# Physics Class 12 NCERT Solutions: Chapter 1 Electric Charges and Fields Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 24. Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude . What is E? : (A) in the outer region of the first plate, (B) in the outer region of the second plate, and (C) Between the plates?

The situation is represented in the following figure.

A and B are two parallel plates close to each other region of plate A is labelled as I, outer region of plate B is labelled as III, and the region between the plates, A and B, is labelled as II.

Charge density of plate

Charge density of plate B,

In the regions, I and III, electric field E is zero. This is because charge is not enclosed by the respective plates.

Electric field E in region II is given by the relation,

Where,

Therefore, electric field between the plates is .

Q: 25. An oil drop of excess electrons is held stationary under a constant electric field of in Millikan՚s oil drop experiment. The density of the oil is . Estimate the radius of the drop. .

Excess electrons on an oil drop,

Electric field intensity,

Density of oil,

Acceleration due to gravity,

Charge on an electron,