On the basis of conductivity ,the materials can be classified in to following categories:-

(a)    Conductors:- These are those materials whose electrical conductivity is very high. Conductors conduct charges very easily . For super-conductor , the value of electrical conductivity is infinite.

Examples. Copper,Silver.Aluminum,Tungsten etc.

(b)   Semiconductors:- They are those materials whose electrical conductivity lies in between insulators and conductors . Semiconductors can conduct charges but not so easily as in case of conductivity.

Examples. Germanium.Silicon etc.

(c)    Insulators:- These are those materials whose electrical conductivity is either very very small or nil. Insulators do not conduct charges.

Examples. Glass,Rubber,Wood etc.


Definition. Electric current is defined as the rate of flow of electric charge q, that is


The unit of current is ampere ,A

If the current is not varying with time , then the current is called steady electric or stationary current.

In the conductors ,current is due to motion of electrons .In gases , the charge is carried by electrons and positive ions.In Liquids, the charge is carried by positive and negative ions. In semiconductors ,the movement in charge is due to electrons and holes.

Types Of Current.The electric current can be classified into the following categories:-

(i) Steady current:- The current whose magnitude does not change with time is called Steady current. The variations between current(I) and time(t) for a steady current will be a straight line.

(ii) Varying current:– The current whose magnitude changes with time is called varying current. T

(iii) Alternating current:-The current whose magnitude changes continuously with time and direction changes periodically is called alternating current. Such a current is represented by a sine curve or cosine curve .The variation of I with t for sinusoidal alternating current is shown in fig