A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the same direction then the electron

Options

A. speed will increase
B. will turn towards left of direction of motion
C. will turn towards right of direction of motion
D. speed will decrease

Explanation

To solve the problem, we need to analyze the forces acting on the electron when it is projected in the presence of both a uniform electric field and a uniform magnetic field, which are both acting in the same direction.

### Step-by-Step Solution:

1. **Identify the Forces**:
- The electric field \( E \) exerts a force on the electron given by \( F_E = qE \), where \( q \) is the charge of the electron (which is negative).
- The magnetic field \( B \) exerts a force on the electron given by the Lorentz force equation \( F_B = q(\mathbf{v} \times \mathbf{B}) \), where \( \mathbf{v} \) is the velocity of the electron.

2. **Direction of Forces**:
- Since the electric field and magnetic field are in the same direction, the force due to the electric field \( F_E \) will act in the same direction as the electric field.
- For the magnetic force, since the electron has a negative charge, the direction of the magnetic force will be opposite to the direction of the velocity. The cross product \( \mathbf{v} \times \mathbf{B} \) will yield a force that acts perpendicular to both \( \mathbf{v} \) and \( \mathbf{B} \). However, since both fields are in the same direction, the angle \( \theta \) between \( \mathbf{v} \) and \( \mathbf{B} \) is 0 degrees, leading to \( F_B = 0 \).

3. **Net Force on the Electron**:
- The net force acting on the electron is therefore only due to the electric field: \( F_{net} = F_E + F_B = qE + 0 = qE \).
- Since \( q \) is negative (for an electron), this force will act in the opposite direction to the electric field.

4. **Effect on the Electron's Motion**:
- The net force acting on the electron will cause it to accelerate in the opposite direction to the electric field.
- As a result, the speed of the electron will decrease because the force is acting against its motion.

5. **Conclusion**:
- Therefore, the correct answer is that the speed of the electron will decrease.

### Final Answer:
The speed of the electron will decrease.