CBSE Class 12 Physics 2025: question 25

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Step-by-step solution

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1. (a) Similarity: both conduction current and displacement current produce a magnetic field (both appear as sources in the Ampere–Maxwell law).

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2. Dissimilarity: conduction current is due to the actual flow of charges in a conductor. Displacement current, $I_d={\epsilon }_0\frac{d{\Phi }_E}{dt}$, arises from a time-varying electric flux, even in vacuum (e.g. between capacitor plates), with no flow of charge.

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3. (b) By Faraday's law, a time-varying magnetic field produces an electric field. By the Ampere–Maxwell law, $\oint \vec{B}\cdot d\vec{l}={\mu }_0{I}_c+{\mu }_0{\epsilon }_0\frac{d{\Phi }_E}{dt}$, a time-varying electric field (displacement current) produces a magnetic field. In free space $I_c=0$, so the changing E and B fields keep regenerating each other. The result is a self-sustaining disturbance of mutually perpendicular E and B fields that propagates as an electromagnetic wave with speed $c=1/\sqrt{{\mu }_0{\epsilon }_0}$.

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Answer: Both produce magnetic fields. Conduction current is a flow of charges; displacement current comes from changing electric flux. Changing E and B fields generate each other via displacement current, so EM waves propagate in free space.

Revise the chapter

• Chapter: Electromagnetic Waves (notes, formulas, where marks are lost and every past question)
• Topic: Displacement Current