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Electromagnetic induction questions usually ask two things: how big is the induced emf, and which way does the current flow? Most students manage the first and guess the second. The guess goes wrong because "opposes" is read as "opposes the field" when it means "opposes the change". This post gives a four-step method for the direction, two worked examples with numbers, and the traps that examiners like.
What do Faraday's and Lenz's laws say?
In the NCERT Class 12 chapter Electromagnetic Induction, Faraday's law gives the induced emf as ε = −dΦ/dt, where Φ is the magnetic flux through the circuit. For N turns, multiply by N. Lenz's law is stated as: "The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it." The minus sign in Faraday's law represents this.
NCERT also explains why it must be so. If the induced current helped the change instead of opposing it, a magnet pushed towards a coil would be pulled in faster and faster, creating energy from nothing. Opposition means you have to do work to move the magnet, and that work turns into heat in the coil. Lenz's law is energy conservation in disguise.
Both laws are in the JEE (Main) 2026 syllabus under Electromagnetic Induction, and the JEE (Advanced) 2026 brochure lists "Faraday's law, Lenz's law". Check the syllabus for your own exam year.
Why do students find induction hard?
Physics education research suggests the difficulty is real and widespread. Zuza and colleagues (2014), designing a teaching sequence on induction, note that research has shown the fundamental concepts of the theory "are barely understood by students", partly because teaching often moves quickly to solving problems "in a more or less rote manner". A rote formula gets you the size of the emf. It does not give you the direction, which needs reasoning about change.
A four-step method for direction
Which way does the existing field point through the loop? Into the page, out of it, left, right.
Is the flux increasing or decreasing? Flux changes if B changes, the area inside the field changes, or the angle between B and the loop changes.
Which way must the induced field point? If flux is increasing, the induced field points opposite to the existing field. If it is decreasing, the induced field points the same way, trying to keep the flux up.
Convert to current. Use the right-hand thumb rule: thumb along the induced field through the loop, fingers curl in the direction of current.
Worked example 1: a field that grows
A square loop of side 10 cm and resistance 0.50 Ω lies in the plane of the page. A uniform field into the page rises steadily from 0.20 T to 0.60 T in 0.20 s. Find the induced current and its direction as seen from the front.
Size. Area A = 0.10 × 0.10 = 0.010 m². Change in flux = (0.60 − 0.20) × 0.010 = 4.0 × 10⁻³ Wb. Emf = 4.0 × 10⁻³ / 0.20 = 0.020 V. Current = 0.020 / 0.50 = 0.040 A, or 40 mA.
Direction. The field is into the page and increasing, so the induced field must point out of the page. With your thumb pointing out of the page, your fingers curl anticlockwise. The current is anticlockwise.
If the same field were instead falling, the induced field would point into the page and the current would be clockwise.
Worked example 2: a sliding rod
A metal rod 0.50 m long slides at 2.0 m/s along two parallel rails joined by a 2.0 Ω resistor, in a 0.40 T field perpendicular to the rails. NCERT derives the motional emf as Blv for this set-up.
Emf = Blv = 0.40 × 0.50 × 2.0 = 0.40 V.
Current = 0.40 / 2.0 = 0.20 A.
Force on the rod = BIl = 0.40 × 0.20 × 0.50 = 0.040 N, opposing the motion, as Lenz's law requires.
Power check: you must push with 0.040 N at 2.0 m/s, which is 0.080 W. Heat in the resistor is I²R = 0.20² × 2.0 = 0.080 W. They match, which is energy conservation again.
For the direction of the force on the rod you need F = IL × B; our post on right-hand-rule mistakes in magnetic force covers that step.
Which mistakes cost the most marks?
Opposing the field instead of the change. Always do Step 2 before Step 3.
Using the total flux instead of the change. A large steady field induces nothing.
Forgetting the number of turns. For a coil, ε = −N dΦ/dt.
Missing area changes. A loop entering or leaving a field region has changing flux even if B is constant. While it is fully inside a uniform field, the flux is steady and the current is zero.
Thinking a magnet falling through a coil keeps accelerating freely. The induced current opposes its motion, so it falls more slowly than it would through air.
Induction is easier to reason about once you can picture the flux changing. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, such as a magnet moving through a coil. If you want to rebuild the idea of flux from the start, the Fermi Concept Library covers every concept in physics with its own lesson.
A practice routine
Day 1: Ten direction-only questions using the four steps, written out in words.
Day 2: Five questions with loops entering, crossing and leaving a field region. Sketch flux against time for each.
Day 3: Three sliding-rod problems with the force and power check.
Day 4: A mixed timed set of induction and magnetic force questions.
Pick one induction question you got wrong this month and redo it, writing Steps 1 to 4 as full sentences before any calculation.
