Self Inductance — L, energy in inductor
EMI
7
JEE Qs
8%
Hard
60
min
Master the transient behavior of inductors (at t=0 and t=infinity) and energy storage calculations, as these are frequently tested in circuit problems.
🧮 Key Formulas
✅ Key Points for JEE
- 1Self-inductance (L) is an intrinsic geometric property of a coil (depends on its shape, size, number of turns, and core material), not on the current flowing through it.
- 2An inductor always opposes any change in the current flowing through it by generating an induced EMF (Lenz's Law), acting like 'electrical inertia'.
- 3Energy is stored in the magnetic field generated by the inductor. This stored energy is released when the current decreases.
- 4At steady-state DC, an ideal inductor behaves as a short circuit because dI/dt = 0, leading to zero induced EMF.
- 5At t=0 (when a circuit is first closed and current was initially zero), an inductor behaves as an open circuit, trying to prevent sudden current flow (dI/dt is initially very large).
⚠️ Common Mistakes
- ✕Confusing the factors affecting self-inductance (geometric factors only) with those affecting magnetic flux or induced EMF (current, dI/dt).
- ✕Incorrectly determining the polarity of induced EMF across an inductor, failing to apply Lenz's Law correctly to oppose the change in current.
- ✕Applying steady-state inductor behavior (short circuit) at t=0 or during transient (charging/discharging) phases.
- ✕Using the energy formula U = (1/2)LI^2 without understanding that I refers to the instantaneous current at which energy is being calculated.
📝 Practice Questions
See allQ42.Regarding self-inductance: A. The self-inductance of the coil depends on its geometry. B. Self-inductance does not depend on the permeability of the medium. C. Self-induced e.m.f. opposes any change in the current in a circuit. D. Self-inductance is electromagnetic analogue of mass in mechanics. E. Work needs to be done against self-induced e.m.f. in establishing the current. Choose the correct answer from the options given below: (1) A, B, C, E only (2) B, C, D, E only (3) A, C, D, E only (4) A, B, C, D only
Q49. A conducting bar moves on two conducting rails as shown in the figure. A constant magnetic field B exists into the page. The bar starts to move from the vertex at time t = 0 with a constant velocity. If the induced EMF is E ∝tn , then value of n is _.
Q35.A rectangular metallic loop is moving out of a uniform magnetic field region to a field free region with a constant speed. When the loop is partially inside the magnate field, the plot of magnitude of induced emf (ε) with time (t) is given by 2025 (22 Jan Shift 2) JEE Main Previous Year Paper (1) (2) (3) (4)
Q48.A parallel plate capacitor of area A = 16 cm2 and separation between the plates 10 cm , is charged by a DC current. Consider a hypothetical plane surface of area A0 = 3.2 cm2 inside the capacitor and parallel to the plates. At an instant, the current through the circuit is 6A. At the same instant the displacement current through A0 is ________ mA .
Q31.A uniform magnetic field of 0.4 T acts perpendicular to a circular copper disc 20 cm in radius. The disc is having a uniform angular velocity of 10πrads−1 about an axis through its centre and perpendicular to the disc. What is the potential difference developed between the axis of the disc and the rim ? (π = 3.14) (1) 0.5024 V (2) V (3) 0.2512V V (4) 0.1256V V
Q27.A coil of area A and N turns is rotating with angular velocity ω in a uniform magnetic field →B about an axis perpendicular to →B. Magnetic flux φ and induced emf ε across it, at an instant when →B is parallel to the plane of coil, are : (1) φ = AB, ε = 0 (2) φ = 0, ε = 0 (3) φ = 0, ε = NABω (4) φ = AB, ε = NABω
NCERT Chapters
- Class 12 Physics Ch 6: Electromagnetic Induction