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Encyclopedia of Electrical Engineering
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Encyclopedia of Electrical Engineering
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Oersted law

Oersted's Law states that when a steady electric current passes through a wire it creates a magnetic field around it.
In 1800, Alessandro Volta invented the voltaic pile, the first electrical battery. The following year, Oersted began to investigate the nature of electricity and to conduct his first electrical experiments. In 1820, while performing classroom demonstrations for his university students, Oersted passed an electric current through a wire which caused a movement in the needle of a magnetic compass placed nearby. This clearly established a relationship between magnetism and electricity making Oersted the first to identify the force of electromagnetism. His discovery was published in July 1820, in a pamphlet titled "Experiments on the Effect of a Current of Electricity on the Magnetic Needle". Through further experimentation, Oersted also found that electric current produces a circular magnetic effect around it.
Hans Christian Oersted
Fig.no.1: Hans Christian Oersted
Illustration of magnetic field around a current carrying conductor
Fig.no.2: Illustration of magnetic field around a current carrying conductor
Danish physicist Hans Christian Oersted (1777-1851), investigated and found the mathematical law which governs how strong the field was, which is now called Oersted's Law. Oersted's discovery was the first connection found between electricity and magnetism, and the first of two laws that link the two; the other is Faraday's law of induction. These two laws became part of the equations that govern electromagnetism, Maxwell's equations. Oersted found that for a straight wire carrying a steady (DC) current:
  • The magnetic field lines encircle the current-carrying wire.
  • The magnetic field lines lie in a plane perpendicular to the wire
  • If the direction of the current is reversed, the direction of the magnetic field reverses.
  • The strength of the field is directly proportional to the magnitude of the current.
  • The strength of the field at any point is inversely proportional to the distance of the point from the wire.
Oersted's discovery of electromagnetism unleashed a series of discoveries that eventually laid the foundation for our modern technology-enabled world. Soon after Oersted's discovery, French physicist Andre-Marie Ampere developed a single mathematical formula to represent the magnetic forces that exist between current-carrying conductors. Then, over 40 years later, Scottish scientist James Clerk Maxwell modified this equation so that it would also apply to situations in which the current is not constant. In this form, it became one of his four famous equations establishing that light is an electromagnetic wave. Electromagnetism is the basis for numerous devices including an electric motor, microphone, electric generator, loudspeaker, and transformers.