Question

The curve represents the distribution of potential along the straight line joining the two charges $Q_{1}$ and $Q_{2}$ (separated by a distance $r$).

1. $\left|Q_{1}\right|>\left|Q_{2}\right|$
2. $Q_{1}$ is positive in nature
3. $A$ and $B$ are equilibrium points
4. $C$ is a point of unstable equilibrium
Then which of the above statements are correct?

• A. 1 and 3
• B. 1,2 and 3
• C. 1,2 and 4
• D. 1,2,3 and 4

Answer 1

Answer: (C)

The potential near a positive charge is positive and that near a negative charge is negative.
From figure, $Q_{1}$ is positive and $Q_{2}$ is negative.

As at point $C$ the potential is positive despite the point being closer to the negative charge.
Hence $\left|Q_{1}\right|>\left|Q_{2}\right|$
Also, point $C$ is a point of local maxima for potential energy.
Hence, it is a point of unstable equilibrium.
Also, at points $A$ and $B, \frac{d U}{d x} \neq 0$,
hence they are not the points of equilibrium.