Assertion : The kinetic energy does not change when a particle moves in uniform magnetic field.
Reason : The velocity of the particle is not affected by magnetic field.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

To solve the problem, we need to analyze both the assertion and the reason provided in the question.

### Step 1: Understand the Assertion
The assertion states that "The kinetic energy does not change when a particle moves in a uniform magnetic field."

- When a charged particle moves in a magnetic field, it experiences a magnetic force given by the formula:
\[ \mathbf{F} = q(\mathbf{v} \times \mathbf{B}) \]
where \( q \) is the charge, \( \mathbf{v} \) is the velocity vector, and \( \mathbf{B} \) is the magnetic field vector.

- This magnetic force is always perpendicular to the velocity of the particle. Since the force does not do any work (work done is given by \( W = \mathbf{F} \cdot \mathbf{d} \) and here \( \mathbf{F} \) is perpendicular to \( \mathbf{d} \)), the speed of the particle remains constant.

- Since kinetic energy (\( KE \)) is given by:
\[ KE = \frac{1}{2} mv^2 \]
and the speed \( v \) remains constant, the kinetic energy also remains constant.

### Conclusion for Step 1
The assertion is correct: The kinetic energy does not change when a particle moves in a uniform magnetic field.

### Step 2: Understand the Reason
The reason states that "The velocity of the particle is not affected by the magnetic field."

- While it is true that the speed (magnitude of velocity) remains constant, the direction of the velocity does change due to the magnetic force acting on the particle. The magnetic force acts as a centripetal force, causing the particle to move in a circular path.

- Therefore, while the magnitude of the velocity remains constant, the direction of the velocity is indeed affected by the magnetic field.

### Conclusion for Step 2
The reason is incorrect: The velocity of the particle does change because its direction changes, even though its speed remains constant.

### Final Conclusion
- The assertion is correct, but the reason is incorrect. Therefore, the correct option is that the assertion is true, but the reason is false.