2.10 Thermal Effect of Electric Current

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.

2.9 Applications of Heating Effect of Electric Current

The heating effect of electric current has wide range of applications in our daily life . Here we shall see a few common applications.

Electric Bulb

                      Have you ever tried to find how light is emitted from an electric bulb ? In an electric bulb actually a resistance wire is heated to very high temperature so that light of all wavelengths are emitted due to  incandescence. Can we make bulb filament using any metal? In order to get a high temperature the resistance of the filament must be high and also at high temperature the filament should not melt . Hence Filament material of a bulb should have high resistivity and high melting point. Power supply systems use constant voltage rather than current values .Hence the resistance of the bulb is inversely proportional to the power of bulb . Thus 100W bulb has lower resistance that 60W bulb.

Electric Fuse

                         Another unavoidable electric device used is electric fuse . Electric fuse is a device used to protect electric circuit from excess of current . The fuse wire material should have low melting point so that it melts off when the current exceeds the permissible value. Commonly used fuse wire material  is alloy of tin and lead. A fuse is basically a wire made up of a material having a suitable low resistivity and low melting point.

 Is there any importance to the length of the fuse wire ?

Note: The above question is quite natural but most of us neglect it. As length increase R increases and hence more heat is developed .But once the area increases the radiation loss also increases. Hence temperature due to current flow is independent of length.

    However if area is more R is lesser and surface area is more . Hence the temperature due to current flow decreases with increase in area of cross-section . Thus melting point will not be reached even though current exceeds permissible value. Hence area of cross section /thickness of  fuse wire  should have an optimum size depending on the permissible value of circuit current, while the length of fuse wire is immaterial.

2.7 Potentiometer

Wheatstone's Bridge

The resistance of a resistor can be determined by measuring potential difference across it and current flowing through it. But when we connect a voltmeter or ammeter to a circuit , due to the finite resistance of voltmeter or ammeter , circuit current and potential difference changes.Hence the measured values will not be accurate. The measurement can be made accurate only when the current through the measuring device is zero.

        It was C.F. Wheatstone's who first suggested a circuit consisting of four resistors and a galvanometer  to zero . The circuit proposed by him  is know as Wheatstone's Bridge Circuit.

Superconductivity

Is it possible to reduce the resistance of any material to zero? the answer is 'yes' .Such materials whose resistance is negligibly small are called superconductors.This phenomenon was discovered by H.Kamerlingh Onnes in 1911. He found that when mercury is cooled to 4.2 K its resistivity suddenly falls to zero.This phenomenon is called superconductivity. The phenomenon by which the resistance certain substances reduce to zero when cooled to extremely low temperature is called superconductivity and the temperature below which superconductivity occurs is called critical temperature.

                              Mercury is not only the substance which exhibit superconductivity . Now researches are going o to obtain superconducting materials with critical temperature of the order of room temperature . Most of the good conductors do not exhibit superconductivity. The element with highest critical temperature is niobium . In 1986 superconducting ceramic was discovered with critical temperature 30 K. As of 2006 the highest temperature superconductor is mercury thallium barium calcium copper oxide at 138 K

Measurement of  E.m.f- Potentiometer

Any practical voltmeter has a finite resistance and draws current from circuit to which it is connected. Hence if we use a voltmeter to measure e.m.f of a cell , due to the finite current through the cell, the potential difference measured will be only terminal voltage and not the e.m.f. E.m.f of a cell can be measured only if no current is drawn from cell while measuring the emf ,This made possible in a potentiometer.

             Potentiometer is a device used to measure potential difference between two points in a circuit. The Working principle of a potentiometer is that the potential  drop per unit length of a uniform resistance wire is constant , if a constant current is maintained through it. Generally ,potentiometer consists of 10 m long uniform resistance wire fixed on a wooden platform. Let r0 be the resistance per unit length .Then potential difference between two points separates by l length of the wire is

V=IR=Ir0l