An electron is moving along the positive x-axis. You want to apply a magnetic field for a short time so that the electron may reverse its direction and move parallel to the nagative x-axis. This can be done by applying the magnetic field along

Options

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

Explanation

To solve the problem of reversing the direction of an electron moving along the positive x-axis to move parallel to the negative x-axis using a magnetic field, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Motion of the Electron**:
The electron is initially moving along the positive x-axis. We denote this direction as the positive x-direction.

2. **Identify the Required Change in Direction**:
To reverse the direction of the electron, we want it to move along the negative x-axis.

3. **Use the Right-Hand Rule for Magnetic Force**:
The force on a charged particle moving in a magnetic field is given by the equation:
\[ \vec{F} = q (\vec{v} \times \vec{B}) \]
where \( \vec{F} \) is the magnetic force, \( q \) is the charge of the particle, \( \vec{v} \) is the velocity vector, and \( \vec{B} \) is the magnetic field vector.

4. **Determine the Direction of the Magnetic Field**:
Since the electron has a negative charge (\( q < 0 \)), the direction of the force will be opposite to that given by the right-hand rule.

5. **Apply the Magnetic Field in the Y-Direction**:
If we apply the magnetic field in the positive y-direction (\( \vec{B} \) along +y), and the velocity of the electron is along the positive x-axis (\( \vec{v} \) along +x), we can use the right-hand rule:
- Point your fingers in the direction of \( \vec{v} \) (positive x).
- Curl them towards \( \vec{B} \) (positive y).
- Your thumb will point in the direction of the force on a positive charge, which is in the positive z-direction. However, since the electron is negatively charged, the force will be in the negative z-direction.

6. **Apply the Magnetic Field in the Z-Direction**:
If we apply the magnetic field in the positive z-direction (\( \vec{B} \) along +z), and the velocity of the electron is still along the positive x-axis, we apply the right-hand rule again:
- Point your fingers in the direction of \( \vec{v} \) (positive x).
- Curl them towards \( \vec{B} \) (positive z).
- Your thumb will point in the direction of the force on a positive charge, which is in the positive y-direction. Since the electron is negatively charged, the force will be in the negative y-direction.

7. **Conclusion**:
Therefore, applying the magnetic field in either the positive y-direction or the positive z-direction will result in a force that can reverse the direction of the electron, allowing it to move along the negative x-axis.

### Final Answer:
The magnetic field can be applied along the positive y-axis or the positive z-axis.