Question

Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R):

Assertion (A): An electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.
Reason (R): The magnetic field in that region is along the direction of velocity of the electron.

In the light of the above statements, choose the correct answer from the options given below:

• A. Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
• B. (A) is false but (R) is true
• C. Both (A) and (R) are true and (R) is the correct explanation of (A)
• D. (A) is true but (R) is false

Hint:

When a charged particle moves in a magnetic field, the force acting on the particle is always perpendicular to its velocity. This means the velocity does not change in magnitude, only direction. If the particle is moving in a straight line, the magnetic field cannot be parallel to the velocity.

Answer 1

The Correct Option is A

- Assertion (A) is true because an electron moving in a straight line with constant velocity in the presence of a magnetic field must not experience any force in the direction of motion. This implies the velocity of the electron is perpendicular to the magnetic field, so there is no magnetic force component along the velocity.

- Reason (R) is also true since the magnetic field must be perpendicular to the velocity for the force to not affect the motion of the electron. However, the statement that the magnetic field is "along the direction of velocity" contradicts the nature of the magnetic force, which acts perpendicular to both the magnetic field and the velocity. Thus, Reason (R) does not correctly explain Assertion (A).

Final Answer: Both (A) and (R) are true, but (R) is not the correct explanation of (A).