Magnetism


In physics, magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, gadolinium and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.

Magnetism also has other definitions/descriptions in physics, particularly as one of the two components of electromagnetic waves such as light.

Aristotle attributes the first of what could be called a scientific discussion on magnetism to Thales, who lived from about 625 BC to about 545 BC. Around the same time in ancient India, the Indian surgeon, Sushruta, was the first to make use of the magnet for surgical purposes.

In ancient China, the earliest literary reference to magnetism lies in a 4th century BC book called Book of the Devil Valley Master: "The lodestone makes iron come or it attracts it." The earliest mention of the attraction of a needle appears in a work composed between AD 20 and 100 (Louen-heng): "A lodestone attracts a needle."

The ancient Chinese scientist Shen Kuo (1031-1095) was the first person to write of the magnetic needle compass and that it improved the accuracy of navigation by employing the astronomical concept of true north (Dream Pool Essays, AD 1088 ), and by the 12th century the Chinese were known to use the lodestone compass for navigation.

Alexander Neckham, by 1187, was the first in Europe to describe the compass and its use for navigation. In 1269, Peter Peregrinus de Maricourt wrote the Epistola de magnete, the first extant treatise describing the properties of magnets. In 1282, the properties of magnets and the dry compass were discussed by Al-Ashraf, a Yemeni physicist, astronomer and geographer.

In 1600, William Gilbert published his De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth). In this work he describes many of his experiments with his model earth called the terrella. From his experiments, he concluded that the Earth was itself magnetic and that this was the reason compasses pointed north (previously, some believed that it was the pole star (Polaris) or a large magnetic island on the north pole that attracted the compass).

An understanding of the relationship between electricity and magnetism began in 1819 with work by Hans Christian Oersted, a professor at the University of Copenhagen, who discovered more or less by accident that an electric current could influence a compass needle. This landmark experiment is known as Oersted's Experiment. Several other experiments followed, with Andrˇ-Marie Amp¸re, Carl Friedrich Gauss, Michael Faraday, and others finding further links between magnetism and electricity. James Clerk Maxwell synthesized and expanded these insights into Maxwell's equations, unifying electricity, magnetism, and optics into the field of electromagnetism. In 1905, Einstein used these laws in motivating his theory of special relativity, requiring that the laws held true in all inertial reference frames.

Electromagnetism has continued to develop into the twentieth century, being incorporated into the more fundamental theories of gauge theory, quantum electrodynamics, electroweak theory, and finally the standard model. Read more ...




In the News ...





How magnetism manifests in the universe   PhysOrg - June 30, 2015

My main interest is in "cosmic magnetism" - magnets in outer space. Incredibly, magnetism is everywhere in the cosmos: planets, stars, gaseous nebulae, entire galaxies and the overall universe are all magnetic. What does it mean to say that a heavenly body is magnetic? For a solid body like the Earth, the idea is reasonably simple: the Earth's core is a giant bar magnet, with north and south poles. But farther afield, things get weird. Our entire Milky Way galaxy is also a magnet. Just like for the Earth, the Milky Way's magnetism is produced by electrical currents. But while the Earth has a molten core to carry these currents, our galaxy's magnetism is powered by uncounted numbers of electrons, slowly drifting in formation through space. The result is a magnet like nothing you've ever seen.




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