• a point charge 'q ' is placed at o as shown in fig .is (Va-Vb) positive , negative or zero if (1)q is positive (2)qis negative O_____________Va__________Vb

plse explain this i know that answer is 1) positive and 2) negative but plse how we know that plse explain properly

A positive charge creates a positive potential around it in space while a negative charge creates a negative potential around it.

as the we move away from the charge radially their potential decrease.for positive charge the positive potential decreases and for negative charge negative potential decreases.

if the charge is positive then the potential at point A is more positive than the potential at B.for example it can be 6v at A and 3v at B.so if you find the potential difference it will be 3v.

if the charge is negative then the potential at point A is more negative than the potential at B.for example it can be -6v at A and -3v at B.so if you find the potential difference it will be [-6-(-3)]=-3v 

I hope it's clear now