Physics · Electromagnetic Induction · NEET
Because EMF depends on the RATE of change of flux, not on the amount of flux. A strong stationary magnet gives a large flux Φ, but Φ is constant, so dΦ/dt = 0 and EMF = 0. You need the flux to be changing (magnet moving, field growing, area or angle changing) to get any EMF.
Flux Φ = BA cosθ tells you how many field lines pass through the coil right now (unit: weber). Rate of change dΦ/dt tells you how fast that number is changing each second (unit: weber/second = volt). Faraday's law connects EMF to the second quantity, not the first. Big flux with no change makes no EMF; small flux changing fast makes a big EMF.
Moving faster means the flux through the coil changes more per second, so dΦ/dt is larger. Since EMF = -N dΦ/dt, a larger dΦ/dt gives a larger EMF. Same magnet, same coil, but double the speed roughly doubles the induced EMF.
dΦ/dt is the instantaneous rate of change (the slope of the flux-time graph at one instant). ΔΦ/Δt is the average rate over a time interval Δt. For NEET numericals where flux changes over a stated time (like 0.1 s), you use the average: EMF(avg) = -N ΔΦ/Δt = -N (Φ_final - Φ_initial)/Δt.
Each turn of the coil links the same changing flux, so each turn produces its own EMF. Connecting N turns in series adds their EMFs, giving N times the single-turn value. That is why the formula has N in front. (Covered fully in the next page on N turns.)
A 800 turn coil of effective area 0.05 m² is kept perpendicular to a magnetic field 5 × 10⁻⁵ T. When the plane of the coil is rotated by 90° about any of its coplanar axes in 0.1 s, the emf induced in the coil will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. This is the mathematical form of Faraday's law of induction. The minus sign is Lenz's law, telling you the induced EMF opposes the change that caused it.
Weber per second (Wb/s), which is exactly equal to the volt. So rate of change of flux directly gives you a voltage.
Use dΦ/dt for instantaneous EMF (given flux as a function of time, differentiate it). Use ΔΦ/Δt for average EMF over a stated time interval, common in NEET numericals.
No. The minus sign only sets the direction (opposition), not the magnitude. For 'how many volts' questions you use the size |N dΦ/dt|; for 'which direction' you keep the minus (Lenz's law).
Yes. EMF exists whenever flux changes, even in an open coil with no closed path. Current flows only if the circuit is complete and has finite resistance.
