Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason(R).
Assertion (A) : A electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.
Reason (A) : 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 :
Assertion (A) : A electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.
Reason (A) : 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 false but (R) is true
- (B)Both (A) and (R) are true and (R) is the correct explanation of (A)✓ Correct
- (C)Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
- (D)(A) is true but (R) is false
Answer: (B)
Solution
The magnetic force on a moving charge is . For the electron to travel with constant velocity in a straight line, the net force on it must vanish, so . This requires and to be parallel or antiparallel — that is, the magnetic field must point along (or opposite to) the direction of motion.
Assertion (A) is therefore true: an electron can indeed move with constant velocity along a straight line inside a uniform magnetic field. Reason (R) is also true, since it states exactly this condition — the field is along the direction of the electron's velocity — and this is precisely why the magnetic force vanishes. Hence (R) correctly explains (A): both statements are true and (R) is the correct explanation of (A).
Assertion (A) is therefore true: an electron can indeed move with constant velocity along a straight line inside a uniform magnetic field. Reason (R) is also true, since it states exactly this condition — the field is along the direction of the electron's velocity — and this is precisely why the magnetic force vanishes. Hence (R) correctly explains (A): both statements are true and (R) is the correct explanation of (A).
JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Single correct option