Consider a closed curve enclosing an area A (as shown in the figure). Let there be a uniform magnetic field B in that region. The magnetic flux through the area A is given by
Φ = B.A = B A CosΘ
where Θ is the angle which the vector B makes with the normal to the surface. If B is perpendicular to A, then the flux through the closed area A is zero.
SI unit of magnetic flux is weber (Wb).
Notes :
- Area vector is ⊥ to the surface.
- For open surface choose one direction as the area vector direction and stick to it for the whole problem.
- For closed surfaces outward normal is taken as area vector direction
- Flux is basically count of number of lines crossing a surface
              [because magnetic field lines exists in closed loop.]
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