A charged particle enters a magnetic field at 90° to the field. Its kinetic energy MCQ with Answer and Explanation

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A charged particle enters a magnetic field at 90° to the field. Its kinetic energy

A. Becomes zero

B. Remains constant

C. Increases

D. Decreases

Answer: Option B

Solution (By JKSSB Mock Tests)

Magnetic force does no work, speed constant, so KE constant. Path circular.

This question belongs to: Science Physics

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Question #1

Sound waves of frequency 20 kHz are called

A. Infrasonic

B. Ultrasonic

C. Audible

D. Supersonic

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Correct Answer: Option B

Explanation:
Above 20 kHz is ultrasonic.

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Question #2

A convex lens is immersed in water. Its focal length

A. Becomes infinite

B. Remains same

C. Decreases

D. Increases

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Correct Answer: Option D

Explanation:
Lens maker's formula: 1/f = (μ_lens/μ_medium - 1)(...). In water, relative refractive index reduces, so power decreases, focal length increases. Glass in water behaves less converging.

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Question #3

The velocity of light in vacuum is 3×10⁸ m/s. In a medium of refractive index 1.5, speed is

A. 3×10⁸ m/s

B. 2×10⁸ m/s

C. 4.5×10⁸ m/s

D. 1.5×10⁸ m/s

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Correct Answer: Option B

Explanation:
v = c/n = 3×10⁸/1.5 = 2×10⁸ m/s.

This question belongs to: Science Physics

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