Magnetic flux (symbol Φ – the Greek letter Phi) is the magnetic field produced by a source of magnetism such as a permanent magnet or electromagnet. Magnetic flux is measured in webers (Wb).

A commonly used graphical convention when drawing magnetic fields is to represent each weber as a single line of flux. The direction of the field is shown by adding arrows which, by convention, exit the North pole and enter the South pole. (The magnetic field is considered to be stationary and the arrows do not imply any physical movement of particles – unlike an electrical circuit.)

The total magnetic magnetic flux may be determined if the perpendicular area enclosing the flux (A) and the magnetic flux density (B) are known.

Equation: Φ = B A

Worked Example 1

Calculate the total magnetic flux passing through a rectangular area 200 mm wide and 100 mm high at right angles to the field, if the magnetic flux density is 0.8 T.
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The first step is to calculate the area in square metres, which is given by A = W × H = 0.2 × 0.1 = 0.02 m2. (See the Maths > Area topic for more information on calculation of areas in engineering.)

Φ = B A

  = 0.8 × 0.02

  = 0.016 Wb or 16 mWb

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