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Magnetic field: the story on HearLore | HearLore
Magnetic field
In 1269, a French scholar named Petrus Peregrinus de Maricourt performed an experiment that would fundamentally change humanity's understanding of the invisible world. He took a spherical magnet and carefully mapped its magnetic field using iron needles, discovering that the field lines crossed at two specific points which he named poles. This was the first time anyone had visualized the structure of a magnetic field, revealing that magnets always possess both a north and south pole, no matter how finely they were sliced. Three centuries later, William Gilbert of Colchester replicated Peregrinus's work and became the first person to state explicitly that the Earth itself is a giant magnet. His 1600 publication, De Magnete, established magnetism as a legitimate science and laid the groundwork for all future discoveries in the field. The Earth's magnetic field is produced by the convection of a liquid iron alloy in its outer core, creating a dynamo process where electric currents generate electric and magnetic fields that act back on the currents. This field shields the Earth's ozone layer from the solar wind and has guided navigators for centuries, though the field is not constant and periodically reverses its orientation, with the most recent reversal occurring 780,000 years ago.
The Electric Connection
For centuries, electricity and magnetism were viewed as separate phenomena until 1820 when Hans Christian Ørsted made a startling discovery that shattered the existing foundation of physics. He demonstrated that a current-carrying wire is surrounded by a circular magnetic field, proving that moving electric charges generate magnetic forces. This revelation was quickly followed by André-Marie Ampère, who showed that parallel wires with currents in the same direction attract one another, while those with opposite currents repel. Jean-Baptiste Biot and Félix Savart then announced empirical results about the forces that a current-carrying wire exerts on a small magnet, determining that these forces are inversely proportional to the perpendicular distance from the wire. Ampère published his own successful model of magnetism in 1825, showing the equivalence of electrical currents to magnets and proposing that magnetism is due to perpetually flowing loops of current rather than the dipoles of magnetic charge that had been previously theorized. In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field, formulating what is now known as Faraday's law of induction. This discovery became the basis for many electrical generators and electric motors, fundamentally changing how humanity harnesses energy.
When did Petrus Peregrinus de Maricourt first visualize the structure of a magnetic field?
Petrus Peregrinus de Maricourt first visualized the structure of a magnetic field in 1269. He performed an experiment using a spherical magnet and iron needles to map the field lines crossing at two points he named poles.
What is the source of the Earth's magnetic field and when did it last reverse?
The Earth's magnetic field is produced by the convection of a liquid iron alloy in its outer core creating a dynamo process. The most recent reversal of this field occurred 780,000 years ago.
Who discovered that electricity and magnetism are related and when did this happen?
Hans Christian Ørsted discovered that electricity and magnetism are related in 1820. He demonstrated that a current-carrying wire is surrounded by a circular magnetic field proving that moving electric charges generate magnetic forces.
When did James Clerk Maxwell publish his equations unifying electricity and magnetism?
James Clerk Maxwell developed and published his famous equations between 1861 and 1865. His first set of equations appeared in a paper entitled On Physical Lines of Force in 1861 and were completed in his 1865 paper A Dynamical Theory of the Electromagnetic Field.
Who patented the electric motor using rotating magnetic fields and when was it granted?
Nikola Tesla received a patent for his electric motor in May 1888. He claimed to have conceived his work on rotating magnetic fields in 1882 and gained patents for this work just two months after Galileo Ferraris published his research.
What is the strongest known magnetic field produced by a naturally occurring object?
Magnetars have the strongest known magnetic fields of any naturally occurring object ranging from 0.1 to 100 gigatesla. These fields measure between 10 to the 8 and 10 to the 11 tesla and can distort the electron clouds of atoms.
James Clerk Maxwell developed and published his famous equations between 1861 and 1865, which explained and united all of classical electricity and magnetism into a single theoretical framework. His first set of equations was published in a paper entitled On Physical Lines of Force in 1861, though these equations were valid but incomplete. Maxwell completed his set of equations in his later 1865 paper A Dynamical Theory of the Electromagnetic Field, demonstrating the fact that light is an electromagnetic wave. Heinrich Hertz published papers in 1887 and 1888 that experimentally confirmed this fact, proving that electric and magnetic disturbances travel at the speed of light in space. The full law including Maxwell's correction to Ampère's law is known as the Maxwell, Ampère equation, which describes how mutually changing electric and magnetic fields interact to sustain each other and form electromagnetic waves. This unification showed that the partition of the electromagnetic force into separate electric and magnetic components is not fundamental, but varies with the observational frame of reference. An electric force perceived by one observer may be perceived by another in a different frame of reference as a magnetic force, or a mixture of electric and magnetic forces. This insight would later become a cornerstone of Albert Einstein's special theory of relativity, which showed that both the electric and magnetic fields are part of the same phenomena viewed from different reference frames.
The Quantum Leap
The twentieth century brought a radical transformation to the understanding of magnetic fields, extending classical electrodynamics to work with quantum mechanics. Albert Einstein, in his paper of 1905 that established relativity, showed that both the electric and magnetic fields are part of the same phenomena viewed from different reference frames. The emergent field of quantum mechanics was merged with electrodynamics to form quantum electrodynamics, which first formalized the notion that electromagnetic field energy is quantized in the form of photons. In quantum electrodynamics, the magnitude of the electromagnetic interactions between charged particles and their antiparticles is computed using perturbation theory, producing complex formulas that yield remarkable pictorial representations as Feyn diagrams in which virtual photons are exchanged. Predictions of quantum electrodynamics agree with experiments to an extremely high degree of accuracy, currently about 10 to the negative 12, limited only by experimental errors. This makes quantum electrodynamics one of the most accurate physical theories constructed thus far, though all equations in classical electrodynamics are less accurate than this quantum description. Under most everyday circumstances, however, the difference between the two theories is negligible, allowing classical models to remain useful for practical applications.
The Engine of Modernity
The practical application of magnetic fields revolutionized the world through the development of rotating magnetic fields in electric motors. In 1888, Galileo Ferraris published his research in a paper to the Royal Academy of Sciences in Turin, and Nikola Tesla gained patents for his work on rotating magnetic fields, which he claimed to have conceived in 1882. Tesla received a patent for his electric motor in May 1888, just two months after Ferraris's publication. A rotating magnetic field can be constructed using two coils at right angles with a phase difference of 90 degrees between their alternating current, though in practice, three-phase systems are used where the three currents are equal in magnitude and have a phase difference of 120 degrees. Three similar coils at mutual geometrical angles of 120 degrees create the rotating magnetic field that powers the majority of the world's electrical power supply systems. Synchronous motors use direct current-voltage-fed rotor windings, which lets the excitation of the machine be controlled, while induction motors use short-circuited rotors that follow the rotating magnetic field of a multicoiled stator. The short-circuited turns of the rotor develop eddy currents induced by the rotating field of the stator, and these currents in turn produce a torque on the rotor through the Lorentz force, driving everything from industrial machinery to household appliances.
The Extreme Universe
The largest magnitude magnetic fields produced in a laboratory over a macroscopic volume reached 1.2 kilotesla by researchers at the University of Tokyo in 2018, though the previous record of 2.8 kilotesla was set at VNIIEF in Sarov, Russia, in 1998. The largest magnitude magnetic fields produced in a laboratory occur in particle accelerators, such as the Relativistic Heavy Ion Collider, inside the collisions of heavy ions, where microscopic fields reach 10 to the 14 tesla. Magnetars have the strongest known magnetic fields of any naturally occurring object, ranging from 0.1 to 100 gigatesla, which is 10 to the 8 to 10 to the 11 tesla. These extreme magnetic fields create conditions that challenge our understanding of physics, as they can distort the electron clouds of atoms and affect the very structure of matter. The magnetic field of arbitrary moving point charges is expressed in terms of retarded time, which is the time at which the particle in the past causes the field at the point, given that the influence travels across space at the speed of light. This principle of locality ensures that causal efficacy propagates no faster than light, maintaining the fundamental structure of spacetime as described by special relativity.
The Measurement Challenge
An instrument used to measure the local magnetic field is known as a magnetometer, with important classes including induction magnetometers, rotating coil magnetometers, Hall effect magnetometers, nuclear magnetic resonance magnetometers, superconducting quantum interference device magnetometers, and fluxgate magnetometers. The finest precision for a magnetic field measurement was attained by Gravity Probe B at 10 to the negative 15 tesla. The Hall effect is often used to measure the magnitude of a magnetic field and is used as well to find the sign of the dominant charge carriers in materials such as semiconductors, whether negative electrons or positive holes. Magnetic field lines can be visualized by a set of magnetic field lines that follow the direction of the field at each point, constructed by measuring the strength and direction of the magnetic field at a large number of points. Iron filings placed in a magnetic field form lines that correspond to field lines, though the use of iron filings presents something of an exception to this picture, as the filings alter the magnetic field so that it is much larger along the lines of iron, because of the large permeability of iron relative to air. Magnetic field lines are also visually displayed in polar auroras, in which plasma particle dipole interactions create visible streaks of light that line up with the local direction of Earth's magnetic field.