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

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Electric Field Formula

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.

Q_24_the Two Parrallel Plates

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,


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,

Radius of the oil drop

Force (F) due to electric field E is equal to the weight of the oil drop (W)


Therefore, the radius of the oil drop is

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