Magnetic flux density (B) is the quantity of magnetic flux (Φ) per unit area (A), measured at right angles to the magnetic field. It is measured in either tesla (T) or weber per square metre (Wb/m2) – the former being more commonly used.

Magnetic flux density may be calculated by using the following equation:

Equation: B = Φ / A

Worked Example 1

A magnet produces a flux of 180 μT through a rectangular region of width 30 mm and height 40 mm, measured at right angles to the field. Find the magnetic flux density.
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The first step is to convert all lengths to metres, so the width, W = 0.03 m and the height, H = 0.04 m. Hence the area, A = W × H = 0.0012 m2. (See the Maths > Area topic for more information on calculation of areas in engineering.)

B = Φ / A

  = 180 × 10–6 / 0.0012

  = 0.15 T

Worked Example 2

Find the total magnetic flux produced by an electromagnet through a circular pole face of radius 50 mm, if the flux density is 1.8 T.
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The area, A = π r2 = π × 0.052 = 0.0078539816 m2 (once again remembering to convert all lengths to metres before use).

The total flux, Φ = B × A

  = 1.8 × 0.0078539816

  = 0.014 Wb or 14 mWb (to 2 d.p.)

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