In our daily life we come accross a number of devices based on effect of electric current. The current may be produce from heating, magnetic, chemical, lighting etc. Heat is produce when a current pass through a Conductor. This phenomenon is called joule effect. It is also possible to convert heat energy into electrical energy. Current can be generated by maintaining a temperature difference either between different parts of the same conductor or different conductor join at their ends. The branch of electricity that deals with the conversion of heat energy into eletrical energy is called thermo electricity.
This effect was discover by a German Physicst Thomas Johann Seebeck. In order to understand Seebeck effect, let us consider a closed circuit consisting of two different metals Cu and Fe. A sensitive galvanometer G is introduced. When one of the junctions is kept hot and the other cold, a current begins to flow from Cu to Fe through the hot junction and from Fe to Cu through the cold junction. This current is called thermoelectric current. The existence of current implies that there is an e.m.f. in the circuit. This e.m.f. is known as thermoelectric e.m.f. The arrangement is called thermoelectric couple or thermocouple. The phenomenon of generation of an electric current in a thermocouple by keeping its junctions at different temperatures is called Seebeck effect or Thermoelectric effect. The thermomoelectric effect is perfectly reversible i.e. if the hot and cold junctions are interchanged, the e.m.f. changes sign. i.e. the direction of the circulating current is reversed.
The current flow in circuit is called thermoelectric current and emf is called thermo emf and the closed circuit is called thermocouple. the emf produce depend upon nature of material and temperature difference between two Junction.
When a thermocouple is formed by joining two different metal wire, if one end the metal wire is kept at lower temperature while other at higher temperature, then an electric current start to flow in the circuit. This is the Principle of Seebeck Effects.
The magnitude and direction of thermo e.m.f. in a thermocouple depends not only on the temperature difference between the hot and cold junctions but also on the nature of metals constituting the thermocouple.
The magnitude of the thermo e.m.f. depends upon the extent of separation of the metals in the series. Larger the separation between the metals in the series, greater is the thermo e.m.f.
According of free electron theory, metal have large number of free electrons which move about freely inside the lattice in a random manner. the free electron density is different metals are brought in contact, the free electrons diffuse from the metal of higher electron density to the one with lower electron density. This diffusion of electron sets up a potential difference across the junction and is called contact potential when one of the two junction is heated, the high rate of diffusion of electron causes in making a high contact potential at the junctions. Which drives a current in the circuit.
1. Measurement of temperature by thermoelectric thermometer
2. Thermoelectric Refrigerator
3. Detection of heat radiation by thermopile.
1. If a thermocouple is formed of wires of any two metals from the series, the direction of current is from a metal occurring earlier in the series to the metal occurring latter in the series.
2. More the two metals are separated in series, the greater is the thermo e.m.f. produced in the thermocouple.
3. Temperature of hot junction of which the thermo e.m.f. produced in a thermocouple become maximum called neutral temperature. For a given thermocouple it is a fixed temperature. This is important features of Thermoelectric Effects.
4. Temperature of inversion is the temperature of hot junction at which the thermo e.m.f. reverses its direction for a given thermocouple, it changes with temperature of the cold junction.
5. Seebeck effect is reversible effect.
6. At neutral temperature thermo e.m.f. is maximum.