Oersted law


What is Oersted's Law?

Oersted's Law states that when a steady electric current passes through a wire it creates a magnetic field around it.

Who developed Oersted's Law?

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.
Hans Christian Oersted
Fig. 1: Hans Christian Oersted

When did Oersted began to investigate?

In 1800, Alessandro Volta invented the voltaic pile, the first electrical battery. The following year, in 1801 Oersted began to investigate the nature of electricity and to conduct his first electrical experiments which resulted in Oersted's Law of electromagnetics.

How did oersted discover electromagnetism?

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 as shown in [Fig. 2].
Illustration of magnetic field around a current carrying conductor
Fig. 2: Illustration of magnetic field around a current carrying conductor
Fig. 3: Right hand thumb shows current direction and curved fingers shows direction of magnetic field around current carrying conductor
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.

Oersted's Formula

Oersted's law only holds for steady currents, which don't change with time. Therefore, it only holds for DC electric circuits, with no capacitors or inductors. It can be seen that it fails for time varying currents by considering the case of a circuit consisting of a battery charging a capacitor through a resistor. As shown in the [Fig. 3], the equation can be stated as,
The line integral of the magnetic field B(x), around any closed curve C, is proportional to the total current I, passing through any surface bounded by the curve.
$$\oint _C B \, dl = \mu_0 I$$
where $\mu_0 = 4x10^{-7} V.s/(A.m)$ is the magnetic constant, and the direction of integration around C, is related to the direction of current by the right hand rule.
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 and Maxwell's equations.

Some key points of Oersted's Law

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 contribution to the modern technology

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.

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