Two pith balls with mass m are suspended from insulating threads. When the pith balls are given equal positive charge Q, they hang in equilibrium as shown. We now increase the charge on the left pith

Question

Two pith balls with mass m are suspended from insulating threads. When the pith balls are given equal positive charge Q, they hang in equilibrium as shown. We now increase the charge on the left pith ball from Q to 2Q while leaving its mass essentially unchanged. Which of the following diagrams best represents the new equilibrium configuration?

TestHub · Accepted Answer

The Coulomb forces on the two balls form an action-reaction pair: both balls feel the same magnitude $F = \frac{k ( 2 Q ) Q}{r ^{2}}$ , whatever the individual charges.

For each ball, $tan ϕ = \frac{F}{m g}$ . Since F and m are the same for both balls, both threads make equal angles with the vertical.

Doubling one charge doubles F, so both angles increase equally: $ϕ > θ$ , symmetric. Hence (D).

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JEE Main 2027 (Class 12+)