2.02 Magnetic field strength.
In the previous section we discovered that magnets possess a property called a magneto motive force
(m.m.f.) and that this m.m.f. generates flux, to form a magnetic field surrounding the magnet.
The distribution of this flux determines the characteristics of the magnetic field. e.g. The flux can be
distributed widely, producing and extended and weak field, or it could be very concentrated, resulting in a strong compact field.
There are two measurable parameters that are effected by the flux distribution, these are field strength and flux density.
A larger m.m.f. will naturally produce a stronger field. With a fixed magnitude of m.m.f., then as
would be logically expected, the more extended the field lines are, then the weaker they will be. (i.e. The field strength is
proportional to the m.m.f. and inversely proportional to the length of the magnetic field lines.)
H = m.m.f./l .
- H = magnetic field strength measured in Amperes per metre (Am-1).
- m.m.f = magneto motive force measured in Ampere turns (At).
- l = average length of magnetic circuit measured in metres (m).
Flux density is another measure of how concentrated the magnetic field is. However, in this case, we are
considering the concentration of field lines, through an area which is perpendicular to their direction.
A way of visualising flux density is by thinking of how close, or far apart, the field lines are from each other.
Flux density can be calculated from the equation below.
B = I/A .
- B = Flux density measured in Webers per metre squared (Wm-2).
- I = Magnetic flux measured in Webers (W).
- A = Cross sectional area perpendicular to the field lines measured in metres squared (m2) .