Magnetic Materials
All the materials in this universe may be classified either magnetic or non-magnetic. Magnetic materials are those which are affected by magnetic field and non magnetic materials are those which are not affected or slightly affected by magnetic field. Maximum types of materials fall under category of non – magnetic material. Non-magnetic material can further be classified as diamagnetic and paramagnetic materials. Some of the non magnetic materials exhibit very slight magnetic effect but it is extremely difficult to detect. Practically, all these materials are referred as totally non-magnetic. On the other hand, all the materials which exhibit magnetic effect strongly are referred as magnetic materials or ferromagnetic materials. The materials based on iron, cobalt and ferrites are normally ferromagnetic materials. 
Magnetic Field
Magnetic fields can be created either by placing, a permanent magnet or by supplying current through a solenoid. Latter is electromagnet. A magnetic field is defined as the space surrounding a permanent magnet or electromagnet where the electric field is felt by other magnet or magnetic material.
Magnetic Flux or Magnetic Lines of Force
Magnetic field is also represented by lines of force as static electric field. These lines of force are referred as magnetic flux. When a unit magnetic pole is placed inside a magnetic field, it will experience both repulsive and attractive force, from similar and opposite poles of the magnet, respectively. The unit pole travels due to resultant of the repulsive and attractive force. The path through which the unit pole travels in the magnetic field is referred as magnetic lines of force. There are numbers of magnetic lines of force in a magnetic field, and these lines of force are collectively called magnetic flux. These flux lines have some specific properties that are described below.
Properties of Magnetic Flux or Magnetic Lines of Force
- They always form complete closed loops. Unlike lines of electric flux, which radiate from and terminate at the charged surfaces, lines of magnetic flux exist all the way through the magnet.
- They behave as if they are elastic. That is, when distorted they try to return to their natural shape and spacing.
- The lines of force of magnetic field are radiated from the north (N) pole to the south (S) pole.
- Flux lines do not cross or interact to each other.
Since the flux lines are something like elastic bands, the lines linking two unlike magnetic poles always try to shorten themselves. This brings two magnets together. 
When two like magnetic poles are brought closer to each other, as the magnetic flux lines do not intersect each other, the magnetic flux lines of both magnetic fields are compressed. As these flux lines behave as elastic bands, they will try to expand to their normal shape. Hence, one magnetic pole will push away other magnetic poles. 
Difference between a Permanent Magnet and an Electromagnet
A permanent magnet does not require any external electric supply to produce the field. But their magnetic field is normally weaker than that of an electromagnet. Hence, permanent magnets are relatively bulky in size. The strength of the field cannot be varied as per requirement. The field of these magnets are also not everlasting, it will be loosened over a period of time. They also lose their magnetism, if they get subjected to physical shock or vibration. Because of these many disadvantages, the applications of permanent magnet in the field of engineering are quite limited. 
Advantages of Electromagnet over Permanent Magnet
In addition to the heating effect of electric current, it also produces a magnetic field surrounding the conductor carrying the current . The field strength is directly proportional to the current flowing through the conductor. Thus, by varying the current, a magnetic field strength of an electromagnet can be varied. It can be turned on or off by switching on or off the supply current , as when required. The direction of field can also be reversed by reversing the direction of current flowing through the conductor. Because of these many advantages, electromagnets are more suitably used in the field of engineering.
Magnetic Flux Pattern of a Solenoid
A magnetic field is a vector quantity. The field pattern produced by a current flowing through a straight conductor is illustrated in figures below. 
The direction of the flux lines created around a current carrying conductor can easily be determined by Right Hand Grip Rule. If we hold the current conductor with our hand as shown in the figure, then the thumb will indicate the direction of current whereas four other fingers of this hand will indicate the direction of flux lines surrounding the conductor. Magnetic flux surrounds the whole length of the current carrying conductor. The flux pattern extends outwards in concentric circles up to infinity. Since, the magnetic field strength at a point in space, is inversely proportional to its perpendicular distance from the axis of the conductor, the field diminishes very rapidly with the distance. When a straight current carrying conductor forms a coil, it produces the flux pattern like a bar magnet. This is which we call a solenoid. The first figure shows the flux patterns produced by adjacent turns of the coil. However, since lines of flux will not intersect, the flux distorts to form complete loops around the whole coil as shown in second figure. 
Unit of Magnetic Flux
The unit of this flux is the Weber (Wb). The unit was named in honor of German scientist Max Weber.
Definition of Magnetic Flux Density
The amount of flux crosses a unit area perpendicularly, in a magnetic field is known as magnetic flux density. Hence, the unit flux density would be expressed as Weber per meter2. This unit of flux density is named as Tesla after the name of famous American scientist, Nikola Tesla. The quantity symbols for magnetic flux and flux density are ψ and B respectively. Therefore, flux density,