When the pointed end of an uncharged optical pin is brought near the cap cf a positively charged electroscope, it is observed that the gold

Analyze the question: A positively charged electroscope has its gold leaf diverged. An uncharged pointed pin is brought near the cap. What happens to the leaf?

Concept: Action of points (point discharge)

• Charge concentrates at sharp points, so the electric field is very strong there.
• The positively charged cap induces negative charge on the pointed end of the pin, which is closer to the cap. The far end of the pin gets positive charge.
• The strong field at the point ionizes the surrounding air molecules.
• The negative ions are attracted to the positive cap and neutralize some of its charge.
• The positive ions are repelled away from the cap.

Solution:

1. The pin is brought near the cap, and the point induces negative charge.
2. Air near the point ionizes, and negative charge flows to the cap by the "wind" of ions.
3. The positive charge on the electroscope is gradually neutralized.
4. The leaf loses charge, so the repulsion between leaf and stem decreases.
5. The gold leaf therefore collapses. This happens gradually, not instantly, because the discharge is by slow leakage through the air without touching.

So the gold leaf collapses slowly.

Answer: A. collapse slowly

Tip: Remember "points discharge." Any question about a pointed conductor near a charged electroscope means the charge leaks away, so the leaf falls. This is also the principle behind lightning conductors. Options like "diverge" would only apply if an oppositely or similarly charged body was brought near the cap without a point discharge effect, and even then they work differently.