CUET · PHYSICS · PYQ PAPER 2023

A polythene piece rubbed with wool is found to have a negative charge of \( 8 \times 10^{-10} \, \text{C} \). What is the number of electrons transferred ?

A \(2 \times 10^{10} \text{; polythene to wool} \)
B \(5 \times 10^9 \text{; polythene to wool} \)
C \( 5 \times 10^9 \text{; wool to polythene} \)
D \(2 \times 10^{10} \text{; wool to polythene} \)

Verified Solution

Answer & Solution

Correct Answer

(C) \( 5 \times 10^9 \text{; wool to polythene} \)

Step-by-step Solution

Detailed explanation

\( n = \frac{|Q|}{e} \) \( n = \frac{8 \times 10^{-10}}{1.6 \times 10^{-19}} = 5 \times 10^9 \text{ electrons} \) Since polythene has negative charge, electrons transferred from wool to polythene.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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