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Section B of the **Class 12 CBSE Physics Solved Question Paper** includes **descriptive questions**, each carrying 2 marks. These questions require students to provide concise yet detailed explanations, derivations, and numerical solutions to demonstrate their understanding of fundamental Physics concepts. Practicing these questions will help develop clarity in explanations and mastery over various topics in Physics.

## Section B – Descriptive Questions

17(a). Four point charges of 1μC, −2μC, 1μC, and −2μC are placed at the corners A, B, C, and D respectively, of a square of side 30 cm. Find the net force acting on a charge of 4μC placed at the center of the square.

17(b). Three point charges, 1 pC each, are kept at the vertices of an equilateral triangle of side 10 cm. Find the net electric field at the centroid of the triangle.

18. Derive an expression for the magnetic force F⃗ acting on a straight conductor of length L carrying current I in an external magnetic field B⃗. Is it valid when the conductor is in zig-zag form? Justify.

19. A telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm. Calculate its magnifying power in normal adjustment and the distance of the image formed by the objective.

20(a). Two energy levels of an electron in a hydrogen atom are separated by 2.55 eV. Find the wavelength of radiation emitted when the electron makes a transition from the higher energy level to the lower energy level.

20(b). In which series of the hydrogen spectrum does this line fall?

21. The earth revolves around the sun in an orbit of radius 1.5×10¹¹ m with an orbital speed of 30 km/s. Find the quantum number that characterizes its revolution using Bohr’s model (mass of Earth = 6.0×10²⁴ kg).

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