JEE Main 2024 · 4 April · morning · Single correct · +4 / −1
An electron is projected with uniform velocity along the axis inside a current carrying long solenoid. Then :
- (a) the electron will experience a force at 45° to the axis and execute a helical path.
- (b) the electron will be accelerated along the axis.
- (c) the electron path will be circular about the axis.
- (d) the electron will continue to move with uniform velocity along the axis of the solenoid.
Answer and solution
Answer (d) the electron will continue to move with uniform velocity along the axis of the…
Solution
Physics sir · 3 steps
Inside the solenoid, B points along the axis, and so does the electron's velocity: no force at all.
- Field direction. Inside a long solenoid the magnetic field is uniform and points along the axis, with B = μ₀ n I.
F⃗ = q(v⃗ × B⃗), |F⃗| = qvB sin θ
- Angle is zero. The electron moves along the axis, so v⃗ is parallel to B⃗ and θ = 0°. Since sin 0° = 0, the magnetic force is zero.
- So. With no force acting, the electron keeps its speed and direction. It moves on with uniform velocity along the axis, which is option (d).
The idea to keep. A magnetic force needs a velocity component perpendicular to B⃗. If a charge moves exactly along the field lines, the field cannot touch it.
Answer from the NTA final answer key. Solution by PrepWiser.