Conversion of temperature differences into electrical voltage and
vice versa is known as thermoelectric
A thermoelectric device generates voltage when there is a temperature
difference on each side. If a voltage is applied, it generates a temperature
Seebeck discovered that
in a closed circuit consisting of two dissimilar metals, when the junctions are
maintained at different temperatures an emf (potential difference) is
developed. The current that flows due to the emf developed is called
thermoelectric current. The two dissimilar metals connected to form two
junctions is known as thermocouple (Figure 2.35).
If the hot and cold
junctions are interchanged, the direction of current also reverses. Hence the
effect is reversible.
The magnitude of the emf
developed in a thermocouple depends on (i) the nature of the metals forming the
couple and (ii) the temperature difference between the junctions.
1. Seebeck effect is
used in thermoelectric generators (Seebeck generators).These thermoelectric
generators are used in power plants to convert waste heat into electricity.
2. This effect is utilized in automobiles as
automotive thermoelectric generators for increasing fuel efficiency.
3. Seebeck effect is
used in thermocouples and thermopiles to measure the temperature difference
between the two objects.
In 1834, Peltier
discovered that when an electric current is passed through a circuit
of a thermocouple, heat is evolved at one junction and absorbed at the other
junction. This is known as Peltier effect.
In the Cu-Fe
thermocouple the junctions A and B are maintained at the same temperature. Let
a current from a battery flow through the thermocouple (Figure 2.36 (a)). At
the junction A, where the current flows from Cu to Fe, heat is absorbed and the
junction A becomes cold. At the junction B, where the current flows from Fe to
Cu heat is liberated and it becomes hot. When the direction of current is
reversed, junction A gets heated and junction B gets cooled as shown in the
Figure 2.36(b). Hence Peltier effect is reversible.
Thomson showed that if
two points in a conductor are at different temperatures, the density of
electrons at these points will differ and as a result the potential difference
is created between these points. Thomson effect is also reversible.
If current is passed
through a copper bar AB which is heated at the middle point C, the point C will
be at higher potential. This indicates that the heat is absorbed along AC and
evolved along CB of the conductor as shown in Figure 2.37(a). Thus heat is
transferred due to the current flow in the direction of the current. It is
called positive Thomson effect. Similar effect is observed in metals like
silver, zinc, and cadmium.
When the copper bar is
replaced by an iron bar, heat is evolved along CA and absorbed along BC. Thus
heat is transferred due to the current flow in the direction opposite to the
direction of current. It is called negative Thomson effect as shown in the
Figure 2.37(b). Similar effect is observed in metals like platinum, nickel,
cobalt, and mercury.