Which of the following while in motion cannot be deflected by magnetic field?

Options

A. Protons
B. Cathode rays
C. Alpha particles
D. Neutrons

Explanation

To determine which particle cannot be deflected by a magnetic field while in motion, we need to analyze the properties of each option given. The deflection of a particle in a magnetic field depends on its charge and velocity. The force acting on a charged particle moving in a magnetic field is given by the equation:

\[ F = Q \cdot V \cdot B \]

Where:
- \( F \) is the magnetic force,
- \( Q \) is the charge of the particle,
- \( V \) is the velocity of the particle,
- \( B \) is the magnetic field strength.

A particle will only be deflected if it has a non-zero charge (\( Q \neq 0 \)). Let's evaluate each option:

1. **Protons**: Protons have a positive charge (\( +1e \)). Since they are charged, they will be deflected by the magnetic field. Therefore, this option cannot be the answer.

2. **Cathode Rays**: Cathode rays are streams of electrons, which carry a negative charge (\( -1e \)). As charged particles, they will also be deflected by a magnetic field. Thus, this option cannot be the answer either.

3. **Alpha Particles**: Alpha particles consist of two protons and two neutrons, giving them a positive charge of \( +2e \). Being charged, they will also be deflected by the magnetic field. Therefore, this option cannot be the answer.

4. **Neutrons**: Neutrons have no charge (\( Q = 0 \)). Since the charge is zero, the force acting on neutrons in a magnetic field will also be zero (\( F = 0 \)). Therefore, neutrons will not be deflected by the magnetic field.

Based on this analysis, the particle that cannot be deflected by a magnetic field while in motion is:

**Answer: Neutrons**