Step 1: Plate A is positively charged and B is neutral.
Step 2: When a neutral plate B is brought near A, charges are induced on B such that the side near A is
negative and the other side is positive. The potential of the system of A and B in step 1 and 2 remains the same because the potential due to positive and negative charges on B cancel out.
Step 3: When the farther side of B is earthed the positive charges on B get neutralised and B is left only
with negative charges.
Now, the net potential of the system decreases due to the sum of positive potential on A and negative potential on B.
To increase the potential to the same value as was in step 2, an additional amount of charges can be given to plate A.
This means, the capacity of storing charges on A increases.The system so formed is called a ‘capacitor’.