Conductors are the materials which possess a large number of free charge carries and also bound charge carriers. Whereas Insulators or dielectrics possess a negligibly small number of free electrons. When a conductor is placed in an external electric field, net electric field in the interior of a conductor is zero , there is no charge in the interior of a conductor i.e. charge always resides on the outer surface of the conductor. The surface of the conductor is an equipotential surface and electric field over a charged conductor is perpendicular to the surface of the conductor at every point.

### Capacitance of a capacitor is the ratio of charge (Q)

Capacitance(c) of a capacitor is the ratio of charge (Q) given and the potential (V) to which it is raised i.e. C=Q/V. The value of C depends on size, shape and relative position of two coatings of the capacitor. It also depends on nature of medium separating the two coatings. Capacity of a parallel plate capacitor with air as dielectric is C = ∈_{0} A/d Where A is area of each plate and d is distance between them. When the insulating medium separating the two conductors in a capacitor is other then air, its capacity becomes K times the capacity in air. Here, K is dielectric constant or relative permittivity of the medium. For mica, K = 3 to 6, for paper K = 3.5, for water, K = 81, for ebonite, K = 2.7 and for glass, K varies from 3 to 4. Capacity of an isolated spherical conductor of radius r in vacuum is C= 4π∈_{0}r

### Series grouping of condensers

In Series grouping of condensers negative plate if one condenser is connected to positive plate of other and so on. Charge on each capacitor is the same, but their potentials are different. The total capacity in series (Cs) is given by the relation: 1/Cs=1/c1+1/c2+1/c3+… Obviously, total capacity in series combination decreases.

### Parallel grouping of condensers

In parallel grouping of condensers positive plates of all the condensers are connected to the other point and negative plates of all condensers are connected to the other point. Potential difference across each conductor is same, but the charges on the capacitors are different. The total capacity in parallel (Cp) is given by the relation: Cp= C1+C2+C3+… Obviously, total capacity in series combination increases.

### A Van de graff generator

A Van de graff generator is a powerful machine used for generating high positive potentials approximately 6 lakes volt. It is based on the action of sharp point (carona discharge) and the property that charge always resides on the outer surface of a hollow conductor.. Such high positive potentials are needed for accelerating positive ions.

### Non-ohmic conductors

1) These are those conductors which do not obey Ohm’s law.

2) Examples are vacuum tubes, semiconductor diode, liquid electrolyte, transistor etc.

3) The relation; V/I = R is valid for ohmic and non ohmic conductors.

4) The value of R for non ohmic conductor is different at different values of I and V.

5) The relation between V and I for non ohmic conductor is non linear.

### Super-conductors

1) Those materials which offer least resistance to the flow of current through them are called super-conductors.

2) Examples: Mercury at temp 4.2 K, led at 7.25 K and niobium at temperature 9.2 K become super-conductors.

3) If a current is once setup in a closed ring of super-conducting material, it continues flowing for several weeks after the source of e.m.f. has been withdrawn.

4) A metal conductor behaves as a super-conductor at a temperature called critical temperature, with the different for different materials.

5) In a super-conductor at critical temperature, the free electrons from a coherent or co-operative cloud of electrons which could not be deflected by the ionic vibration of the conductor at the temperature.

6) Super conductors are used-

a) In making very strong electromagnets.

b) to produce very high speed computers

c) in transmission of electric powers

d) in the study of high energy particle physics.

### Resistance of a conductor

1) It is the obstruction posed by the conductor to the flow of current through it.

2) Resistance of a conductor(R) is the ratio of potential difference applied (V) across the ends of conductor to the current flowing through it. i.e. R = V/ I.

3) S.I unit of resistance is ohm (denoted by Ω).

4) An international ohm is defined as the resistance of 106.3 cm long mercury column of 1m.mm cross sectional area and mass 14.4521 gram at 0 ° C.

5) Resistance of a conductor changes with temperature.

6) The value of resistance of a superconductor is zero.

7) The inverse of resistance is called conductance ( denoted by G), whose unit is Ω^{-1} or Siemen ( denoted by S) or mho.