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Flux and flux density

To understand the meaning of magnetic flux (Φ) and magnetic flux density (B) think first about an ordinary bar magnet.

Around the magnet there is a magnetic field and this gives a ‘flow of magnetic energy’ around the magnet. It is this flow of energy that we call magnetic flux (Φ). We think of magnetic flux as flowing from the north pole of a magnet round to its south pole as shown by the arrows on the lines in the diagram. Looking at the diagram you should see that there is as much flux flowing ‘from the north pole’ as there is ‘flowing into the south pole’.

Magnetic flux is given the symbol Φ and is measured in units called Webers (Wb).

However the amount of magnetic flux flowing through a given area will change from one point to another around the magnet and you can understand this by thinking about a loop of wire placed in the field at two different points (A and B). You can see that in position B there are a smaller number of magnetic field lines passing through the loop than there is when it is in position A.

We call the amount of flux passing through a unit area at right angles to the magnetic field lines the flux density (B) at that point.

Flux density is measured in Tesla (T) where 1 T = 1 Wbm-2


Flux (Φ) = Flux density (B) x area through which flux passes (A)    Φ = BA

If we now use more than one loop of wire, in others words a coil of N turns as shown in position C the flux flowing through the N turns is simply N times that flowing through the single loop. The quantity NΦ is called the the flux linkage for the coil at that point.


Flux linkage = NΦ = NBA

© Keith Gibbs 2020