Given below are two statements : One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In an uniform magnetic field, speed and energy remains the same for a moving charged particle.
Reason (R) : Moving charged particle experiences magnetic force perpendicular to its direction of motion.

Options

A. Both (A) and (R) are true and (R) is the correct explanation of (A).
B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
C. (A) is true but (R) is false.
D. (A) is false but (R) is true.

Answer

To solve the given question, we need to analyze both the Assertion (A) and the Reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that in a uniform magnetic field, the speed and energy of a moving charged particle remain the same.
- This is true because a charged particle moving in a magnetic field experiences a magnetic force that acts perpendicular to its velocity. This force does not do work on the particle, hence the speed (and thus the kinetic energy) remains constant.

2. **Understanding the Reason (R)**:
- The reason states that a moving charged particle experiences a magnetic force that is perpendicular to its direction of motion.
- This is also true. The magnetic force on a charged particle is given by the formula \( F = q(\mathbf{v} \times \mathbf{B}) \), where \( q \) is the charge, \( \mathbf{v} \) is the velocity vector, and \( \mathbf{B} \) is the magnetic field vector. The cross product ensures that the force is perpendicular to both the velocity and the magnetic field.

3. **Analyzing the Relationship**:
- The reason explains why the assertion is true. Since the magnetic force is always perpendicular to the direction of motion, it does not change the speed of the particle, thus maintaining its energy (kinetic energy).

4. **Conclusion**:
- Both the assertion and the reason are true, and the reason provides a valid explanation for the assertion. Therefore, the correct answer is that both statements are true, and the reason explains the assertion.

### Final Answer:
Both Assertion (A) and Reason (R) are true, and the reason explains the assertion.

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