Thermoelectric effect

                            Thermoelectric effect

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side.

This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.

The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect. Textbooks may refer to it as the Peltier–Seebeck effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Baltic German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a current is passed through a resistive material, is related, though it is not generally termed as thermoelectric effect. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not.

             OR,

When two wire of different material are joint to form closed circuit and the two jointed end are placed at different temperature then a small emf is develop in that closed circuit and small current flow through the closed circuit this process is called seebeck’s effect or thermoelectric effect.

A couple of wires of dissimilar metals forming a loop and producing thermoelectricity is called 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 junction. 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.

The production of electricity by keeping the junction of two dissimilar metals at different temperatures is called thermoelectric effect. The emf thus produced across the junction is called thermo emf and the current obtained from the thermo emf is called thermoelectric current.

The two factors on which thermoelectric e.m.f.produce in a thermocouple depends on are given below:

1. Nature of two metals forming the thermocouple and

2. Temperature difference between two junctions of the thermocouple.

Neutral temperature:

The temperature of hot junction at which the thermo emf generated in the thermocouple becomes maximum is called neutral temperature. It depends on the nature of the materials of the thermo couple.

The uses of thermoelectric effect:

(i) It is used to make solid – state refrigerator device.

(ii) It is used to sense temperature difference.

(iii) It is used to convert thermal energy directly into electricity.

Variation of thermo emf with temperature

 For explanation of the variation of the thermo emf with temperature

The thermo emf generated by the thermocouple varies with temperature.  So, the temperature is maintained in such a way that there will be generation of maximum emf. The temperature at which there is a generation of maximum emf is called neutral temperature. For this, the temperature of one of the junction is kept constant. On increasing further temperature, the thermoemf decreases and again becomes zero. After this, the emf becomes negative. This is called temperature of inversion.

Let us consider an iron-copper thermocouple is taken and fitted with a galvanometer as shown in figure. Here, one junction is kept in oil bath and another is immersed in ice. The temperature of ice is kept constant. When the temperature of both junction is same, the galvanometer shows null deflection as there is no generation of thermoemf.

Now, the temperature of hot junction is made increased by heating the oil bath provided with the burner.  The temperature of hot junction is increased by keeping the temperature of cold junction constant. At certain temperature, the galvanometer shows the maximum deflection.  This temperature is called the neutral temperature. On increasing the temperature further, the deflection of galvanometer goes on decreasing.

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

E=ɵα+ βθ22$\frac{\beta {\theta }^2}{2}$

$\frac{\mathrm{<span\; style="text-align:\; justify;\; white-space:\; normal;">Where\; \alpha \; and\; \beta \; are\; constant\; whose\; value\; depend\; upon\; the\; material\; of\; conductor </span>}}{}$

and temperature difference of two junction.

Let ɵC be the temperature of the cold junction then,

ɵi- ɵn= ɵn- ɵC

$\frac{\mathrm{}}{}$

or, ɵn=θc+θi2

so the neutral temperature lies between the inversion temperature and temperature of cold junction

Thermocouple:

A thermocouple is an electrical device consisting of two different conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. Thermocouples are a widely used type of temperature sensor.

Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered major appliances.

Neutral temperature:

The temperature of hot junction at which the thermo emf becomes maximum is called neutral temperature.

Temperature of inversion:

The temperature of the hot junction at which the thermo emf is zero and reverses the direction is called temperature of inversion.

Relation between Neutral temperature and Temperature of inversion:

If θC be the temperature of cold junction and θi is the temperature of inversion, then it is found that,

Or, θn= θi+θc2$\frac{{\theta }_i+{\theta }_c}{2}$

Similarly, At the neutral temperature,

Or, dedθ$\frac{\mathrm{d}\mathrm{e}}{\mathrm{d}\theta }$ = 0, i.e a + 2bθn = 0

So, θn = a2b$\frac{\mathrm{a}}{2\mathrm{b}}$

$\frac{\mathrm{<br>}}{}$

$\frac{\mathrm{ \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; SEEBECK\; Effect}}{}$

 The Seebeck effect is the conversion of heat directly into electricity at the junction of different types of wire. It is named for the Baltic German physicist Thomas Johann Seebeck, who in 1821 discovered that a compass needle would be deflected by a closed loop formed by two different metals joined in two places, with a temperature difference between the joints. This was because the electron energy levels in each metal shifted differently and a voltage difference between the junctions created an electrical current and therefore a magnetic fieldaround the wires. Seebeck did not recognize there was an electric current involved, so he called the phenomenon "thermomagnetic effect." Danish physicist Hans Christian Orsted rectified the oversight and coined the term "thermoelectricity".

The Seebeck effect is a classic example of an electromotive force (emf) and leads to measurable currents or voltages in the same way as any other emf. Electromotive forces modify Ohm's law by generating currents even in the absence of voltage differences (or vice versa); the local current density is given by

${\displaystyle \mathbf {J} =\sigma (-{\boldsymbol {\nabla }}V+\mathbf {E} _{\rm {emf}})}$

where ${\displaystyle \scriptstyle V}$ is the local voltage^[2] and ${\displaystyle \scriptstyle \sigma }$ is the local conductivity. In general, the Seebeck effect is described locally by the creation of an electromotive field

${\displaystyle \mathbf {E} _{\rm {emf}}=-S{\boldsymbol {\nabla }}T}$

where ${\displaystyle \scriptstyle S}$ is the Seebeck coefficient (also known as thermopower), a property of the local material, and ${\displaystyle \scriptstyle {\boldsymbol {\nabla }}T}$ is the gradient in temperature ${\displaystyle \scriptstyle T}$.

The Seebeck coefficients generally vary as function of temperature, and depend strongly on the composition of the conductor. For ordinary materials at room temperature, the Seebeck coefficient may range in value from −100 μV/K to +1,000 μV/K (see Seebeck coefficient article for more information).

If the system reaches a steady state where ${\displaystyle \scriptstyle \mathbf {J} \;=\;0}$, then the voltage gradient is given simply by the emf: ${\displaystyle \scriptstyle -{\boldsymbol {\nabla }}V\;=\;S{\boldsymbol {\nabla }}T}$. This simple relationship, which does not depend on conductivity, is used in the thermocouple to measure a temperature difference; an absolute temperature may be found by performing the voltage measurement at a known reference temperature. A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition is kept at a constant temperature and held in contact with the unknown sample that is locally heated to the probe temperature. It is used commercially to identify metal alloys. Thermocouples in series form a thermopile. Thermoelectric generators are used for creating power from heat differentials.

                                                                  Peltier effect

The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors and is named after French physicist Jean Charles Athanase Peltier, who discovered it in 1834. When a current is made to flow through a junction between two conductors, A and B, heat may be generated or removed at the junction. The Peltier heat generated at the junction per unit time, ${\displaystyle \scriptstyle {\dot {Q}}}$, is equal to

${\displaystyle {\dot {Q}}=\left(\Pi _{\mathrm {A} }-\Pi _{\mathrm {B} }\right)I}$

where ${\displaystyle \scriptstyle \Pi _{A}}$ (${\displaystyle \scriptstyle \Pi _{B}}$) is the Peltier coefficient of conductor A (B), and ${\displaystyle \scriptstyle I}$ is the electric current (from A to B). The total heat generated is not determined by the Peltier effect alone, as it may also be influenced by Joule heating and thermal gradient effects.

The Peltier coefficients represent how much heat is carried per unit charge. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ${\displaystyle \scriptstyle \Pi _{A}}$ and ${\displaystyle \scriptstyle \Pi _{B}}$ are different. The Peltier effect can be considered as the back-action counterpart to the Seebeck effect (analogous to the back-emf in magnetic induction): if a simple thermoelectric circuit is closed then the Seebeck effect will drive a current, which in turn (via the Peltier effect) will always transfer heat from the hot to the cold junction. The close relationship between Peltier and Seebeck effects can be seen in the direct connection between their coefficients: ${\displaystyle \scriptstyle \Pi \;=\;TS}$.

A typical Peltier heat pump device involves multiple junctions in series, through which a current is driven. Some of the junctions lose heat due to the Peltier effect, while others gain heat. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators.

                                                   Thomson effect:

In different materials, the Seebeck coefficient is not constant in temperature, and so a spatial gradient in temperature can result in a gradient in the Seebeck coefficient. If a current is driven through this gradient then a continuous version of the Peltier effect will occur. This Thomson effect was predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient.

If a current density ${\displaystyle \scriptstyle \mathbf {J} }$ is passed through a homogeneous conductor, the Thomson effect predicts a heat production rate ${\displaystyle \scriptstyle {\dot {q}}}$ per unit volume of:

${\displaystyle {\dot {q}}=-{\mathcal {K}}\mathbf {J} \cdot {\boldsymbol {\nabla }}T}$

where ${\displaystyle \scriptstyle {\boldsymbol {\nabla }}T}$ is the temperature gradient and ${\displaystyle \scriptstyle {\mathcal {K}}}$ is the Thomson coefficient. The Thomson coefficient is related to the Seebeck coefficient as ${\displaystyle \scriptstyle {\mathcal {K}}\;=\;T\,{\frac {dS}{dT}}}$ This equation however neglects Joule heating, and ordinary thermal conductivity.

Important question: 

Difference between Seeback effect and petlier effect

Petlier effect When two wire of different material are joint to form closed circuit and the two jointed end are placed at different temperature then a small emf develops in that closed circuit and small current flow through the closed circuit this process is called seebeck’s effect or thermoelectric effect.	Seeback –effect The Peltier effect is a temperature difference created by applying a voltage between two electrodes connected to a sample of semiconductor material. This phenomenon can be useful when it is necessary to transfer heat from one medium to another on a small scale.
In this effect ,heat energy absorbed from external source is converted into electrical energy	In this effect, one junction evolves heat, whereas the other junction absorbs heat.
When difference of temperature is maintained between the two junction or at a joint point of a thermocouple, the Peltier'se.m.f. is developed	Seebeck effect is the resultant of peltier's effect and Thomson effect.
Peltier-effect devices are used for thermoelectric cooling in electronic equipment and computers when more conventional cooling methods are impractical.	This effect can be used to generate electricity, measure temperature or change the temperature of objects.

SOURCE:WIKIPEDIA

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