A neutron is moving with a velocity of \( \vec{v} \) in non-uniform magnetic field as shown in figure.
Velocity \( \vec{v} \) of neutron would be
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Same at A, B and C
A magnetic field acts on a moving particle only through its charge, by the Lorentz force \( \vec{F} = q\,(\vec{v} \times \vec{B}) \).
A neutron has no charge, so \( q = 0 \) and the force on it is zero everywhere, however strong or weak the field is. With no force, its velocity does not change in size or direction, so it is the same at A, B and C.
NCERT (2026-27):
Magnetic Field, Lorentz Force – “… The force on an electric charge q due to both of them can be written as
\( \displaystyle \mathbf{F} = q\,[\mathbf{E}(\mathbf{r}) + \mathbf{v} \times \mathbf{B}(\mathbf{r})] \equiv \mathbf{F}_{\text{electric}} + \mathbf{F}_{\text{magnetic}} \qquad (4.3) \)
This force was given first by H.A. Lorentz based on the extensive experiments of Ampere and others. It is called the Lorentz force.”
Magnetic Field, Lorentz Force – “… (i) It depends on q, v and B (charge of the particle, the velocity and the magnetic field). Force on a negative charge is opposite to that on a positive charge.”