A positively charged particle is moving along the positive `X`-axis.You want to apply a magnetic field for a short time so that the particle may reverse its direction and move parallel to the negative `X`-axis.This can be done be applying the magnetic field along.

Options

A. `y`-axis
B. `z`-axis
C. `y`-axis only
D. `z`-axis only

Answer

To solve the problem of reversing the direction of a positively charged particle moving along the positive X-axis so that it moves parallel to the negative X-axis, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Motion of the Charged Particle:**
- The positively charged particle is initially moving in the positive X direction. We need to apply a magnetic field to change its direction to the negative X direction.

2. **Using the Right-Hand Rule:**
- The force acting on a charged particle moving in a magnetic field is given by the Lorentz force law:
\[ \vec{F} = q(\vec{v} \times \vec{B}) \]
where \( q \) is the charge, \( \vec{v} \) is the velocity vector, and \( \vec{B} \) is the magnetic field vector.

3. **Determining the Required Force Direction:**
- To reverse the direction of the particle from positive X to negative X, the magnetic force must act in the negative X direction.

4. **Choosing the Direction of the Magnetic Field:**
- If we apply the magnetic field in the positive Y direction (\( \vec{B} \) along +Y), we can use the right-hand rule:
- Point your fingers in the direction of the velocity (positive X).
- Curl your fingers in the direction of the magnetic field (positive Y).
- Your thumb will point in the direction of the force, which will be in the negative Z direction.
- This force will cause the particle to move in the negative Z direction, eventually allowing it to turn and move parallel to the negative X direction.

5. **Verifying Other Directions:**
- If we apply the magnetic field in the negative Y direction (\( \vec{B} \) along -Y), the force will act in the positive Z direction, which will also allow the particle to turn and move parallel to the negative X direction.
- If we apply the magnetic field in the Z direction (either positive or negative), the force will act in the Y direction, which will not help in reversing the direction of the particle effectively.

### Conclusion:
To reverse the direction of the positively charged particle moving along the positive X-axis to move parallel to the negative X-axis, the magnetic field should be applied along the Y-axis (either positive or negative).