Physics · Electromagnetic Induction · NEET
No. As long as the whole loop stays inside the uniform field, B is the same, the area A inside the field is the same, and the angle θ is the same. So Φ = BA cosθ does not change and no EMF is induced. Flux only changes when the loop is entering or leaving the field, because then the area A that is actually inside the field is changing.
In the formula Φ = BA cosθ, θ is always the angle between B and the normal (the perpendicular to the loop's surface), not the plane. This is the most common NEET trap. When the plane is perpendicular to B, the normal is parallel to B, so θ = 0 and cosθ = 1 (maximum flux). When the plane is parallel to B, θ = 90° and flux is zero.
You have three dials, not just motion. You can keep everything still and only change B — for example by increasing the current in a nearby coil or electromagnet. A rising current means a rising B, which means rising flux, which induces an EMF. This is exactly how a transformer works without any moving parts.
Rotating changes the angle θ, not the area. As the coil turns, cosθ changes from 1 down to 0 and back, so Φ = BA cosθ oscillates. This is the third way (angle) and is the working principle of an AC generator. Changing area (way 2) means physically shrinking, stretching, or partly pulling the loop out of the field.
An AC generator changes the angle θ — a coil spins in a fixed magnetic field, so cosθ keeps changing. A transformer changes the field B — an alternating current makes B rise and fall through a shared core, with no moving parts. Both give a changing Φ, but through different dials of the same formula Φ = BA cosθ.
A square loop of side 1 m and resistance 1 Ω is placed in a magnetic field of 0.5 T. If the plane of loop is perpendicular to the direction of magnetic field, the magnetic flux through the loop is:
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° around any of its coplanar axis in 0.1 s, the emf induced in the coil will be:
A rectangular wire loop of sides 8 cm and 3 cm with a small cut is moving out of a region of uniform magnetic field of magnitude 0.3 T directed normal to the plane of the loop. The emf developed across the cut, if the velocity of the loop is 2 cm s⁻¹ in a direction normal to the shorter side, will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
From Φ = BA cosθ: (1) change the magnetic field B, (2) change the area A of the loop inside the field, or (3) change the angle θ between B and the loop's normal. Any one changing changes the flux.
A changing flux is the only thing that induces an EMF (Faraday's law). Almost every EMI numerical asks you to spot WHICH of the three factors is changing — B, A, or θ — and then apply emf = N|ΔΦ/Δt|. Identifying the factor first makes the problem easy.
No. N does not change the flux through one loop — flux is Φ = BA cosθ per loop. But the total flux linkage is NΦ, and the induced EMF is N times bigger. So N scales the EMF, not the single-loop flux.
It means change the area that is actually inside the field. You can shrink or stretch the loop, or slide it so part of it leaves the field region. In a uniform field, only the part of the loop inside the field counts toward A.
The angle θ. A coil spins in a fixed field, so cosθ keeps changing between 1 and −1, giving a sinusoidal flux and a sinusoidal EMF ε = ε₀ sin(ωt).
