In the previous section we found that on passing current through a resistance heat is developed . Now it is natural to think that, whether a current is developed if a resistor heated in some way . A thermal motions electrons are random no resultant current can be established on a resistance by simple heating . Thus Joules heating effect is irreversible.
About 20 years earlier to the Joules experimentally study on the heating effect of current Thomas Johann Seebeck in 1821 discovered that electric current is developed in a circuit consisting of two dissimilar metals forming two junctions one of which is kept at high temperature. He called this arrangement as thermocouple and the phenomenon as Seebeck effect . Thermocouple is a closed circuit forming two junctions of two dissimilar metals.The Phenomenon in which an emf is developed across the junctions, when the two junctions of a thermocouple are kept at different temperature is called Seebeck effect
Explanation
We know that all metals have free electrons which are in random motion . The free electron density at all portion of a metal is same But free electron densities of different metals are different . When two dissimilar metals are joined to each other free electrons have a tendency to move from metal of more electron density to that of lesser concentration. The rate of this diffusion increases with temperature of junction. This diffusion result in a potential difference across the junctions of dissimilar metals. The potential difference developed across the junctions of two dissimilar metals is called contact potential
When the junctions of a thermocouple are kept at the same temperature the contact potential at both the junctions are equal and potential difference between the junction is zero.But this is not the case when they are kept at different temperatures The contact potential at the hot junction is more than that at the cold junction. Hence an effective potential difference is developed between the junctions . This is the cause of thermal electricity . The thermo emf developed across a thermocouple depends on the metals of the couple and also on the temperature difference between the junctions
Thermoelectric Power or Seebeck-Coefficient
The rate of change of thermo emf with temperature is called thermoelectric power or Seebeck Coefficient S. The thermo electric power is the change in emf when the temperature difference between the junctions change by 1 degree C.
Variation of Thermo-emf with Temperature
When both the junctions of thermocouple are kept at same temperature the contact potential developed at both the junctions are the same. Hence no thermoelectric emf is obtained between the junctions.
Neutral Temperature and Inversion Temperature
The temperature of the hot junction for which the thermo-emf is maximum is known as the neutral temperature . The temperature of the hot junction at which the thermo-emf is zero and changes its polarity thereafter is known as inversion temperature At inversion temperature E=0.
About 20 years earlier to the Joules experimentally study on the heating effect of current Thomas Johann Seebeck in 1821 discovered that electric current is developed in a circuit consisting of two dissimilar metals forming two junctions one of which is kept at high temperature. He called this arrangement as thermocouple and the phenomenon as Seebeck effect . Thermocouple is a closed circuit forming two junctions of two dissimilar metals.The Phenomenon in which an emf is developed across the junctions, when the two junctions of a thermocouple are kept at different temperature is called Seebeck effect
Explanation
We know that all metals have free electrons which are in random motion . The free electron density at all portion of a metal is same But free electron densities of different metals are different . When two dissimilar metals are joined to each other free electrons have a tendency to move from metal of more electron density to that of lesser concentration. The rate of this diffusion increases with temperature of junction. This diffusion result in a potential difference across the junctions of dissimilar metals. The potential difference developed across the junctions of two dissimilar metals is called contact potential
When the junctions of a thermocouple are kept at the same temperature the contact potential at both the junctions are equal and potential difference between the junction is zero.But this is not the case when they are kept at different temperatures The contact potential at the hot junction is more than that at the cold junction. Hence an effective potential difference is developed between the junctions . This is the cause of thermal electricity . The thermo emf developed across a thermocouple depends on the metals of the couple and also on the temperature difference between the junctions
Thermoelectric Power or Seebeck-Coefficient
The rate of change of thermo emf with temperature is called thermoelectric power or Seebeck Coefficient S. The thermo electric power is the change in emf when the temperature difference between the junctions change by 1 degree C.
Variation of Thermo-emf with Temperature
When both the junctions of thermocouple are kept at same temperature the contact potential developed at both the junctions are the same. Hence no thermoelectric emf is obtained between the junctions.
Neutral Temperature and Inversion Temperature
The temperature of the hot junction for which the thermo-emf is maximum is known as the neutral temperature . The temperature of the hot junction at which the thermo-emf is zero and changes its polarity thereafter is known as inversion temperature At inversion temperature E=0.
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