Explain Faraday law of electromagnetic induction with experiment

Question

State Faraday’s laws of electromagnetic induction. Describe an experiment to demonstrate these laws.

Solution — Step by Step

Faraday’s First Law: Whenever the magnetic flux linked with a circuit changes, an electromotive force (EMF) is induced in the circuit. This induced EMF persists as long as the change in flux continues — it stops when the flux stops changing.

In simple terms: a changing magnetic field creates an electric current (or voltage). A steady magnetic field, no matter how strong, does NOT induce EMF.

Faraday’s Second Law: The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linked with the circuit.

\varepsilon = -\frac{d\Phi_B}{dt}

Where:

$\varepsilon$ = induced EMF (Volts)
$\Phi_B$ = magnetic flux = $B \cdot A \cdot \cos\theta$ (Weber)
The negative sign comes from Lenz’s law (induced EMF opposes the change in flux)

For $N$ turns in a coil:

\varepsilon = -N\frac{d\Phi_B}{dt}

Faster change in flux → larger induced EMF.

Apparatus: A coil (primary) connected to a battery and switch, placed near a second coil (secondary) connected to a galvanometer.

Observations:

When the switch is closed (current increases from 0 to steady value): galvanometer deflects momentarily — induced EMF in secondary coil.
When current is steady: galvanometer shows no deflection — no change in flux, no induced EMF.
When the switch is opened (current decreases to 0): galvanometer deflects in the opposite direction — the decreasing current (and flux) induces an EMF that opposes the decrease.

This demonstrated that it’s the change in current/flux that matters, not the current itself.

Simpler experiment:

Hold a bar magnet near (but not in) a coil connected to a galvanometer
Push magnet into the coil: galvanometer deflects one way — flux increases, EMF induced
Hold magnet stationary inside coil: galvanometer reads zero — flux constant, no EMF
Pull magnet out of coil: galvanometer deflects the other way — flux decreases, EMF induced in opposite direction
Push magnet in faster: galvanometer deflects more — higher rate of flux change = larger EMF

This directly demonstrates both laws: change in flux causes EMF, and faster change causes larger EMF.

Why This Works

Faraday’s law reveals a fundamental connection between electricity and magnetism. Changing magnetic flux creates an electric field — even in empty space. This is not intuitive, but it’s a foundation of Maxwell’s equations and explains everything from generators to transformers to wireless charging.

The flux $\Phi_B = BA\cos\theta$ can change three ways:

Changing $B$ (field strength changes — moving magnets, changing current)
Changing $A$ (area of coil changes — like a compressing spring)
Changing $\theta$ (coil rotates — this is how electric generators work)

For CBSE Class 12 and JEE, always remember the formula $\varepsilon = -N\frac{d\Phi_B}{dt}$ and the negative sign (Lenz’s law). In numerical problems, if they ask for “magnitude of induced EMF,” use $|\varepsilon| = N\frac{\Delta\Phi}{\Delta t}$ . The sign just indicates direction.

Common Mistake

A very common conceptual error is writing: “EMF is induced when current flows through the circuit.” No — EMF is induced when flux changes. Flux can change due to a moving magnet, changing external field, rotating coil, etc. The current (if any) flows as a result of the induced EMF — it’s the effect, not the cause. Also: flux is $\Phi = BA\cos\theta$ , not just $BA$ . If the coil is perpendicular to the field ( $\theta = 90°$ ), flux is zero even though B and A are both large.

Question

Solution — Step by Step

Why This Works

Common Mistake

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