Physics · Electromagnetic Induction · NEET
| Symbol / quantity | Φ (magnetic flux) | B (magnetic field) |
| SI unit | weber (Wb) = T·m² = V·s | tesla (T) = Wb/m² |
| Meaning | total field through an area | field strength at a point |
| Formula link | Φ = B A cosθ | B = Φ / (A cosθ) |
Magnetic flux Φ has the SI unit weber (Wb). Magnetic field B has the unit tesla (T). They are linked by Φ = B × A (for a field perpendicular to area). So 1 weber = 1 tesla × 1 metre² = 1 T·m². Also 1 Wb = 1 V·s (volt-second), which you can see from EMF = dΦ/dt. Trap to avoid: tesla is field strength at a point; weber is total flux through a whole area. NEET options often swap them.
The full formula is EMF = −N dΦ/dt. The N is the number of turns of the coil: each turn feels the same changing flux, so their EMFs add up, giving N times more. The minus sign is Lenz's law — the induced current opposes the change in flux (this comes from energy conservation). For NEET numericals, use the size EMF = N dΦ/dt and decide direction separately using Lenz's law. The unit of EMF is volt (V).
Self-inductance L relates a coil's own flux to its own current: Φ = LI, and back EMF = −L dI/dt. Mutual inductance M relates flux in coil 2 to current in coil 1: Φ₂ = MI₁, and EMF₂ = −M dI₁/dt. Both have the same SI unit, the henry (H), where 1 H = 1 Wb/A = 1 V·s/A. For a solenoid, L = μ₀ n² A l. Remember M₁₂ = M₂₁ (reciprocity).
Use EMF = Bvl (motional EMF) when a straight conductor of length l physically moves with speed v across a magnetic field B — the three must be mutually perpendicular. Use EMF = −N dΦ/dt (Faraday's law) whenever the flux through a coil changes for any reason: B changing, area changing, or angle changing. Motional EMF is actually a special case of Faraday's law where the area changes because the rod sweeps out area. If confused, always fall back to dΦ/dt.
Energy stored in an inductor is U = ½ L I², with the SI unit joule (J). Here L is in henry and I in ampere. This energy sits in the magnetic field of the coil. Compare it with a capacitor's ½ C V² — same 'half times constant times variable squared' pattern. A common NEET trap: forget the ½, or use I instead of I². Also the energy per unit volume (energy density) of a magnetic field is u = B²/(2μ₀).
Consider a long solenoid of length l and radius r. If n is the number of turns per unit length and μ₀ the permeability of free space, the inductance of the solenoid is:
Two identical inductors (each of inductance L) are connected: configuration P in series and configuration Q in parallel, carrying a time-varying current I(t). Neglecting mutual inductance, the ratio E_P/E_Q of induced EMFs is (for the arrangement shown):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The high-yield ones are: flux Φ = BA cosθ; Faraday's law EMF = −N dΦ/dt; motional EMF = Bvl; rotating rod EMF = ½ B ω R²; self-inductance Φ = LI and back EMF = −L dI/dt; solenoid L = μ₀ n² A l; mutual inductance Φ₂ = MI₁; energy U = ½ LI²; and AC generator peak EMF ε₀ = NBAω. Learn each with its SI unit.
The SI unit of both self-inductance (L) and mutual inductance (M) is the henry (H). One henry equals one weber per ampere (1 H = 1 Wb/A), which is also 1 V·s/A. A coil has 1 H if a current changing at 1 ampere per second induces 1 volt across it.
No — despite the name 'electromotive force', EMF is not a force. It is the energy given to each unit charge by the source, so its SI unit is the volt (V = joule/coulomb), the same as potential difference. In induction, EMF = rate of change of flux, and 1 volt = 1 weber per second.
For a rod of length R rotating about one end with angular speed ω in a field B perpendicular to the plane of rotation, EMF = ½ B ω R². It comes from integrating Bvl along the rod, where speed increases from 0 at the pivot to ωR at the tip, giving the average factor of ½.
No. Inductance L depends only on the coil's geometry (number of turns, area, length) and the core material. Current changes the flux (Φ = LI) and the stored energy (½LI²), but L itself is a fixed property of the coil, measured in henry.
