| Column 1 | Column 2 | Column 3 | |||
|---|---|---|---|---|---|
| (I) | Electron with | (i) | (P) | ||
| (II) | Electron with | (ii) | (Q) | ||
| (III) | Proton with | (iii) | (R) | ||
| (IV) | Proton with | (iv) | (S) | ||
- A
- B
- C
- D
View written solutionFree
Correct answer: A
Step-by-step Solution:
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Analyze the condition for a helical path: A charged particle follows a helical path when it moves in a uniform magnetic field, and its velocity vector has components both parallel () and perpendicular () to the magnetic field (). The axis of the helix is always parallel to the direction of the magnetic field vector .
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Identify the required direction of the magnetic field: The question states that the axis of the helical path must be along the positive z-direction. This implies that the magnetic field must be directed along the z-axis. Looking at the options in Column 3:
- (P) (along negative x-axis)
- (Q) (along positive x-axis)
- (R) (along positive y-axis)
- (S) (along positive z-axis) Only option (S) has the magnetic field along the z-axis. Therefore, the correct combination must include (S) from Column 3.
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Evaluate the given options: Now, let's check which of the multiple-choice options includes (S) from Column 3.
- A: - This option includes (S).
- B: - This option includes (R), so the axis would be along the y-direction. Incorrect.
- C: - This option includes (P), so the axis would be along the x-direction. Incorrect.
- D: - This option includes (R), so the axis would be along the y-direction. Incorrect.
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Conclusion from elimination: Based on the direction of the magnetic field, only option A is plausible. Let's verify that this combination indeed results in the described motion.
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Detailed analysis of Option A: (IV)(i)(S):
- Particle and Initial Velocity (IV): Proton with charge and initial velocity . The initial velocity is purely in the xy-plane (perpendicular to the z-axis).
- Electric Field (i): . The electric field is along the positive z-axis.
- Magnetic Field (S): . The magnetic field is along the positive z-axis.
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Calculate the Lorentz Force: The total force on the proton is given by the Lorentz force law:
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Analyze the components of motion:
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Motion parallel to B (z-direction): The electric field is parallel to the magnetic field . The electric force on the proton is . This force is constant and directed along the positive z-axis. The magnetic force, , is always perpendicular to , so it has no z-component. Therefore, the net force in the z-direction is . This causes a constant acceleration in the positive z-direction. Since the proton starts at the origin with , it will move along the positive z-axis.
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Motion perpendicular to B (xy-plane): The initial velocity of the proton, , is entirely perpendicular to . The magnetic force component in the xy-plane, , acts as a centripetal force, causing the proton to execute circular motion in the xy-plane. The electric field has no component in this plane.
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Combine the motions: The resulting trajectory is a superposition of circular motion in the xy-plane and accelerated linear motion along the positive z-axis. This combined motion describes a helical path whose axis is the z-axis. Since the particle accelerates in the +z direction, it moves along the positive z-axis. The pitch of the helix increases with time, but it is still a helical path.
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Final Conclusion: The combination (IV)(i)(S) correctly describes a particle moving in a helical path with its axis along the positive z-direction. Therefore, option A is the correct answer.
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