The total number of magnetic lines of force of a magnet is called magnetic flux. It is represented by phi (ϕ) and its unit is Weber.
Magnetic flux per unit area is called magnetic flux density. It is given as B = ϕ/A.
Where "B" is the magnetic flux density, (ϕ) is the magnetic flux and "A" is the area. The unit of magnetic flux density is Weber/m2.it is also called tesla.
When current is passed through a coil having length of m, meter and N, turn then the magnetizing force is given by the fallowing formula
H = NI / L A/m
Where "H" number of turns, "I" current "L" length of coil. Its unit is A/m.
The phenomena of magnetism and electro magnetism are dependent upon a certain property of the medium called its permeability. Every medium is supposed to posse's two permeability.ABSOLUTE AND RELATIVE PERMEABILITY
For measuring relative permeability, vacuum or free space is chosen as the reference medium. It is allotted an absolute permeability of υ0 = 4π x 10-7 H/m. absolutely, relative permeability of vacuum with reference to its self-only. Hence, for free space,
Absolute permeability = υ0 = 4π x 10-7 H/m
Relative permeability = υ0 = 1
Now take other medium than vacuum. If its relative permeability, as compared to vacuum is υr, then its absolutes permeability is
υ = υ0 υr H/m
When a current flows through a conductor, magnetic lines of force are set up which surround the conductor. If the conductor is a straight wire the magnetic lines of force are in the form of concentric circle around the wire. As shown in the fig
When current is passed through a coil than the collective magnetic field excess as shown in the figure. We see that this field is similar to that of a permanent magnet. The magnetic lines of force, which are emitting from North Pole and entering at the south pole of the magnet.
If we warped a copper wire around an iron rod then the iron rod is called iron core. In the given fig a coil with an iron rod in side it is shown. We know that the reluctance (opposition to the flow of magnetic flux is called reluctance) of air is greater than the reluctance of iron. It means that the magnetic flux can be easily propagated in the iron as compare to air. So by using iron core the magnetic flux density increases.