Q. 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) (A) is true but (R) is false
B) Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
C) Both (A) and (R) are true and (R) is the correct explanation of (A)
D) (A) is false but (R) is true
Explanation
[Solution by Dr. Abubacker] The magnetic force $q\vec v\times\vec B$ vanishes when $\vec B\parallel\vec v$, so the electron moves undeflected along a straight line. Both statements are true and (R) explains (A). Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.