The phenomenon of production of electric current in a coil due to the relative motion between coil and magnet is called electromagnetic induction. The current so produced is called induced current and corresponding emf is called induced emf.

Magnetic flux and magnetic flux density:

A group of magnetic field lines penetrating some surface perpendicularly is called magnetic flux. It is given by the dot product of magnetic field (B) and area vector (A).

Magnetic flux is denoted by phi (Φ).

Φ= B A cos θ

Since, flux is the scalar product of area vector and magnetic field, so it is a scalar quantity.

Case I: For θ= 90^o

Φ= B A cos 90

Φ= B A × 0 = 0

If the angle between magnetic field and normal to the surface is 90^o then no flux is linked with the surface.

Case II:  For θ= 0^o

Φ= B A cos 0

Φ= B A × 1= B A

If the angle between magnetic field and normal to the surface is 90^o then flux linked with the surface is maximum. It is called positive flux.

Case I: For θ= 180^o

Φ= B A cos 180

Φ= B A × -1 = - B A

If the angle between magnetic field and normal to the surface is 180^o then the flux linked with the surface is minimum called negative flux.

Since, Φ= B A

The SI unit of magnetic flux is weber (wb).

The magnetic flux is said to be one weber when a magnetic field of one Tesla act on a plane area of 1 m² placed normally to the direction of magnetic field.