Class 12 Physics · NCERT notes for NEET

Electromagnetic Induction: NCERT Notes, Formulas and Tricks for NEET

How a changing magnetic flux produces an emf: Faraday's and Lenz's laws, motional emf, eddy currents, the AC generator, and self and mutual inductance. It is a short, formula-rich chapter, and NEET usually asks one or two direct questions from it, often a numerical on flux change, a moving rod or inductance.

• 1. Faraday's and Lenz's laws
• 2. Motional EMF and eddy currents 🔒
• 3. Self and mutual inductance 🔒

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Topic 1

Faraday's and Lenz's laws

Tap your metro card or charge your phone on a wireless pad: no wire touches anything, yet a current flows. A changing magnetic flux is doing all the work.

• Faraday and Henry found that an emf is induced in a coil whenever the magnetic flux through it changes: move a magnet near a coil, move a current-carrying coil, or switch the current in a nearby coil on or off.
• No change, no emf: a magnet held still inside a coil or a steady current in the nearby coil gives zero galvanometer deflection. An iron rod inside the coils makes the deflection larger.
• Magnetic flux ${\Phi }_B=\vec{B}\cdot \vec{A}=BA\cos \theta$, where $\theta$ is the angle between $\vec{B}$ and the normal to the area. Scalar; SI unit weber, $1\text{Wb}=1{\text{T m}}^2$.
• Flux can change in three ways: change $B$, change the area $A$, or change the angle $\theta$ (rotate the coil).
• Faraday's law: $\epsilon =-N\frac{d{\Phi }_B}{dt}$. The emf lasts only while the flux is changing. $N{\Phi }_B$ is called the flux linkage.
• Induced current $I=\epsilon /R$, and the charge that flows is $q=N\Delta {\Phi }_B/R$: it depends only on the total flux change, not on how fast it happens.
• Lenz's law: the induced current opposes the change in flux that produces it. The minus sign in Faraday's law expresses this.
• N pole approaching a coil: the near face becomes N (anticlockwise current seen from the magnet) and repels it. N pole receding: the near face becomes S and attracts it.
• Lenz's law is energy conservation: you must do work against the opposing force, and that work appears as electrical energy (heat in the coil). If the induced current helped the change, energy would come from nothing.
• An open coil still has an induced emf; only the current is zero. NCERT example: a loop of area $2{\text{cm}}^2$ inside a solenoid of 15 turns/cm, current changed by 2 A in 0.1 s, gives $\epsilon \approx 7.5\times 10^{-6}$ V.

Formulas

• ${\Phi }_B=BA\cos \theta$
  Uniform $B$, flat area; $\theta$ between $\vec{B}$ and the normal
• $\epsilon =-N\frac{d{\Phi }_B}{dt}$
  Faraday's law for $N$ turns; minus sign = Lenz's law
• $I=\frac{|\epsilon |}{R}$
  Induced current in a closed coil of resistance $R$
• $q=\frac{N\Delta {\Phi }_B}{R}$
  Charge that flows; independent of time taken
• ${\epsilon }_{avg}=\frac{N|{\Phi }_2-{\Phi }_1|}{\Delta t}$
  Average emf for a flux change in time $\Delta t$

Tips, tricks and hacks

Common mistakes

NEET traps

Weber is the unit of flux, tesla (${\text{Wb m}}^{-2}$) is the unit of $B$. Options often swap them; the dimensions of flux are $[{\text{M L}}^2{\text{T}}^{-2}{\text{A}}^{-1}]$.

"The induced charge depends on the rate of change of flux": FALSE. Emf and current depend on the rate; charge does not.

A ring dropped over a vertical magnet, or a magnet dropped through a ring: it is opposed both while entering and while leaving, so its acceleration is less than $g$ on both sides.

Practise Electromagnetic Induction questions

Topic-wise practice from Level 1 to Level 3, NCERT-based questions and NEET PYQs, with a full solution after every answer.