Question

A point charge is placed in a moving train. A passenger A sitting in the train and person B on the ground observe the fields due to this charge. Then

• A. A observes both electric and magnetic fields
• B. B observes both electric and magnetic fields
• C. A observes only magnetic field
• D. B observes only electric field

Hint:

Electric and magnetic fields are not absolute entities; they are frame-dependent components of the single electromagnetic field tensor. A field that appears purely electric in its source's rest frame will generally appear to have both electric and magnetic components in any frame moving relative to it.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The manifestation of electromagnetic fields is relative and depends on the state of motion of the observer with respect to the source charge.
Step 3: Detailed Explanation:
1. Observer A (Passenger): Since A is sitting in the same moving train as the point charge, the charge is at rest relative to A. According to electrostatics, a stationary charge produces only an electric field in its local rest frame.
2. Observer B (Person on ground): From the perspective of person B, the train and the charge within it are in motion. A moving point charge constitutes a convective current. According to Maxwell's equations, a moving charge generates a magnetic field in addition to its electric field.
Therefore, observer B will detect both electric and magnetic field components.
Step 4: Final Answer:
The person B on the ground observes both electric and magnetic fields.