When a charged ebonite rod is brought near a charged glass rod, there will be

Analyze the question: We are asked what happens when a charged ebonite rod is brought near a charged glass rod. The key is to know what charge each material usually carries.

Concept: Two kinds of charge exist, positive and negative.

• Like charges repel.
• Unlike charges attract.

By the standard triboelectric convention taught in the syllabus:

• Ebonite rubbed with fur or flannel gains electrons, so it becomes negatively charged.
• Glass rubbed with silk loses electrons, so it becomes positively charged.

Solution:

1. The ebonite rod carries a negative charge.
2. The glass rod carries a positive charge.
3. The charges are opposite (unlike), so the force between the rods is attractive.

Why not the other options?

• B (repulsion): This would happen only if both rods had the same type of charge.
• C (spark): A spark needs a large potential difference and discharge across a gap. It is not the expected result for ordinary charged rods.
• D (no force): Charged bodies always exert electrostatic forces on each other.

Answer: The rods attract each other, so the correct option is A.

Exam tip: Remember "Glass is Positive, Ebonite is Negative" (think "Good People, Every Night"). Also remember that the only sure test for charge is repulsion, since attraction can also occur between a charged and an uncharged body. In this question, though, both rods are known to be charged with opposite signs, so attraction is the answer.