Observer A and a point charge Q are both at rest in an inertial reference frame F1. Observer B is at rest in a different inertial frame F2, which moves with constant velocity relative to F1. Which of the following statements is correct?

Both A and B will observe electric fields.
Both A and B will observe magnetic fields.
Neither A nor B will observe magnetic fields.
B will observe a magnetic field, but A will not.

Correct answer: B will observe a magnetic field, but A will not.

Solution

Observer A sees the charge at rest, so only an electrostatic field exists in F1. Observer B sees the charge moving (relative to F2), which is equivalent to a current and therefore produces a magnetic field in addition to an electric field.

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