why current flows when temperature difference is created across the junctions?

Thermoelectricity is electricity produced directly from heat. The production of electricity from heat is called the Seebeck effect, after the German physicist Thomas J. Seebeck, who discovered the phenomenon in the 1820's. Thermoelectricity arises in an electric circuit in which two different conductors (for example, metals) are joined at their ends, and when one of the junctions is at a different temperature than the other, a direct electric current will flow in the circuit. For a given thermoelectric circuit operating in a given temperature range, the magnitude of the current depends mainly on the temperature difference between the two junctions, the greater the temperature difference, the larger the current. (see the website below for a diagram)

Thermoelectric circuits have been used in small thermoelectric generators to furnish power in remote areas, in space probes for radio transmitters and receivers, and other devices that require relatively small amounts of electric power. The thermocouple, an important temperature-measuring device, also uses a thermoelectric circuit.

The Seebeck effect can be reversed, when a direct current is sent through a circuit in which two different conductors are joined at their ends, heating will take place at one of the junctions and cooling at the other. This thermoelectric effect is called the Peltier effect, after the French physicist Jean C. A. Peltier, who discovered it in the 1830's. Small heaters and refrigerators whose operation is based on this effect have been developed.

An explanation of the Seebeck effect requires an understanding of the behavior of electrons inside a metal. Not all the electrons inside a metal are bound to specific atoms; some are free to move about. These free electrons behave like a gas. The density of the "free" electrons (the number per unit volume) differs from metal to metal. Consequently, when two different metals are placed in contact, their electron gases diffuse into one another. Because of the different densities of the electron gases and because electrons carry an electrical charge, the metals at the junction become oppositely charged. This difference in charge produces a potential difference (voltage like a battery) across the junction. The extent of diffusion of the "electron gases" depends on the temperature. If the two junctions are at different temperatures, a potential difference will exist between the junctions and a current will flow.

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