We have seen that magnet has its influence on the surrounding medium. The region around a magnet within which the influence of the magnet can be experienced is called magnetic field. Existence of such field can be experienced with the help of compass needle, iron or pieces of metals or by bringing another magnet in vicinity of a magnet.
What is Magnetic Lines of Force?
The magnetic field of a magnet is represented by imaginary lines around it which are called magnetic lines of force. Note that these lines have no physical existence, these are purely imaginary and were introduced by Michael Faraday to get the visualization of the distribution of such lines of force.
Direction of Magnetic Field:
The direction of magnetic field can be obtained by conducting small experiment. Let us place a permanent magnet on table and cover it with a sheet of cardboard. Sprinkle steel or iron fillings uniformly over the sheet. Slight tapping of cardboard causes fillings to adjust themselves in a particular pattern as shown in figure. The shape of this pattern projects a mental picture of the magnetic field present around a magnet.
Properties of Magnetic Lines of Force:
Magnetic lines of force are closed continuous curves.
Magnetic lines of force are always originating on a N-pole and terminating on a S-pole, external to the magnet.
Magnetic lines of force never intersect each other.
Magnetic lines of force, are like stretched rubber bands and always try to contract in length.
Magnetic lines of force, which are parallel and travelling in the same direction repel each other.
Magnetic lines of force always prefer a path offering the least opposition.
Crowding of Magnetic lines of force represents a stronger magnetic field and vice-versa.
Important Note- In electricity, an isolated charge can exist whereas, in magnetism, an isolated magnetic pole does not exist.
What is Magnetic Flux?
The total number of lines of force existing in a particular magnetic field is called magnetic flux. Lines of force can be called lines of magnetic flux. The unit of flux is weber and flux is denoted by the symbol (Φ). The unit weber is denoted as Wb.
1 Weber = 108 lines of force
What is Magnetic Dipole?
A magnetic dipole consists of two unlike poles of equal strength (m) and separated by a small distance (2l). Example- a bar magnet, a compass needle etc.
Magnetic dipole moment is defined as the product of pole strength and the distance between two poles. If ‘m’ be the strength of each pole and ‘2l’ be the distance between two poles, then
Magnetic dipole moment is a vector quantity directed from the South to the North pole. The S.I. units of magnetic dipole are Joule/Tesla or ampere-m2.