When a negatively charged rod is brought near the cap of a charged gold leaf electroscope which has positive charges, the leaf

Analysis

This question tests your understanding of electrostatic induction and the behavior of charges in a gold leaf electroscope.

Concept Explanation

A gold leaf electroscope works on the principle that like charges repel.

1. Initial State: The electroscope is positively charged, meaning the metal cap and the gold leaf have an excess of positive charges. Because they have the same charge, they repel each other, causing the leaf to diverge (stand away from the metal plate).
2. The Interaction: When you bring a negatively charged rod near the cap, it exerts an electrostatic force on the charges in the electroscope.
3. Neutralization: The negative rod attracts the positive charges from the leaf up towards the cap. As the positive charges move from the leaf to the cap, the amount of positive charge on the leaf decreases.
4. Result: Since the leaf loses its positive charge, the repulsive force that was keeping it diverged weakens. Consequently, the leaf falls back toward the metal plate.

Step-by-Step Solution

1. Identify the initial state: The electroscope is positively charged, so the leaf is diverged.
2. Apply the external influence: A negatively charged rod is brought near the cap.
3. Analyze the charge movement: The negative rod attracts the positive charges in the electroscope toward the cap.
4. Observe the effect on the leaf: As the positive charges leave the leaf to move toward the cap, the leaf becomes less positively charged (or neutral).
5. Conclusion: With less charge on the leaf, the electrostatic repulsion decreases, causing the leaf to collapse.

Identifying the Answer

The leaf collapses because the negative rod neutralizes the positive charge on the leaf by attracting the positive charges toward the cap.

Exam Tip

Always remember the "Opposites Attract" rule in electrostatics. If the rod has the opposite charge to the electroscope, the leaf will collapse. If the rod had the same charge as the electroscope, the leaf would diverge further because the repulsion would increase!