Assertion If a charged particle enters from outside in uniform magnetic field, then it will keep on rotating in a circular path.
Reson Magnetic force is always perpendicular to velocity vector.

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 correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If Assertion is false but Reason is true.

Answer

To solve the question, we need to analyze the assertion and the reason provided:

**Assertion:** If a charged particle enters from outside in a uniform magnetic field, then it will keep on rotating in a circular path.
**Reason:** Magnetic force is always perpendicular to the velocity vector.

### Step-by-Step Solution:

1. **Understanding the Magnetic Force:**
The magnetic force acting on a charged particle moving in a magnetic field is given by the Lorentz force equation:
\[ \mathbf{F} = q (\mathbf{v} \times \mathbf{B}) \]
where \( q \) is the charge of the particle, \( \mathbf{v} \) is the velocity vector, and \( \mathbf{B} \) is the magnetic field vector.

2. **Analyzing the Direction of Forces:**
The magnetic force is always perpendicular to both the velocity vector (\( \mathbf{v} \)) and the magnetic field vector (\( \mathbf{B} \)). This means that the magnetic force does not do work on the charged particle, as work is defined as the force component in the direction of displacement.

3. **Condition for Circular Motion:**
For a charged particle to move in a circular path, the velocity vector must be perpendicular to the magnetic field vector. This condition ensures that the magnetic force provides the necessary centripetal force to keep the particle in circular motion.

4. **Considering Different Scenarios:**
- **Case 1:** If the velocity is perpendicular to the magnetic field, the particle will move in a circular path.
- **Case 2:** If the velocity is parallel to the magnetic field, the magnetic force will be zero, and the particle will continue in a straight line.
- **Case 3:** If the velocity makes an angle with the magnetic field, the particle will follow a helical path, combining circular motion and linear motion.

5. **Conclusion on the Assertion:**
Since the assertion states that a charged particle will keep on rotating in a circular path regardless of the angle of entry, this is not true. The particle will only rotate in a circular path if it enters perpendicular to the magnetic field. Therefore, the assertion is **false**.

6. **Conclusion on the Reason:**
The reason states that the magnetic force is always perpendicular to the velocity vector, which is true. This is a fundamental property of the magnetic force acting on a charged particle.

### Final Answer:
- The assertion is **false**.
- The reason is **true**.