In the circuit shown below- a capacitor is being used in an AC circuit. A magnetic needle shows deflection, both at point P and Q. Which of the options among the following correctly specifies the reason for the presence of magnetic field at both the points?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Flow of charges in the wire and varying electric field between capacitor plates A and B.
By the Ampere-Maxwell law, a magnetic field is produced by a conduction current (flow of charges) and also by a displacement current, which is due to a changing electric field. P is on the wire, where charges flow; Q is between the plates, where no charge flows but the electric field changes as the capacitor charges and discharges.
Option (1) Due to conduction current and displacement current at Q and P respectively. — Incorrect. The two are interchanged: the conduction current is at P and the displacement current at Q.
Option (2) Flow of charges in the wire and constant electric field between the plates A and B. — Incorrect. With an ac source the field between the plates keeps changing; a constant field gives no displacement current.
Option (4) At Q the magnetic needle will show deflection due to magnetic field of the earth and at P due to the current flowing in the wire. — Incorrect. The earth's field is present everywhere and cannot explain the deflection at Q.
NCERT (2026-27):
Displacement Current – “… The source of a magnetic field is not just the conduction electric current due to flowing charges, but also the time rate of change of electric field. … In explicit terms, this means that outside the capacitor plates, we have only conduction current ic = i, and no displacement current, i.e., id = 0. On the other hand, inside the capacitor, there is no conduction current, i.e., ic = 0, and there is only displacement current, so that id = i.”