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Thermoelectric Phenomena: 

Thermoelectric Phenomena Electrically conductive materials exhibit three types of thermoelectric phenomena: the Seebeck effect, the Peltier effect, and the Thomson effect.


SEEBECK EFFECT Differential Thermocouple


SEEBECK EFFECT The Seebeck effect is the conversion of temperature differences directly into electricity It is named after German physicist Thomas Johann Seebeck in 1821 he discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field.

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Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the mistake and coined the term "thermoelectricity".

Cause of seebeck effect: 

Cause of seebeck effect As we know that no of free electrons per unit volume and their average velocity varies from one metal to another . At the junction of two dissimilar metals, electrons migrate in one direction across the junction. It sets up an opposing electric field . As a result a fixed potential difference is developed .

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This effect is called Seebeck effect . The combination of two metals in such a manner is called thermocouple The e.m.f generated in a thermocouple is known as thermo e.m.f The current which flows along the closed circuit is called thermo electric current

Cause (Contd.): 

Cause (Contd.) Since a migration of charges creates an electric potential, the buildup of charged carriers onto the cold side eventually ceases at some maximum value since the electric field is at equilibrium. An increase in the temperature difference resumes a buildup of charge carriers on the cold side, leading to an increase in the thermoelectric voltage, and vice versa.

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The voltage created by this effect is on the order of several micro volts per Kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolt per Kelvin at room temperature. S is positive when the direction of electric current is same as the direction of thermal current

Thermoelectric series: 

Thermoelectric series For a given temperature range, the thermo e.m.f induced is different for different metal combinations. Seebeck arranged 35 metals in a series in such away when any two form a thermocouple:- Current will flow through the cold junction from the one that occurs first in series to the other that occurs later in series Between 0 o C & 100 0 C the thermoelectric series is: Antimony, Fe, Cd , Zn, Ag, Au, Rb,Mo,Cr,Sn,Pb,Hg,Mn,Cu,Pt,Cob, Ni, Bismuth .

Variation of thermoelectric E.M.F. with temperature: 

Variation of thermoelectric E.M.F. with temperature T n =neutral temperature(constant for a given thermocouple) T i =inversion temperature(beyond which e.m.f. starts increasing in opp. direction) T i – T n = T n – T 0 Thermo e.m.f.

Law of thermoelectric effects: 

Law of thermoelectric effects Law of intermediate metal: if the junction of the thermocouple is opened and a third metal is inserted, the e.m.f. for the couple AB remains the same i.e. without introducing C provided both the junction of the third metal C are at the same temperature.

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2 . Law of intermediate or successive temperatures : the e.m.f for a thermo-couple between temperatures t 1 and t 2 is equal to the sum of e.m.fs (seebeck) for any number of successive steps into which the given range of temperatures may be divided. Thus e ᶿ1 ᶿn =e ᶿ1 ᶿ2 + e ᶿ2 ᶿ3 + e ᶿ3 ᶿ4 +…..e ᶿn-1 ᶿn

Experimental demonstration of seebeck effect: 

Experimental demonstration of seebeck effect In this experiment , two rods of bismuth & antimony are arranged in the form of cross “X” Firstly, closed key “K 1 ”. Current starts flowing, making the junction hot. Temperature difference is obtained. Open K 1 and close K 2 . Deflection is shown by galvanometer , thus confirming the seebeck effect

Peltier Effect: 

Peltier Effect Difference in ε F between Materials A and B Material A Material B Heat Absorbed or Expelled Electric Current

Peltier effect: 

Peltier effect In 1834, a French watchmaker and part time physicist, Jean Peltier found that an electrical current would produce a temperature gradient at the junction of two dissimilar metals when the current direction was r reversed, the cold junction w would get hot while the hot junction w would get cold.

Cause of peltier effect: 

When a current flows across the junction of two metals, it gives rise to an absorption or liberation of heat, depending on the direction of the current. i.e. Applying a current (e - carriers) transports heat from the warmer junction to the cooler junction. Cause of peltier effect

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As an example of the Peltier effect, consider the circuit shown here. Under these conditions, it is observed, as indicated in the diagram, that the right-hand junction is heated. It shows, in other words, that electrical energy is being transformed into heat energy. Meanwhile, heat energy is transformed into electrical energy at the left junction, thereby causing it to be cooled. When the current is reversed, heat is absorbed at the right junction and produced at the left one.

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The rate of evolution of or absorption of heat is proportional to the current &thus the peltier effect is reversible. If direction of current is changed then the peltier effect is also reversed. The energy absorbed or evolved at one of the junctions of the two dissimilar metals when one ampere of current flows for one second is called Peltier coefficient ,denoted by ∏

Experimental demonstration of peltier effect : 

In this experiment, two rods of bismuth and antimony are joined as shown. A differential air thermometer is placed with its two bulbs at junctions A and B. Experimental demonstration of peltier effect

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When no current flows through the thermocouple , the junctions A & B are at same temperature. Also Hg(mercury) pellet in the thermometer lies symmetrically at the centre. When large current is passed through the thermocouple , then there will be: Evolution of heat at junction A. Absorption of heat at junction B. Hg pellet moves to one direction If direction of current is changed the pellet moves in the opp. Direction indicating that now A has become cold and B has become hot.

Applications : 

Applications Seebeck effect The Seebeck effect is used in the thermoelectric generator , which functions like a heat engine These have a use in power plants for converting waste heat into additional power (a form of energy recycling). in automobiles as automotive thermoelectric generators (ATGs) for increasing fuel efficiency. Space probes often use radioisotope thermoelectric generators with the same mechanism but using radioisotopes to generate the required heat difference.

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Peltier effect The Peltier effect can be used to create a refrigerator which is compact and has no circulating fluid or moving parts such refrigerators are useful in applications where their advantages out weigh the disadvantage of their very low efficiency.


25 Applications Water/Beer Cooler Cooled Car Seat Electronic Cooling Cryogenic IR Night Vision Laser/OE Cooling TE Si bench

Temperature measurement : 

Temperature measurement Thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects, one connected to a voltmeter and the other to the probe. The temperature of the voltmeter, and hence that of the material being measured by the probe.

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