JEE Mains Physics · Teaching notes
Electromagnetic Induction — JEE Mains Physics
Electromagnetic Induction has 90 past-year questions from 2021 to 2026, and 41 of them ask for a number rather than an option. Almost all of them come down to one law: the emf is the rate at which flux changes. The work is in seeing what changes, the field, the area, the angle or a conductor cutting across the field, and writing that change down before reaching for a formula. The arithmetic is short. Marks are lost on factors: an angle taken from the plane instead of the normal, the wrong component of the earth's field, the half in ½Bωl², rpm left unconverted, or a current change that crosses zero.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Magnetic Flux, Faraday's Law and Lenz's Law
30 PYQsFlux is NBA cos θ with θ measured from the normal; the induced emf is the rate at which the flux changes, ε = −dΦ/dt, and the induced current always flows so as to oppose that change.
Motional EMF: Rods, Rails and Moving Loops
19 PYQsA conductor of length l moving at v across a field B develops an emf Blv; on rails the current it drives feels a force B²l²v/R that opposes the motion, and a loop crossing the edge of a field has an emf only while its flux is changing.
Rotating Coils, Rods and Discs
15 PYQsA coil spinning in a field has an emf NBAω sin ωt, largest when its plane lies along the field; a rod turning about one end sweeps out area and develops ½Bωl², and a disc does the same between its axle and its rim.
Self and Mutual Inductance, Energy and LR Circuits
26 PYQsA coil opposes a change in its own current with an emf −L dI/dt and induces −M dI/dt in a neighbour; it stores energy ½LI², and in an LR circuit the current grows or decays with time constant L/R.
Formula & revision sheet
10 formulas · 1 reference tables · 34 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
10 formulas · 1 reference tables · 34 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Reference tables (1)
Lenz's law and the direction of the induced current9 rows
| Situation | What the flux does | Induced current or effect |
|---|---|---|
| Field into the page, increasing | Flux into the page grows | Anticlockwise, so its own field points out of the page |
| Field into the page, decreasing | Flux into the page falls | Clockwise, so its own field points into the page |
| North pole pushed towards a loop | Flux from the magnet grows | Near face of the loop becomes a north pole; magnet repelled |
| North pole pulled away from a loop | Flux from the magnet falls | Near face becomes a south pole; magnet attracted back |
| Bar magnet passing right through a loop | Rises as it enters, falls as it leaves | Two emf pulses of opposite sign, with a gap while it is inside |
| Coil moved through a uniform field | Unchanged | No emf and no current |
| Coil rotated in a uniform field | Changes with the angle | Alternating emf |
| Field reversed in direction | Changes by twice BA | Emf while it reverses |
| Magnet dropped down a long copper tube | Changes in every ring of the tube | Eddy currents brake it; it falls at a nearly constant speed A non-magnetic bar of the same size falls freely and arrives first. |
Watch out for (9)
- Angle measured from the plane→ Induced emf from a changing magnetic flux
- Using the solenoid's area for a loop inside it→ Induced emf from a changing magnetic flux
- Putting the time into the flux instead of its derivative→ Induced emf from a changing magnetic flux
- Reversing the field is not 'no change'→ Charge, heat and power from an induced current
- Charge does not depend on the time taken→ Charge, heat and power from an induced current
- Power of a sinusoidal emf uses half the square of the peak→ Charge, heat and power from an induced current
- Opposing the flux instead of its change→ Lenz's law and the direction of the induced current
- Motion alone does not induce an emf→ Lenz's law and the direction of the induced current
- Eddy currents need a conductor→ Lenz's law and the direction of the induced current
Formulas (3)
Watch out for (9)
- Using the total field instead of the cut component→ Motional emf of a moving rod
- Sine and cosine of the dip swapped→ Motional emf of a moving rod
- Leaving the speed in km/h or the field in gauss→ Motional emf of a moving rod
- Using the rod's length instead of the rail gap→ Force, power and terminal speed for a rod on rails
- Forgetting that the force goes as v→ Force, power and terminal speed for a rod on rails
- Work done is not the stored energy→ Force, power and terminal speed for a rod on rails
- An emf while the loop is fully inside→ Loop crossing a field boundary or a non-uniform field
- Adding the two sides in a non-uniform field→ Loop crossing a field boundary or a non-uniform field
- Using the arc for a ring at an edge→ Loop crossing a field boundary or a non-uniform field
Formulas (2)
Watch out for (6)
- Leaving rpm unconverted→ Emf of a coil rotating in a magnetic field
- Zero flux does not mean zero emf→ Emf of a coil rotating in a magnetic field
- Angle with the plane versus angle with the normal→ Emf of a coil rotating in a magnetic field
- Forgetting the half→ Emf of a rod or disc rotating about one end
- Adding the emfs of fan blades→ Emf of a rod or disc rotating about one end
- Total field for a horizontal fan→ Emf of a rod or disc rotating about one end
Formulas (3)
Watch out for (10)
- Change of current across zero→ Self-inductance and the back emf
- Using the battery emf as the induced emf→ Self-inductance and the back emf
- Thinking L depends on the current→ Self-inductance and the back emf
- Working out the flux through the wrong loop→ Mutual inductance of two coils
- Field at a square's centre→ Mutual inductance of two coils
- Sign of 2M in series→ Mutual inductance of two coils
- Time constant upside down→ Energy stored in an inductor and current growth in an LR circuit
- Energy fraction taken as current fraction→ Energy stored in an inductor and current growth in an LR circuit
- Sign of dI/dt for a falling current→ Energy stored in an inductor and current growth in an LR circuit
- Using μ₀ in a filled solenoid→ Energy stored in an inductor and current growth in an LR circuit
PYQ weightage by concept
11 concepts · 90 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
11 concepts · 90 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Induced emf from a changing magnetic flux | 15 | 17% |
| Lenz's law and the direction of the induced current | 9 | 10% |
| Charge, heat and power from an induced current | 6 | 7% |
| Concept | PYQs | Share |
|---|---|---|
| Motional emf of a moving rod | 8 | 9% |
| Force, power and terminal speed for a rod on rails | 6 | 7% |
| Loop crossing a field boundary or a non-uniform field | 5 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| Emf of a rod or disc rotating about one end | 8 | 9% |
| Emf of a coil rotating in a magnetic field | 7 | 8% |
| Concept | PYQs | Share |
|---|---|---|
| Mutual inductance of two coils | 10 | 11% |
| Energy stored in an inductor and current growth in an LR circuit | 10 | 11% |
| Self-inductance and the back emf | 6 | 7% |
Test yourself on Electromagnetic Induction
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.