Which of the following statements is correct?

Options

A. If the moving charged particle enters into a region of magnetic field form outside, it does not complete a circular path.
B. If a moving charged particle traces a helical path in a uniform magnetic field, the axis of the helix is parallel to the magnetic field.
C. The power associated with the force exerted by a magnetic field on a moving charged particle moving charged particle is always equal to zero.
D. If in a region a uniform magnetic field and a uniform electric field both exist, a charged particle moving in this region cannot trace a circular path.

Explanation

To determine which of the statements is correct, let's analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** If the moving charged particle enters into a region of magnetic field from outside, it does not complete a circular path.

**Explanation:** When a charged particle enters a magnetic field perpendicularly, it experiences a magnetic force that acts perpendicular to its velocity. If it enters from outside, it will not complete a full circular path; instead, it will trace a half-circle before exiting the field. Therefore, this statement is correct.

### Step 2: Analyze the second statement
**Statement:** If a moving charged particle traces a helical path in a uniform magnetic field, the axis of the helix is parallel to the magnetic field.

**Explanation:** In a uniform magnetic field, a charged particle can move in a helical path if it has a component of velocity parallel to the magnetic field. The axis of the helix is indeed parallel to the magnetic field direction. Thus, this statement is also correct.

### Step 3: Analyze the third statement
**Statement:** The power associated with the force exerted by the magnetic field on a moving charged particle is always equal to zero.

**Explanation:** The magnetic force on a charged particle is always perpendicular to its velocity. The power (P) is given by the dot product of force (F) and velocity (V): P = F · V. Since the angle between F and V is 90 degrees, the power is zero (cos 90° = 0). Therefore, this statement is correct.

### Step 4: Analyze the fourth statement
**Statement:** If in a region, a uniform magnetic field and a uniform electric field both exist, a charged particle moving in this region cannot trace a circular path.

**Explanation:** For a charged particle to trace a circular path, it must experience a net centripetal force, which can be provided by the magnetic field. However, if an electric field is also present, it exerts a force on the charged particle that can alter its trajectory, preventing it from tracing a circular path. Therefore, this statement is correct.

### Conclusion
All four statements are correct based on the analysis above.

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