Brief history of Magnetism Laws | applied electronics engineering


Brief history of Magnetism Laws

By Applied Electronics - Saturday, April 1, 2017 No Comments
Magnetic phenomenon is one of the oldest physical phenomena observed by humans. One of the earlier magnetic effect observed was the attraction and repulsion of pieces of some lodestones. Futhermore, rotation of the rock to align with the north and south pole was observed early. This led to now what we know now as the compass. It has been recorded that the Chinese in 300 to 200BC used Si Nan as compas by Campbell. Around 640 to 546BC, Thales of Miletus has been reported to investigate the magnetic effect between lodestone and iron. Socrates in between 470–399 BC has been reported to write "that lodestone not only attracts iron rings, but imparts to thema similar power" (see Keithley 1999).

Pierre de Maricourt made a more detailed study of magnetism and compass in around 1269 and he published his work "Epistola de magnete". It is in this paper that the polarity of magnets was first described.

Wiliam Gilbert (1544–1603) continued the work of Peregrinus and in his work "De magnete" he had  described the movement of a magnetic needle in a compass being caused by the Earth’s magnetic field—he concluded that Earth acts as huge spherical magnet.

Then in 1736–1806, Charles-Augustin de Coulomb constructed a torsion balance and investigated electrostatic and magnetic forces of attraction. He formulated what is now popularly known as the Coulomb’s law which describes mathematically the force, F, between hypothetical magnetic poles m1, m2 distanced by r as shown in the figure below.

The mathematical formula of force F was shown to be,

where μ0 is permeability of free space (μ0=4π×10−7Wb/Am).

Then in 1820, Hans Christian Oersted discovered that a  magnetic needle moved near a wire with electric current— thus that electric current generates a magnetic field that encircles the wire. This revolutionary discovery inspired Andre Marie Ampère to formulate the hypothesis that an electric current is a source of every magnetic field, also generated by a magnet as shown in figure below.

In 1820, he demonstrated that two parallel wires conducting current attract each other.

In the same year, Jean-Baptiste Biot and Felix Savart proposed the rule, now known as the Biot–Savart law, enabling the determination of the magnetic field strength around a current conducting wire. The magnetic field strength δH generated by current i in infinitesimal length δl of a
conductor at a radial distance a is

where u is a unit vector along the radial direction. Thus, the magnetic field strength H generated by a current flowing in circuit C is,

In 1826, Ampère formulated one of the fundamental laws of magnetism—Ampere’s circuital law:

In 1831, Michael Faraday discovered another fundamental law of magnetism. He stated that if a magnetic flux Φ linking an electrical circuit changes, it induces in this circuit electromotive force (voltage) V proportional to the rate of change of the flux:

This effect is called electromagnetic (EM) induction. The minus sign in Equation 2.8 shows that the voltage is induced in a direction opposing the flux change, which produces it. This phenomenon is called Lenz’s law.

In 1873, James Clerk Maxwell in this paper "A Treatise on Electricity and Magnetism" proposed a set of 20 equations which are fundamental to all analyses of magnetic and electric fields. These set of equations are summarized to a set of just four equations which are popularly known as Maxwell's Equations:

See Download The Electromagnet and Electromagnetic Mechanism PDF free.


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