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Displacement Current

Mediumphysics

A parallel-plate capacitor is being charged so that the electric flux through the region between its plates increases at a steady rate of 6.0 \times 10^{9}\ \text{V·m/s}. What displacement current flows in the gap?

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About This Question

Subject
physics
Chapter
electromagnetic waves
Topic
displacement current
Difficulty
Medium
Year
2025
Tags
displacement currentelectric flux ratepermittivity of free spacecapacitor chargingcurrent continuity

Solution

Correct Answer:

53 mA

The displacement current that Maxwell added is not carried by moving charges but by the rate of change of electric flux through a surface, written as i_d = \varepsilon_0 \frac{d\Phi_E}{dt}. In the insulating gap of a charging capacitor this quantity exactly matches the conduction current in the connecting wires, which is why the magnetic field stays continuous. Substituting \varepsilon_0 = 8.85 \times 10^{-12}\ \text{C}^2/\text{N·m}^2 and d\Phi_E/dt = 6.0 \times 10^{9}\ \text{V·m/s} gives i_d = 8.85 \times 10^{-12} \times 6.0 \times 10^{9} = 5.31 \times 10^{-2}\ \text{A}, which is about 53 mA. The value 5.3 mA is ten times too small and would follow from a flux rate of 6 \times 10^{8}. The value 0.53 A overstates the result by a factor of ten. The 530 mA option corresponds to an even larger arithmetic slip in the exponent. This calculation follows directly from the Ampere-Maxwell formulation in the NCERT Electromagnetic Waves chapter, and it shows that even a purely field-based quantity can be expressed cleanly in amperes. The same displacement current would also equal the conduction current charging the plates, so one could verify it independently from the rate at which charge accumulates if that data were given. As a check, multiplying a permittivity (\sim10^{-11}) by a flux rate (\sim10^{9}) should land near 10^{-2}\ \text{A}, consistent with the 53 mA answer.

This medium difficulty physics question is from the chapter electromagnetic waves, covering the topic of displacement current. It appeared in the 2025 exam.

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