i) Conductance of a material is the property of a material to aid the flow of charges and hence, the passage of current in it. The conductance of a material is mathematically defined as the reciprocal of its resistance (R). Hence, the conductance ‘G’ of a conductor is given by
ii) G = 1/R
iii) Its unit is ohm^-1. It is also represented as ‘mho’. The reciprocal of electrical resistivity of a material is called its electrical conductivity.
iv) \(\sigma =1/\rho \) 
v) Its unit is onm^-1 metre^-1 It is also represented as mho metre^-1.
vi) The conductivity is a constant for a given material. Electrical conductivity of a conductor is a measure of its ability to pass the current through it.
vii) Some materials are good conductors of electric current. Example: copper, aluminium, etc. While some other materials are nonconductors of electric current (insulators). Example: glass, wood, rubber, etc. Conductivity is more for conductors than for Insulators. But the resistivity is less for conductors than for insulators.

i) The resistance of any conductor ‘R’ is directly proportional to the length of the conductor ‘L’ and is inversely proportional to its area of cross section ‘A’
ii) R\(\alpha\) L, R\(\alpha\) 1/A,
Hence, R\(\alpha\) L/A
Therefore, R = \(\rho \) L/A
Where, \(\rho \) (rho) is a constant, called as electrical resistivity or specific resistance of the material of the conductor.
From equation (1), \(\rho \) =RA/L
If L= 1m, A= 1m² then, from the above equation),  \(\rho \) =R
iii) Hence, the electrical resistivity of a material is defined as the resistance of a conductor of unit length and unit area of cross section. Its unit is ohm metre.
iv) Electrical resistivity of a conductor is a measure of the resisting power of a specified material to the passage of an electric current. It is a constant for a given material.

A German physicist, Georg Simon Ohm established the relation between the potential difference and current, which is known as Ohm’s Law.
According to Ohm’s law, at a constant temperature, the steady current ‘I’ flowing through a conductor is directly proportional to the potential difference ‘V’ between the two ends of the conductor.
I \(\alpha\) V. Hence, I/V = constant.
The value of this proportionality constant is found to be 1/R
Therefore, I = (I /R) V

V = IR Here, R is a constant for a given material (say Nichrome) at a given temperature and is known as the resistance of the material. Since, the potential difference V is proportional to the current I, the graph between V and I is a straight line for a conductor.

i) The electric potential difference between two points is defined as the amount of work done in moving a unit positive charge from one point to another point against the electric force.

ii) Electrical potential moved a charge Q from a point A to another point B. Let ‘W’ be the work done to move the charge from A to B. Then, the potential difference between the points A and B is given by the following expression.
iii) Potential Difference (V) = Work done (w)/ Charge (Q).
iv) Potential difference is also equal to the difference in the electric potential of these two points. If V_A and V_B represent the electric potential at the points A and B respectively, then, the potential difference between the points A and B is given by:
V= V_A- V_B  (If V_A is more than V_B)
V = V_B- V_A  (If V_B is more than V_A).