What Is the Theory Behind Magnetic Field?

The strength of the magnetic field is determined by the number of lines of force per unit area of space. Magnetic fields are created either by the flow of current through a conductor or by the presence of some magnetised material known as magnets. Magnetic force is polarised, i.e., the magnetic force is the strongest at the poles and develops opposite signs at the two ends and becomes weak as we move far away from the poles. Forces or magnetic flux lines represent the magnetic field, and these lines always originate from the north pole and end at the south pole. The direction of the magnetic field is taken to be from the north pole to the south pole. A magnetic field causes permanent magnets and compass needles to align in the direction of the field.

History of Magnetism

Before magnetism was actually discovered, people observed its properties in the mineral magnetite, also known as lodestone. Greeks found that this mineral could attract and repel iron placed near it, calling it a magnet. 

The first person to observe the effect of magnetism was William Gilbert. Earth being a weak magnet was also a part of his discovery. Hans Christian Oersted(1777-1851) suggested that there’s a connection between electricity and magnetism. Still, later on, experiments were conducted by Michael Faraday and Andre Marie Ampere to study the impact of electric and magnetic fields. Later on, in the nineteenth century, James Clerk Maxwell provided a theoretical foundation for electrodynamics, and he proved that both electricity and magnetism emerge from the same force field.

Presently, the theory of magnetism is based on the work of two German scientists, Ernest Ising and Werner Heisenberg. 

Origin of Magnetism

An electron in an atom revolves around the nucleus, known as orbital motion, and rotates about its own axis, known as spin motion. The two ways of movement of electrons give rise to magnetism. A magnetic moment is defined as the torque experienced by a magnet when it is placed in an external magnetic field of unit strength. According to the Pauli exclusion principle, an atomic orbital can only have two electrons of opposite spin. So, eventually, the magnetic moment gets cancelled out because of the Pauli exclusion principle. Even though this is true, some transition elements like nickel, cobalt, iron, etc., possess magnetic moments and behave as magnetic materials.

Magnetic Fields

In the presence of a magnet or electric current, another field comes into existence in which magnetic forces are present, known as a magnetic field. A magnetic field causes permanent magnets and compass needles to align in the direction of the field. The magnetic field created around a permanent magnet remains constant, and thus, it’s known as a stationary field. In the case of alternating currents, the value of the current keeps on fluctuating; therefore, the magnetic field also changes accordingly. Forces or magnetic flux lines represent the magnetic field, and these lines always originate from the north pole and end at the south pole. The direction of the magnetic field is taken to be from the north pole to the south pole. The field is stronger near the poles and becomes weak as we move far from the pole. The more dense the lines are, the stronger the field is. 

Vectors are used to represent magnetic fields. Two different vectors, such as magnetic flux density (B) and magnetic field strength (H), are used to represent the magnetic field. Magnetic flux is the total number of field lines passing through a given area. The SI unit of magnetic flux is Weber.

Measurement of Magnetic field

The S.I unit of the magnetic field is Tesla (T). The magnetic field strength is measured in an ampere per metre (A/m) unit. Gauss metres and magnetometers are used to measure the magnetic fields.

Conclusion

A magnetic field is produced by moving charges and is associated with two poles of the magnet; north and south. The direction of the magnetic field is determined by force acting on the north pole. The continuous loop formed by the magnetic field is directed from the north to the south pole outside the magnet, and inside it, the magnetic field lines run from south to north. Magnetic force is polarised, i.e., the magnetic force is strongest at the poles and develops opposite signs at the two ends and becomes weak as we move far away from the poles. Magnetic fields and magnetism have various applications such as electromagnets, electric generators, transformers, MRI etc.