An electron is moving in electron field and magnetic field it will gain energy from

Options

A. Electric field
B. Magnetic field
C. Both of these
D. None of these

Answer

To determine in which field an electron will gain energy while moving, we need to analyze the effects of both electric and magnetic fields on the electron.

### Step-by-Step Solution:

1. **Understanding the Electric Field:**
- When an electron (which has a charge of -e) moves in an electric field (E), it experiences an electrostatic force given by:
\[ F_E = Q \cdot E = -e \cdot E \]
- The negative sign indicates that the force acts in the opposite direction to the electric field.

2. **Acceleration of the Electron:**
- The acceleration (a) of the electron can be calculated using Newton's second law:
\[ a = \frac{F_E}{m_e} = \frac{-e \cdot E}{m_e} \]
- Since the force is in the same direction as the velocity of the electron (due to the negative charge), the electron will accelerate in the direction of the electric field.

3. **Change in Velocity:**
- The velocity of the electron will increase as it accelerates:
\[ V' = V_0 + a \cdot t \]
- This increase in velocity indicates that the kinetic energy of the electron will also increase.

4. **Kinetic Energy in Electric Field:**
- The kinetic energy (KE) of the electron is given by:
\[ KE = \frac{1}{2} m_e V^2 \]
- As the velocity increases, the kinetic energy will also increase.

5. **Understanding the Magnetic Field:**
- When the electron moves in a magnetic field (B), it experiences a magnetic force given by:
\[ F_B = Q \cdot (V \times B) = -e \cdot (V \times B) \]
- The direction of this force is always perpendicular to both the velocity of the electron and the magnetic field.

6. **Effect of Magnetic Force:**
- Since the magnetic force is always perpendicular to the velocity, it does not do work on the electron. Therefore, the speed (and thus the kinetic energy) of the electron remains constant:
\[ KE = \frac{1}{2} m_e V^2 \quad \text{(constant)} \]

7. **Conclusion:**
- In the electric field, the electron gains energy due to the work done by the electric force, leading to an increase in kinetic energy.
- In the magnetic field, the electron does not gain energy because the magnetic force does not change its speed.

### Final Answer:
The electron will gain energy from the **electric field**.

---