conductance and conductivity

Conductance is defined as the reciprocal of resistance of a conductor, i.e., the ratio of the current i and voltage V.

Specific resistance or resistivity

In order to understand the behaviour of the materials, it is necessary to study certain properties like (a) Resistivity (b) Conductivity and (c) conductance.

The resistance (R) of a conductor depends on its length, area of cross section.

In the above equation if l=1m and A=1sq.m, then p(rou)=R. There is the difference between the resistance and resistivity i.e., resistance is the bulky property of a material and resistivity is specific property of the material.Since, specific resistance is proportionality constant, so that it is independent of length and area of cross section for that material.

For conductors, the specific resistivity like follows


From the above figure we found that for mercury, the resistivity is high.
For semi conductors, insulators resistivities like follows

From the above the above tables, we see that resistivity of insulators is about 10power22 times the metallic conductors. Since metals like silver, copper and aluminium have lowest resistivities, so that there are used in manufacture electric cables, connecting wires etc.

graphical properties of non-ohmic resistances

This figure shows that the relation between the current i and v in case of vaccum tubes i.e., an electronic device
This figure shows the V-i characterstics of a P-N junction diode rectifier which shows that non-linear relation between the voltage and the current in positive direction and in the reverse direction. This figure shows increase in the current is not possible beyond zeroth value of the voltage, so that either in positive value of V or negative value of V, for large value of the voltage, there is increase in the current.
This figure shows that the V-i characterstics of thermistor. The detail study of thermistor we can see in electronics subject.

Ohmic and Non-ohmic resistances(devices)

Resistances that obey the Ohm's law are called ohmic resistances. A metallic conductor at a constant temperature is called Ohmic resistance. For such ohmic resistances, the V-I characterstic curve will be a straight line passing through the origin.
In ohmic resistance, current is reversed in the direction when the potential difference is reversed, but the magnitude of the current remain the same.

There are many resistances that do not obey the Ohm's law. These are called Non-ohmic resistances. For these resistances, the V-I characterstic curve will not be linear. The function of modern electronic devices that do not obey the Ohm's law.

Ohm's law

Generally, for a metallic conductor has a constant resistance R when other physical conditions remain same. For a metallic conductor the current passing through conductor will be directly proportional to the potential difference V applied across its ends. George Simon Ohm gave the relationship between V and i, which is called ohm's law.
Ohm's law is just a empirical relationship. It is not a fundamental physical principle and does not specify any general property of matter. For electrolytes, another essential condition required for application of Ohm's law is that the physical state must remain the same. Ohm's law is valid only for metallic conductors in which V/i has a constant value irrespective of the magnitudes of V and i

Resistance and units

The resistance is defined as the ratio of the potential difference 'V' across it to the current 'i' which flows through the conductor.

Resistance (R)=V/i

When we apply a potential difference 'V' between the ends of a conductor, an electric field 'E' will be set up inside the conductor and as a result a current 'i' flows through the conductor. This causes root notion for the resistance.
Resistance is a characteristic of the conductor as a whole. Resistance depends in general on nature of material, its dimensions(length,area of cross section), its temperature.

In some type of conductors the resistance 'R' increases when 'V' is increased. In another type of conductors the resistance 'R' decreases when 'V' is decreased. In some type of conductors 'R' depends on the direction of current flows through it.

current and units

Current is defined as the rate of flow of charge through any cross section of a conductor. Electric current is defined as the net charge passing through any cross section per unit time.
Suppose the net charge 'q' passes through any cross section of the conductor in time 't', then the current 'i' is given by

i=q/t

Where 'q' is in coloumbs and 't' is in seconds and then 'i' is in coloumb/sec or ampere. Like the mass, current is macroscopic quantity in SI system and is dimensionally denoted as I or A