The ability of magnets to attract metal objects was already known more than 2000 years ago in ancient Greece, China and India. Our modern understanding of magnetism has its origin in the early 19th century, when the Danish physicist Hans Christian Oersted discovered its connection with electricity. His research led to the Scottish physicist James Clerk Maxwell's first unification of forces in 1873. He described electricity and magnetism as two aspects of the same phenomenon. Maxwell's four equations of electromagnetism have become so iconic that nowadays you can find them printed on a T-shirt (check out a bookstore at a college or a university near you).
We're all familiar with magnets - pieces of metal (usually iron but also nickel or cobalt) that attract other metallic objects. (The flexible fridge magnets are also made of these metals, or their compounds, by mixing them with plastic.) Every magnet has a North pole and a South pole, near which the magnetic forces are strongest. The poles always come in pairs. Even if you cut the magnet in half, the two halves will each have a North pole and a South pole. The same poles on two magnets will repel each other while the opposite poles will attract. One can trace the invisible lines of magnetic force starting in one of the poles and ending in the other, as illustrated by the directions of the compass needles in this image. Strictly speaking the magnetic field lines don't end at the poles. They continue inside the magnet and emerge from the opposite pole thus forming closed loops.
Oersted was the first to observe how a metal wire with electrical current running through it affected a compass needle. His experiments led to the conclusion that electrical current - a stream of electrons speeding along the wire - also generated magnetic field. In this case we cannot talk about magnetic poles though, since the lines of magnetic forces encircle the wire like hula hoops.
An electron, the negatively-charged elementary particle, has its own intrinsic magnetic field. You can think of it as a result of the electric current caused by the electron itself spinning on its axis. Electrons in an atom interact on the quantum level and these interactions force them to always be parallel to one another, their magnetic fields aligned in the same or the opposite direction. Electrons within an atom pair up to neutralize each other's magnetic field, but if an atom has an unpaired electron, it becomes a tiny magnet itself. Under the influence of external magnetic forces these atoms tend to align the directions of their magnetic fields and thus the whole piece of metal becomes magnetic. We call it ferromagnetism.
The Earth itself has a magnetic field, thanks to electric currents in its outer core. Its magnetic poles don't coincide with its geographic poles. The magnetic poles keep moving around and they even reverse every few hundred thousand years. The Magnetic North Pole is currently some 800 km away from the Geographic North Pole, in the Northern Canada. The Chinese were the first to use Earth's magnetic field for navigation, having invented the compass more than one thousand years ago.
The most recent twist to our understanding of magnetism (ignoring quantum mechanics for the moment) came in 1905, with Albert Einstein publishing his Special Theory of Relativity. He devoted much of his seminal paper to the analysis of movement of charged particles (electrons) with speeds approaching the speed of light. In it, he showed that electrostatic and magnetic forces are linked by the same math that links space dimensions and time in his Special Theory of Relativity. Based on this, it is possible to understand magnetic forces, and to correctly derive their properties, as a relativistic effect of fast motion of electrostatic charges. This reasoning doesn't strictly apply to ferromagnetism, which is a quantum phenomenon, but with a dose of poetic licence you could say that your fridge poetry is held in place by relativity.
We're all familiar with magnets - pieces of metal (usually iron but also nickel or cobalt) that attract other metallic objects. (The flexible fridge magnets are also made of these metals, or their compounds, by mixing them with plastic.) Every magnet has a North pole and a South pole, near which the magnetic forces are strongest. The poles always come in pairs. Even if you cut the magnet in half, the two halves will each have a North pole and a South pole. The same poles on two magnets will repel each other while the opposite poles will attract. One can trace the invisible lines of magnetic force starting in one of the poles and ending in the other, as illustrated by the directions of the compass needles in this image. Strictly speaking the magnetic field lines don't end at the poles. They continue inside the magnet and emerge from the opposite pole thus forming closed loops.
Oersted was the first to observe how a metal wire with electrical current running through it affected a compass needle. His experiments led to the conclusion that electrical current - a stream of electrons speeding along the wire - also generated magnetic field. In this case we cannot talk about magnetic poles though, since the lines of magnetic forces encircle the wire like hula hoops.
An electron, the negatively-charged elementary particle, has its own intrinsic magnetic field. You can think of it as a result of the electric current caused by the electron itself spinning on its axis. Electrons in an atom interact on the quantum level and these interactions force them to always be parallel to one another, their magnetic fields aligned in the same or the opposite direction. Electrons within an atom pair up to neutralize each other's magnetic field, but if an atom has an unpaired electron, it becomes a tiny magnet itself. Under the influence of external magnetic forces these atoms tend to align the directions of their magnetic fields and thus the whole piece of metal becomes magnetic. We call it ferromagnetism.
The Earth itself has a magnetic field, thanks to electric currents in its outer core. Its magnetic poles don't coincide with its geographic poles. The magnetic poles keep moving around and they even reverse every few hundred thousand years. The Magnetic North Pole is currently some 800 km away from the Geographic North Pole, in the Northern Canada. The Chinese were the first to use Earth's magnetic field for navigation, having invented the compass more than one thousand years ago.
The most recent twist to our understanding of magnetism (ignoring quantum mechanics for the moment) came in 1905, with Albert Einstein publishing his Special Theory of Relativity. He devoted much of his seminal paper to the analysis of movement of charged particles (electrons) with speeds approaching the speed of light. In it, he showed that electrostatic and magnetic forces are linked by the same math that links space dimensions and time in his Special Theory of Relativity. Based on this, it is possible to understand magnetic forces, and to correctly derive their properties, as a relativistic effect of fast motion of electrostatic charges. This reasoning doesn't strictly apply to ferromagnetism, which is a quantum phenomenon, but with a dose of poetic licence you could say that your fridge poetry is held in place by relativity.
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