When a charged particle enters a uniform magnetic field its kinetic energy

Options

A. remains constant
B. increases
C. decreases
D. becomes zero

Answer

To solve the question regarding the behavior of a charged particle when it enters a uniform magnetic field, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Motion of Charged Particles in a Magnetic Field**:
- When a charged particle (like an electron or proton) enters a magnetic field, it experiences a magnetic force due to its charge and velocity.

2. **Direction of the Magnetic Force**:
- The magnetic force acts perpendicular to the velocity of the charged particle. This is described by the right-hand rule and is given by the equation \( \mathbf{F} = q(\mathbf{v} \times \mathbf{B}) \), where \( q \) is the charge, \( \mathbf{v} \) is the velocity vector, and \( \mathbf{B} \) is the magnetic field vector.

3. **Effect of Perpendicular Force on Kinetic Energy**:
- Since the magnetic force is always perpendicular to the velocity, it does not do any work on the particle. Work done is defined as \( W = \mathbf{F} \cdot \mathbf{d} \), and since the force is perpendicular to the displacement, the work done is zero.

4. **Kinetic Energy Calculation**:
- The kinetic energy (KE) of a particle is given by the formula \( KE = \frac{1}{2} mv^2 \), where \( m \) is the mass and \( v \) is the speed of the particle.
- Since the magnetic force does not change the speed of the particle (only its direction), the magnitude of the velocity remains constant.

5. **Conclusion**:
- As the speed of the particle remains constant, the kinetic energy also remains constant. Therefore, the correct answer to the question is that the kinetic energy of the charged particle remains constant when it enters a uniform magnetic field.

### Final Answer:
The kinetic energy of the charged particle remains constant.