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Magnetic Flux Formula

Magnetic flux formula: Explore more about the magnetic flux formula with solved examples.

Magnetic flux formula

Magnetic flux refers to the total number of magnetic field lines passing through a given surface. Magnetic flux is represented by ΦB and the unit is Weber (Wb).

Magnetic flux is the product of the average magnetic field and the perpendicular area through which it passes. The unit of magnetic flux is either Weber (Wb) or Tesla – meter2. The magnetic field flux value could be a vector quantity depending upon the flux. The magnetic flux can be described mathematically as,

              ΦB = BA cos θ


          ΦB = Magnetic flux,

          B = Magnetic field force,

          A = Area of the surface through which flux passes.

          Θ = Angle between the surface and magnetic field line.

If a coil with n turns of area A, is placed in a magnetic field of strength B, then magnetic flux is given by,

ΦB = nBA cos θ

Conditions for maximum and minimum magnetic flux

·        The magnetic flux obtained is maximum when the angle between the magnetic field line and the surface is 0˚.

According to the formula,

                 ΦB = BAcos 0˚ = BA

·        The magnetic flux obtained is minimum when the angle between the magnetic field line and the surface is 90.

 According to the formula,

                 ΦB = BAcos 90 = 0

Properties of magnetic flux

·        Magnetic flux always forms a closed loop.

·        Magnetic flux never intersects with one another.

·        Magnetic flux is a scalar quantity.

·        Magnetic flux starts from the north pole and ends at the south pole.

Solved examples

Example 1: A magnetic field of 8.9 T passes perpendicular to a disc with a radius of 5 cm. Find the magnetic flux of the disc.


            Magnetic field is given by, B = 8.9 T,

             Radius is given by, r = 5 cm = 5 × 10-5 m,

             As the magnetic field is perpendicular to the disc, θ = 0,

             Area is given by = π(5 x 10-2)2

             ΦB =?

By applying the formula,

             ΦB = BAcos θ = 8.9 x π(5 x 10-2)2 x cos 0

                  = 139.7 x 10-4 Wb.

Example 2: The dimensions of a square loop is 0.40m x 0.40m. B and θ are 0.05T and 60° respectively. Determine the magnetic flux.


              B = 0.05 T,

              A = 0.4 x 0.4 = 0.16 m2,

              Θ = 60°

              ΦB =?

By applying the formula,

              ΦB = BAcos θ = 0.05 x 0.16 x cos 60°

                    = 0.004 Wb.


Frequently asked questions

Get answers to the most common queries related to the Magnetic Flux Formula.

What is the SI unit of magnetic flux?

Ans. The SI unit of magnetic flux is given by Weber (Wb).

Is magnetic flux a vector or scalar quantity?

Ans. Magnetic flux is a scalar quantity.

What is the magnetic flux in a closed surface?

Ans. Magnetic flux in a closed surface is always zero.