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:
Answer: (C) Both (A) and (R) are true and (R) is the correct explanation of (A)
The magnetic force is $\vec F = q\vec v\times\vec B$. If $\vec B$ is parallel (or antiparallel) to $\vec v$, then $\vec v\times\vec B = 0$ and the electron feels no force, so it keeps moving in a straight line with constant velocity.
Both statements are true, and (R) is exactly why (A) happens.
Solution by Sreeraj P, M.Sc Physics