Thermoelectric Effects
The phenomenon of production of electricity when two junction
of different metal plates are kept at different temperature is known as
thermoelectric effect.
1.
Seebeck’s Effect
2. Peltier Effect
3. Thomson’s Effect
Seebeck Effect
The phenomenon of generation of an electric current in a
thermo couple by keeping its two end at different temperature is known as
seebeck effect.
Thermocouple:
A pair of wires of different metals forming a loop and producing thermo electricity is known as thermo couple. Iron-copper; Antimony-Bismuth are some example of thermocouple.
Thermoelectric series
An arrangement of metals in series in which any two metals
can be used to form a thermocouple is known as thermo electric series. These series
contains the metal Antimony, iron, zinc, silver, gold, tin, lead, copper,
platinum, nickel, Bismuth.
i.e Sb, Fe, Zn, Ag, Au, Sn, Pb, Cu, Pt, Ni, Bi
Peltier Effect
When an electric current is passed through a thermocouple,
heat is absorbed or released at the junction, depending upon the direction of
current. This effect is called peltier effect.
Thomson Effect
The phenomenon of evolution or absorption of heat along the
length of conductor on passing current through it, when its two ends are kept
at different temperature is known as Thomson effect.
If electric current is passed through copper wire from hot
to cold end, the heat is released and wire get heated then such conductor is
known as positive Thomson effect. But in the case of iron, heat is absorbed and
wire get cooled when current is passed from hot to cold end. So, iron is said
to be negative thomson’s effect.
Variation of thermo emf with temperature
Let us consider a iron-copper thermo-couple to study the variation
of thermo emf with temperature. One junction of the thermocouple is immersed
into hot oil bath and another junction is kept into cold ice. When the temperature
of both junction are at same temperature 0 C, there is no flow of current and
no emf is produced. When the temperature of oil bath is increased keeping cold
junction at 0 C, the emf is produced and becomes maximum at temperature `\theta_n`
called neutral temperature.
When the temperature of oil bath is increased beyond neutral
temperature, the thermo emf goes on decreasing and becomes zero at temperature
`\theta_i` called the temperature of inversion. Finally the direction of thermo
emf is changed into opposite direction. If `\theta_c` be the temperature of
cold junction then,
`\theta_i - \theta_n = \theta_n -\theta_c`
`2\theta_n = \theta_c + \theta_i`
`\therefore \theta_n = \frac{\theta_c + \theta_i}{2}`
Neutral temperature is the mean of temperature of cold junction & temperature of inversion.
The variation of thermo emf(E) with temperature (`\theta`) is
given by
`E = \alpha\theta + \frac{1}{2} \beta \theta^2`
Where `\alpha` and `\beta` are constant.
Relation between `\alpha` , `\beta` , `\theta_n` and `\theta_i` :
The equation of thermo emf with temperature is given by
`E = \alpha\theta + \frac{1}{2} \beta \theta^2`
`\frac{dE}{d\theta} = \frac{d(\alpha\theta+ \frac{1}{2}\beta\theta^2)}{d\theta}`
`\frac{dE}{d\theta} = \alpha + \beta\theta`
E is maximum at `\theta` = `\theta_n` and so `\frac{dE}{d \theta} =0`
`0 = \alpha + \beta\theta_n`
`\theta_n = -\frac{\alpha}{\beta}`
thermo emf(E) = 0 at `\theta = \theta_i` and
`E = \alpha\theta_i + \frac{1}{2} \beta (\theta_i)^2`
`\frac{1}{2}\beta(\theta_i)^2 = -\alpha\theta_i`
`\theta_i \neq 0`,
`frac{1}{2}\beta(\theta_i)^2 = - \alpha\theta_i`
`\theta_i = -\frac{2\alpha}{\beta}`
Neutral temperature (`\theta_n`) : The temperature of hot
junction at which the thermo emf becomes maximum is known as neutral
temperature. It depends upon the nature of metals. It is independent of
temperature of junction.
Temperature of inversion (`\theta_i`): The temperature of
hot junction at which the thermo emf becomes zero and reverses its direction is
known as temperature of inversion. It depends upon the nature of metals and the
temperature of cold junction.
Magnitude of thermo
emf depends on the nature of the metals forming a thermocouple and difference
in temperature of two junction.
Application of thermoelectric effect:
1. Thermopile:
Thermopile is a device used for the detection and
measurement of heat radiation. It works on the principle of thermoelectric
effect.
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