
      • Numerical
  • Chemistry
  • Biologi
• SLC Maths
• slc.Opt.Math
• +2Science
  • Wave Motion

---

Thermoelectric Effect/Seebeck Effect :
Variation of Thermo emf with Temperature :
Relation Connecting Thermoelectric Constants α, β , θ _n, θ _i :

Thermoelectric Effect(Peltier and Thomson's Effect):Click here
Thermoelectric Effect(Short Question and Answer):Click here

Introduction

In the same way as the heat energy can be obtained from electrical energy, the electrical energy can also be generated from the heat energy. The electricity thus generated is called thermoelectricity. The phenomenon in which electrical energy produced by means of thermal energy is called thermoelectric effect. This effect involves following three related effects:
(a) Seebeck effect (b) Peltier effect (c) Thomson's effect.

Seebeck's Effect

If two different metal wires are joined to form a closed circuit and two junctions are kept at different temperatures, a small emf is set up in the circuit and small current flows in the circuit in a definite direction. This effect is called thermoelectric effect or Seebeck effect which is discovered by a German physicist Thomas J Seebeck. The emf developed in the circuit is called thermo emf and the current called thermoelectric current.

Thermocouples

A couple of wires of dissimilar metals forming a loop and producing thermoelectricity is called thermocouple. As an iron-copper thermocouple. The magnitude of emf produced and the direction of current depends on the pair of metals selected from the thermoelectric series and temperature of the junctions. In iron-copper thermocouple, current flows from iron to copper at the cold junction. The direction of current flow changes if heating and cooling of the junction are reversed.

Thermoelectric Series

An arrangement of metals in series in which any two metals can be used to form thermocouple is called thermoelectric series. When a thermocouple is formed from a pair of metals in the sires, the direction of the current flow through the cold junction is from the metal which occurs earlier in the series to the one which occurs later. Greater the difference in the order in the series, higher is the value of emf produced. The thermoelectric series is given below.
Antimony, Iron, Zinc, Silver, Lead, Copper, Platinum, Cobalt, Bismuth.
Thus for copper-iron thermocouple, the current will flow from iron to copper through the cold junction. Similarly in antomony-bismuth thermocouple, the direction of thermoelectric current is from antimony to bismuth throught the cold junctions. Also, for the same temperature difference between the two juncitons, the thermo emf developed in antimony-bismuth temperature is more than that in copper-iron thermocouple.
Thermoelectric effect is used in measurement of temperature and radiant energy. This effect produce trouble in circuit used for precise measurement of current or detecting small current.

Variation of Thermo emf with temperature

To study the variation of thermo emf with temperature, an iron-copper thermocouple is taken as shown in fig. One junction is immersed in an oil bath and the other junction is kept melting ice whose temperature is kept constant. The temperature of oil bath is increased gradually by heating it. When the temperatures of both junctions are at the same (0⁰C), the galvanometer shows on deflection and so, no emf is produced.
...................................
As the temperature of the hot junction is increased, and the cold junction is kept at (0⁰C), the deflection of galvanometer also increases i.e. emf also increase till it becomes maximum at θ _n called neutral temperature. The temperature of the hot junction at which the thermo emf becomes maximum is known as neutral temperature (θ _n).
As the temperature of hot junction is increased beyond neutral temperature, thermo emf starts to decrease and ultimately becomes zero at temperature θ _i called temperature of inversion. The temperature of the hot junction at which thermo emf is zero and changes its polarity is called the temperature of inversion θ _i.

If the temperature is increase beyond θ _i, the direction of thermo emf is reversed. The inversion temperature depends upon the temperature of cold junction and nature of meals used in the thermocouple. For copper-iron thermocouple, neutral temperature is about 250 ⁰ and temperature of inversion is about 500 ⁰C. The variation of thermo emf with temperature is shown in fig. The variation of thermo emf with temperature θ is given by
        $E=\alpha \; \theta \; +\frac{1}{2}\beta \theta \; ²$
where α and β are constants. The values of these constants depends on the materials of conductor and the temperature difference of two junctions.
If θ _c is the temperature of the cold junction, then we have
          θ _i − θ _n = θ _n − θ _c
          or, 2 θ _n = θ _c + θ _i
          or, θ _n = $\frac{\mathrm{\theta \; _c\; <mo>+</mo>\; \theta \; _i}}{2}$ $$
So the neutral temperature lies between the inversion temperature and temperature of cold junction.

Relation Connecting Thermoelectric Constants α, β , θ _n, θ _i

The cold junction of thermocouple is kept at 0⁰C and θ is the temperature of the hot junction. The thermo emf is $E=\alpha \; \theta \; +\frac{1}{2}\beta \theta \; ²$. Differentiating this equation with temperature, we get
          $\frac{\mathrm{dE}}{\mathrm{d\; \theta }}=\alpha \; +\; \beta \; \theta$
At α = α _n, E is maximum and so $\frac{\mathrm{dE}}{\mathrm{d\; \theta }}=\; 0$. It can be seen that the slope of E-θ graph, $\frac{\mathrm{dE}}{\mathrm{d\; \theta }}$ is zero at point P. So,
          0 = α + β θ
          or, θ _n = $-\frac{\alpha }{\beta }$
When θ = θ _i, then E = 0 and
          $0=\alpha \; \theta \; +\frac{1}{2}\beta \theta \; ²$
          θ _i(α +$\frac{1}{2}$ β θ _i) = 0
Since θ _i can't be zero, then (α +$\frac{1}{2}$ β θ _i) = 0
(θ _i = $-\frac{\mathrm{2\; \alpha }}{\beta }$

Thermoelectric power

The rate of change of thermo emf with temperature is called thermoelectric power. It is denoted by P and given by
            $P\; =\frac{\mathrm{dE}}{\mathrm{dT}}$