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Exam level3 past questions

Faraday's Law and Lenz's Law

Faraday's law explains how a changing magnetic field induces an electromotive force (EMF), while Lenz's law determines the direction of the induced current to oppose the change causing it.

Why this shows up in the exam

NEET tests your understanding of how EMF is induced and the direction of induced currents in various setups.

How NEET tests this

Numerical · 4 QsStatement analysis

Learn the idea

A changing magnetic flux through a circuit produces an emf equal to the negative rate of change of that flux; the negative sign (Lenz’s law) tells us the induced current always tries to oppose the change.

🧠 Memory hook: Think of Faraday’s F as a ‘Fall’: the emf falls opposite to the rise of flux – F = – dΦ/dt.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Faraday’s law: ε = – dΦ/dt (for a single turn)
  • For N turns: ε = – N dΦ/dt
  • Magnetic flux Φ = B·A·cosθ, unit = Weber (Wb)
  • Lenz’s law gives the direction of the induced current – opposite to the change in flux
  • Magnitude of emf is |dΦ/dt|, unit = volt (V)

How to approach it

  1. 1Write the given flux Φ as a function of time
  2. 2Differentiate Φ with respect to t to obtain dΦ/dt
  3. 3Apply ε = – N dΦ/dt (use N=1 unless stated otherwise)
  4. 4If the question asks for magnitude, take the absolute value of the result

Worked example — watch it click

The magnetic flux linked with a coil, in webers, is given by the equation φ = 3t² + 4t + 9. Then the magnitude of induced e.m.f. at t = 2 second will be:

  • A)2 volt
  • B)4 volt
  • C)8 volt
  • ✅16 volt

The concept behind this problem

The problem requires converting the time‑dependent flux into its rate of change and then using Faraday’s law; it checks that you can differentiate the flux expression and remember the magnitude rule.

Step by step

  1. 1Induced emf = -dφ/dt.
  2. 2Given φ = 3t² + 4t + 9.
  3. 3Therefore dφ/dt = 6t + 4.
  4. 4At t = 2 s: dφ/dt = 6(2) + 4 = 16.
  5. 5Magnitude of induced emf = |−16| = 16 volt.

Watch out

Students often mis‑differentiate 3t² as 3t, leading to a wrong emf value.

Common slip-ups that cost marks

  • •Missing the negative sign and confusing “value” with “magnitude”
  • •Ignoring the number of turns N when it is given
  • •Differentiating the flux incorrectly (e.g., treating 3t² as 3t)

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3

The magnetic flux linked with a coil, in webers, is given by the equation φ = 3t² + 4t + 9. Then the magnitude of induced e.m.f. at t = 2 second will be:

Push further

More challenging

5 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 5

Consider a metallic ring falling freely under gravity towards a region with a uniform magnetic field perpendicular to the plane of the ring. As the ring enters the magnetic field, an induced current flows. Which statement accurately describes the forces acting on the ring?

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