Electromagnetic Induction for NEET UG
A changing magnetic field produces a voltage. This one discovery, electromagnetic induction, gives us generators, transformers and wireless chargers.
PhysicsClass 1260 MCQs3 lessons11 flashcards3 memory tricks
Practise Electromagnetic Induction free
What you'll learn
- Faraday's and Lenz's laws
Calculate induced EMF from a change in flux and find its direction with Lenz's law. - Motional EMF and eddy currents
Find the EMF across a rod moving in a field and explain eddy currents. - Self and mutual inductance
Calculate inductance, induced EMF in an inductor, and energy stored.
Key formulas
| Magnetic flux | Φ = BA cos θ |
|---|---|
| Faraday's law | ε = −N dΦ/dt |
| Average EMF | ε = −N ΔΦ/Δt |
| Rod moving perpendicular to B | ε = Blv |
| Rod rotating about one end | ε = ½Bωl² |
| Self-induced EMF | ε = −L dI/dt |
| Inductance of a solenoid | L = μ₀n²Al |
| Mutual inductance | ε₂ = −M dI₁/dt |
Memory tricks
The induced emf opposes the change in flux that produces it (hence the minus sign in e = −dΦ/dt). This follows from conservation of energy.
An inductor opposes any change in current. That's why switching off an inductive circuit can produce a spark.
A changing magnetic field induces eddy currents in the metal pan, and their I²R heating cooks the food. Eddy currents are also used in magnetic brakes in trains.
Try 3 NEET-style questions
Q1. The mutual inductance of two coils is 0.2 H. If the current in one changes at 10 A/s, the emf induced in the other is
- 2 V
- 0.02 V
- 20 V
- 50 V
Show answer
Answer: (1) 2 V
ε = M dI/dt = 0.2 × 10 = 2 V.
Q2. An emf can be induced in a coil by changing
- the magnetic field, the area of the coil, or the orientation of the coil relative to the field
- only the resistance of the coil
- only the temperature of the coil
- only the colour of the wire
Show answer
Answer: (1) the magnetic field, the area of the coil, or the orientation of the coil relative to the field
Since Φ = BA cos θ, changing B, A or θ changes the flux and induces an emf.
Q3. A coil of area 0.05 m² is in a uniform field of 0.4 T, with the normal to the coil at 60° to the field. The magnetic flux through the coil is
- 0.02 Wb
- 0.017 Wb
- 0.01 Wb
- 0.005 Wb
Show answer
Answer: (3) 0.01 Wb
Φ = BA cos θ = 0.4 × 0.05 × cos 60° = 0.02 × 0.5 = 0.01 Wb.
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