Lenz's Law — Direction of induced current
EMI
7
JEE Qs
8%
Hard
60
min
Always identify the direction of the *change* in magnetic flux first, then determine the induced magnetic field that opposes this change, and finally use the Right-Hand Rule to find the induced current.
🧮 Key Formulas
✅ Key Points for JEE
- 1Lenz's Law states that the direction of the induced current (or EMF) is always such that it opposes the *cause* producing it, i.e., it opposes the *change* in magnetic flux.
- 2The opposition mentioned in Lenz's Law is a direct consequence of the principle of conservation of energy; work must be done against the opposing force to induce current.
- 3To determine the induced current direction: (1) Identify the direction of the external magnetic field. (2) Determine if the magnetic flux is increasing or decreasing. (3) The induced magnetic field will be in a direction that opposes this *change* in flux. (4) Use the Right-Hand Rule for current loops/solenoids to find the current direction that produces this induced magnetic field.
- 4It is critical to identify the *cause* of the flux change (e.g., magnet approaching/receding, loop entering/leaving field, current increasing/decreasing in a primary coil).
- 5The negative sign in Faraday's Law (E = -dΦ_B/dt) mathematically represents Lenz's Law.
⚠️ Common Mistakes
- ✕Confusing 'opposing the magnetic flux' with 'opposing the *change* in magnetic flux'. The induced current doesn't try to nullify existing flux, but rather to counteract its variation.
- ✕Incorrectly applying the Right-Hand Rule to determine the direction of the induced magnetic field from the induced current, or vice-versa.
- ✕Failing to systematically analyze the change in flux (increase/decrease) and the subsequent direction of the induced magnetic field required to oppose that change.
📝 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