________________
conductor. Other factors that contribute to the resistance are the impurities in the conductor and its constitutional and mechanical defects.
2. Ohm's law and the Resistivity
A German scientist named Ohm deduced in 1828 a law that " under a given (fixed) physical conditions, the ratio of the electrical potential difference to the current is constant." This constant is known as the resistance of the conductor. Mathematically expressed, the law is,
R = V/I
where V is the potential difference between the ends of a conductor and I is the current flowing through it. With V measured in volts and I in amperes, we have R in units called ohm, Thus,
Ohm volt/ampere
The constant Rappearing in equation depends on the dimensions, nature and the temperature of the conductor.
39. (b) Satish K. Gupta op.cit, p. 113
"Conductors, Insulators and Dielectrics
The carriers of current in metals are free electrons and the free electron model for the metal can explain some of the observed properties of the metals in a qualitative manner.
The elements, in which the valence shell is filled less than half, are found to be good conductors. For example, in metals such as copper, aluminium, silver, etc., the valence shell contains three or less electrons. Since an atom has a tendency to have a filled valence shell, the valence electrons in the atoms of a metal leave the atoms and are free to move through the metal lattice in a random manner. They constatnly collide among themselves and with the positive ions located in the metal lattice. They have practically no affinity to their parent atoms. However, the average velocity of free electrons in a metal is zero.
When an external electric field is applied across the two ends of a metal, the free electrons experience force and get accelerated. There is a net flow of electrons through the metal. It is found that as the strength of the applied electric field is increased, more and more free electrons cross through a section of the metal.
The material which do not have free electrons in them are unable to conduct electricity and are termed as insulators. In fact, the same material may possess the following two properties:
तुलसी प्रज्ञा अप्रेल - सितम्बर, 2003
Jain Education International
For Private & Personal Use Only
81
www.jainelibrary.org
