Page 475 - Basic Electrical Engineering

The length of the field lines corresponds to the mean length of a coil. Fig. 5.4
shows two electromagnets having the same number of turns wound on two
cores of different diameters. If the same amount of current is passed through

both the coils, the MMF, i.e., N × I will be the same.

Magnetic field strength

It can be seen from Fig. 5.4 (a) and (b) that the mean length of the flux path is
different. In the case of Fig. 5.4 (b) the length is higher. The MMF which is a
product of N and I is the same.

Figure 5.4 Mean length of the flux path in toroidal cores. The mean length of the flux path is more in
(b) as compared to that in (a)

For the same MMF, the magnetic flux, ϕ or the magnetic flux density, B
will be dependent upon the length of the field lines. The longer is the length,

the weaker will be the flux density. Magnetic field strength, H is expressed as

The longer is the flux line length, the weaker will be the magnetic field

strength for the same amount of MMF. Since the magnetic field strength, H
depends upon the number of turns of the magnetizing coil, the current

flowing through the coil, and the flux line length, there would exist a
relationship between the magnetic field strength H and the magnetic flux