A negatively charged rod is brought near the cap of an electroscope. The cap is earthed momentarily while the…

Analyze the question: This is charging by induction. A negatively charged rod is brought near the cap, the cap is earthed briefly, and then the rod is removed. We need the final charge on the electroscope.

Concept: Charging by induction gives the electroscope a charge opposite to that of the inducing rod. The whole electroscope (cap, rod and leaf) ends up with the same type of charge.

Step-by-step solution:

1. Bring the negative rod near the cap. The negative rod repels the free electrons in the cap, so they move down to the leaf and stem. The cap becomes positive by induction, and the leaf and stem become negative. The leaf diverges.

2. Earth the cap while the rod is still near. The repelled electrons have a path to the ground, so they flow out of the electroscope to the earth. The leaf collapses because the electroscope now has a net deficit of electrons.

3. Remove the earth connection first, then the rod. The electroscope is left with a net positive charge, with no excess electrons. When the rod is removed, the remaining positive charge spreads over the whole electroscope, including the cap, stem and leaf. The leaf diverges again because it is now positively charged.

Result: The cap and the leaf are both positively charged.

Option A is wrong because the leaf cannot be negative, since electrons were drained away. Options B and D are wrong because the electroscope has lost electrons, so it cannot be negative. Option C is correct.

Answer: C

Exam tip: Remember the rule "Induction gives the opposite charge." Then follow the order: bring the rod near, earth, remove the earth, remove the rod. The charge left on the electroscope is always opposite to that of the rod, and it spreads over the cap and leaf alike.