Physics · Electromagnetic Induction · NEET
No. The magnetic field B (also called flux density) tells you how strong the field is at one point, measured in tesla (T). Magnetic flux Φ is the field spread over an area — it is B multiplied by the area the field passes through, measured in weber (Wb). Think of B as 'how heavy the rain is' and flux as 'how much rain falls on the whole bucket'. A strong field over a tiny area can give the same flux as a weak field over a large area.
Only the part of the field that goes straight through the surface counts as flux. θ is the angle between the field B and the normal (the arrow pointing straight out of the surface), NOT the angle with the surface itself. When B is straight through (θ = 0), cos 0 = 1 and flux is maximum. When B is parallel to the surface, it grazes along it and no lines poke through, so θ = 90°, cos 90° = 0, and flux is zero. This is the most common NEET trap.
Tesla is the unit of the magnetic field B (field per unit area). Weber is the unit of flux, which is field times area. So 1 weber = 1 tesla × 1 metre² = 1 T·m². In NEET, if a value is in Wb it is a flux; if it is in T it is a field. Never write flux in tesla — that is a marking mistake.
Yes. If the loop lies flat along the field lines (plane parallel to B), the field skims across the loop and nothing passes through it, so Φ = BA cos 90° = 0, no matter how strong B is. Flux depends on B, area, AND the angle. This is why 'no lines through the loop' means zero flux, not zero field.
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 the loop is perpendicular to the direction of the magnetic field, the magnetic flux through the loop is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the number of magnetic field lines passing straight through a given area. More lines through the surface means more flux. Its formula is Φ = BA cosθ.
The weber (Wb). One weber equals one tesla times one square metre, so 1 Wb = 1 T·m². It is named after Wilhelm Weber.
Φ = B·A = BA cosθ, where B is the magnetic field, A is the area, and θ is the angle between B and the normal to the surface.
Magnetic flux is a scalar quantity. Even though B and A are vectors, flux is their dot product (B·A), which gives a single number that can be positive, negative, or zero.
Faraday's law says induced EMF equals the rate of change of flux. So every electromagnetic induction problem — EMF, Lenz's law, inductance — starts by finding the flux correctly.
