Magnetic field
A magnetic field is an area of influence created by magnets or electric currents, arising from the movement of electrical charges, such as electrons. It has two key components: direction and force, with the force being calculable using the equation F = qvB, where F represents the force, q the moving charge, v its velocity, and B the magnetic field strength, measured in teslas (T). The concept of magnetism dates back to Ancient Greece and India, with notable early discoveries made by figures like Thales and Sushruta. Over the centuries, the understanding of magnetism evolved, notably by scientists such as William Gilbert, who identified Earth's magnetic field, and John Michell, who established the inverse square law of magnetism.
In contemporary science, magnetic fields play a crucial role in theories that seek to unify fundamental forces in the universe. They also find application in health care, where magnetic therapy is utilized for pain management and various ailments, despite lacking FDA approval and scientific backing for its efficacy. Magnetic resonance imaging (MRI), a common medical diagnostic tool, employs strong and consistent magnetic fields to produce detailed images of the body’s internal structures. Some animals possess a unique ability known as magnetoreception, allowing them to navigate using Earth's magnetic field.
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Magnetic field
A magnetic field is the field of influence produced by a magnet or electrical current. It is created by moving electrical charges, such as the motion of electrons around the nucleus of an atom, and by the natural properties of elementary particles. In addition, electrical currents can be used to create a magnetic field, and magnetic fields can be used to create and otherwise affect an electrical current.
There are two aspects of a magnetic field: direction and force. Force can be expressed by the equation F = qvB, where F is the force exerted by magnetic field B on the moving electrical charge q (if qis moving perpendicular to the field), v is the velocity of charge q, and B is the strength or magnitude of magnetic field B. F is measured in teslas (T), named after Austrian-born engineer Nikola Tesla (1856–1943). One tesla is equal to one weber per square meter (Wb/m2), or one kilogram per second squared–ampere (kg/s2·A).
Background
Magnetic properties were first discovered in Ancient Greece and India during the fifth century BCE. Greek philosopher Thales of Miletus and Indian surgeon Sushruta both independently discovered magnetism, though neither could understand it or explain how it worked. Sushruta used magnets for their healing properties, while Thales used them to explain physical philosophy.
In the tenth century CE, the Chinese used magnets for navigation. Lodestone compasses were used, as the naturally magnetized lodestone was attracted to the Earth’s magnetic North Pole. Such compasses and navigation techniques were not used in Europe until nearly two hundred years later.
European scientists were unclear about the relationship between magnetism and electricity until 1550, when Italian scientist and philosopher Gerolamo Cardano (1501–76) distinguished electricity and magnetism for the first time. English scientist and philosopher William Gilbert (1544–1603) followed up on Cardano’s theories, establishing the Latin term electricus, which is derived from the Greek word for amber. In 1600, Gilbert published De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth), which established magnetism as a field of study within physics. While preparing his experiments in advance of his book, Gilbert deduced that Earth has its own magnetic field.
English philosopher and clergyman John Michell (1724–93) mathematically demonstrated the inverse square law of magnetism and magnetic fields, which states that as distance increases between magnetic fields, the strength decreases in inverse proportion to the square of the distance. Between Michell’s time and that of Albert Einstein (1879–1955), much of the work with magnetic fields was closely intertwined with electrical fields. Einstein used magnetic fields in formulating his theories of general and special relativity.
Overview
Today, magnetic fields are fundamental to the formulation of so-called theories of everything, such as unified field theories, which attempt to mathematically tie together the fundamental forces of the universe: electromagnetism, gravity, and the strong and weak nuclear forces. Many scientists believe that such a unified theory would help explain the origins of the universe and how it operates at both the macro and the quantum level.
Magnets and magnetic fields are often used by health care providers to reduce the pain associated with such conditions as fibromyalgia, arthritis, and migraines. Magnet therapy has also been found to be successful in treating depression and depressive disorders. Others have used magnetism to reduce vertigo and balance issues.
Magnet therapy and magnetic field therapy were developed during the middle part of the twentieth century. These therapies are not recognized or approved by the US Food and Drug Administration, though many people use them as alternative, holistic therapies. Proponents of magnet therapy claim that the magnetic fields stimulate blood flow and relax muscles. Scientific studies have found no medical basis for these claims, but they have observed notable placebo effects.
Magnetic resonance imaging (MRI) is a medical imaging technique that uses magnetic fields and water within the body. The patient lies down on a table that can slide in and out of the MRI machine. The machine creates a magnetic field around the individual, allowing a technician to observe the functionality of the person’s body via a computer screen. The magnetic field used in MRI needs to be strong and consistent. Variations on MRI include functional magnetic resonance imaging (fMRI), nuclear magnetic resonance imaging (NMRI), and magnetic resonance tomography (MRT).
Some animals, including homing pigeons, dolphins, bats, moles, and certain species of fox and cattle, have a sense known as magnetoreception, which allows them to use magnetic fields to navigate and orient themselves and determine direction and distance. Studies have been conducted to determine whether homing pigeons navigate primarily by the sun or by Earth’s magnetic field. Researchers found that while a pigeon’s ability to navigate was enhanced when the sun was used as a reference point, the birds were still able to accurately navigate on overcast days and at night. However, when a magnet was attached to a pigeon’s body, it became disoriented and was unable to find its way.
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