CBSE Class 12 Physics 2025: question 24

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1. (a) Faraday's law: the magnitude of the emf induced in a circuit equals the time rate of change of magnetic flux linked with it: $\epsilon =-\frac{d{\Phi }_B}{dt}$ (for N turns, $\epsilon =-N\frac{d{\Phi }_B}{dt}$).

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2. Role of the negative sign (Lenz's law): the induced emf, and hence the induced current, is in a direction that opposes the change in flux that produces it.

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3. (b) Example: push the north pole of a bar magnet towards a closed coil. The induced current makes the near face of the coil a north pole, which repels the magnet. When the magnet is pulled away, that face becomes a south pole and attracts it. In both cases, work must be done against this opposing force. That mechanical work appears as electrical energy (heat in the coil). If the induced current aided the motion, energy would be created from nothing, violating conservation of energy. Hence Lenz's law is consistent with energy conservation.

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Answer: ε = −dΦ/dt; the minus sign shows the induced emf opposes the change in flux. Work done against the opposition (e.g. magnet approaching a coil) is converted into electrical energy, as energy conservation requires.

Revise the chapter

• Chapter: Electromagnetic Induction (notes, formulas, where marks are lost and every past question)