Kinetic Energy and Momentum in Magnetic Field – Rankers Physics

Force Acting on Moving Charges: Practice Problem & Solution

Assertion (A): A charged particle moves perpendicular to magnetic field. Its kinetic energy will remain constant but momentum changes. Reason (R): Magnetic force acts perpendicular to velocity of particle.  
Both (A) & (R) are true and the (R) is the correct explanation of the (A)
Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(A) is true but (R) is false
Both (A) and (R) are false

Short Solution (Best for NEET):

To solve this problem, we apply the core principles of Force Acting on Moving Charges. Understanding the underlying formula is key to arriving at the correct answer below:

The magnetic force is always perpendicular to the velocity (\( \vec{F} \perp \vec{v} \)). Thus, the work done by the magnetic force is zero (\( W = \vec{F} \cdot \vec{v} t = 0 \)), implying no change in kinetic energy. However, since there is a force, it changes the direction of momentum. Hence, both A and R are true, and R correctly explains A.

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