Two charges q1 and q2 are placed 30 cm apart, as shown

Two charges q₁ and q₂ are placed 30 cm apart, as shown in the figure. A third charge q3 is moved along the arc of a circle of radius 40 cm from C to D. The change in the potential energy of the system is (q₃/4πε₀)k where k is

We know that potential energy of discrete system of charges is given by

U =	1		q₁q₂	+	q₂q₃	+	q₃q₁
	4πε₀		q₁₂		q₂₃		q₃₁

According to question,

U_initial =	1		q₁q₂	+	q₂q₃	+	q₃q₁
	4πε₀		0.3		0.5		0.4

U_final =	1		q₁q₂	+	q₂q₃	+	q₃q₁
	4πε₀		0.3		0.1		0.4

U_final - U_initial =	1		q₁q₂	-	q₂q₃
	4πε₀		0.1		0.4

=	1	[10q₂q₃ - 2q₂q₃] =	q₃	(8q₂)
	4πε₀		4πε₀