NCERT Solutions
Class 12 Physics
Electrostatic Potential and Capacitance

Q1.
Two charges 5 × 10–8 C and –3 × 10–8 C are located 16 cm apart. At what point(s) on the line joining the two charges is the electric potential zero? Take the potential at infinity to be zero.
Case 1:
Let q1 =5 × 10–8 C
q2 = +3x10–8 C
Distance between the two charges, d =16cm =0.16m
Distance of point P from charge q1 = r
The electric potential (V) at point P will be 0
Therefore, Potential at point P is the sum of potentials caused by charges q1 and q2 respectively.
∴ V = (1/ 4πε0) x (q1/r) + (1/ 4πε0) x [q2/ (d -r)].......... (1)
Where ε0 = Permittivity of free space
For V=0, equation (1) changes to:
0 = (1/ 4πε0) x (q1/r) + (1/ 4πε0) x [q2/ (d -r)]
=> (1/ 4πε0) x (q1/r) = - (1/ 4πε0) x [q2/ (d -r)]
=> (q1/r) = - (q2)/ (d -r)
=> (5 x 10-8)/(r) = - (-3 x 10-8)/ (0.16-r)
=> 5 (0.16-r) = 3r
=> 0.8 = 8r
=> r =0.1m
=10cm
This shows, the potential is 0 at a distance of 10cm from the positive charge between the charges.
Consider a point P on the outside the system of two charges at a distance of s from the negative charge, where potential is 0.
Then potential is given as:
V = ((1)/ (4πε0)) x ((q1)/s)) + ((1)/ (4πε0)) x (q2)/ (s -d)).......... (2)
Where ε0 = Permittivity of free space
For V=0, equation (2) changes to:
0 = ((1)/ (4πε0)) x ((q1)/s)) + ((1)/ (4πε0)) x (q2)/ (s -r))
=> (1/ 4πε0) x (q1/s) = - (1/ 4πε0) x [q2/ (s -r)]
=> (q1/s) = - [q2/ (s -r)]
=> (5 x 10-8)/(s) = = - (-3 x 10-8)/ (s - 0.16)
=> 5(s - 0.16) = 3s
=> 0.8 = 2s
=> s= 0.4m
Or s=40cm
This shows, the potential is 0 at a distance of 40cm from the positive charge between the charges outside the system of charges.