A parallel plate capacitor with circular plates is being charged. The…

The correct answer is Option (3) → along circles in planes parallel to the plates, centred on the axis of the plates

While the capacitor charges, no charge crosses the gap, but the electric field there grows. This changing field acts as a displacement current \( i_d=\varepsilon_0\,d\Phi_E/dt \) across the gap, along the axis. Like a conduction current, it is a source of magnetic field.

Option (1) perpendicular to the plates, along the electric field — Incorrect. The displacement current points along the electric field. A current produces a magnetic field that goes around it, so \( \vec{B} \) is perpendicular to \( \vec{E} \).

Option (2) nowhere, because the magnetic field between the plates is zero — Incorrect. There is no conduction current in the gap. But the displacement current there is equal to the current in the wires, so a magnetic field is present between the plates.

Option (3) along circles in planes parallel to the plates, centred on the axis of the plates — Correct. By symmetry, the field lines around the axial displacement current are circles parallel to the plates. They are like the field lines around a straight wire. So \( \vec{B} \) is perpendicular to \( \vec{E} \) at every point.

Option (4) radially outward from the axis of the plates, parallel to the plates — Incorrect. A radial field would point away from the current. The magnetic field of a current is always along circles drawn around it.

NCERT (2026-27):
Nature of Electromagnetic Waves – “… The electric field inside the plates of the capacitor is directed perpendicular to the plates. The magnetic field this gives rise to via the displacement current is along the perimeter of a circle parallel to the capacitor plates. So B and E are perpendicular in this case. This is a general feature.”