When is the parallelogram rule used?

The parallelogram rule asks that you put the tails (end without the arrow) of the two vectors at the same point, (just the a vector and b vector on the left of the diagram) then it asks you to close the parallelogram by drawing the same two vectors again (the b vector and a vector to the right of the diagram).

Explanation:

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach