A coil's back emf
"A coil's back emf" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Self-inductance and magnetic energy" in Electromagnetic Induction. A solenoid on a switch: current growth and decay with time constant L/R, the back emf −L dI/dt, and the stored energy ½LI². NEET has asked about this idea 4 times in 10 years of papers. JEE Main asked 29 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Self-inductance and magnetic energy in Electromagnetic Induction. Change the values and watch what happens.
What it shows
NCERT §6.7.2
A solenoid on a switch: current growth and decay with time constant L/R, the back emf −L dI/dt, and the stored energy ½LI².
Controls
4 to try
- Sliderturns per metre 200–2000
- Slidercore μr 1–1000
- Sliderseries R 1–50 Ω
- Buttonclose / open switch
Try this
Choose turns and core so L = 0.5 H and read 1.0 J stored at 2 A.
Common mistake
Thinking the back emf opposes only a rising current.
In NEET
10 years of papers
Asked 4 times in 10 years of NEET.
See the past questions: Faraday's Law, Electromagnetic Induction (chapter)
In JEE Main
42 shifts analysed
29 questions from Electromagnetic Induction across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §6.7.2
- A changing current in a coil changes the coil's own flux, inducing an emf in the same coil: self-induction. The flux linkage is NΦB = LI, where L is the self-inductance.
- The self-induced emf is ε = −L dI/dt. It opposes any change in the current, rise or fall, and is called the back emf.
- For a long solenoid of cross-section A, length l and n turns per metre, L = μ₀n²Al. With a core of relative permeability μr, such as soft iron, L = μrμ₀n²Al.
- L behaves like inertia, the electrical analogue of mass: it resists both the growth and the decay of current.
- Work done against the back emf while the current builds from 0 to I is stored as magnetic energy: W = ½LI², just as a mass stores ½mv².
- With two coils carrying currents, the emf in coil 1 is ε₁ = −L₁ dI₁/dt − M₁₂ dI₂/dt.
- Example 6.9: for a solenoid the stored energy is (B²/2μ₀)Al, so the energy per unit volume is B²/2μ₀. This matches the electric energy density ½ε₀E²: both grow as the square of the field, and both hold for any region of space.
Questions
5 answers
- What does the "A coil's back emf" simulation show?
- A solenoid on a switch: current growth and decay with time constant L/R, the back emf −L dI/dt, and the stored energy ½LI².
- What mistake does the "A coil's back emf" simulation fix?
- A common mistake it clears up: thinking the back emf opposes only a rising current.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 4 times in 10 years of papers.
- Does JEE Main ask questions from Electromagnetic Induction?
- Yes. JEE Main asked 29 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Self-inductance and magnetic energy" in Electromagnetic Induction (Class 12 Physics).
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