Question

three point charges q -2q and -2q are placed at the vertices of an equilateral triangle of side a find the work done by external force to increase their separation to 2a

Answer 1

answer : workdone by external force = 0

explanation : initial electrostatic potential energy , $U_i$ = k(q)(-2q)/a + k(-2q)(-2q)/a + k(-2q)(q)/a

= -2kq²/a + 4kq²/a - 2kq²/a

= 0

final electrostatic potential energy,$U_f$ = k(q)(-2q)/2a + k(-2q)(-2q)/2a + k(-2q)(-q)/2a

= -2kq²/2a + 4kq²/2a - 2kq²/2a

= -kq²/a + 2kq²/a - kq²/a

= 0

according to conservation of energy theorem.

we know, change in potential energy = workdone by external force

or, workdone by external force = $U_f-U_i=0$

hence, answer is 0