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Facts of the Matter Richard Brill |
Research on magnetism
can be quite attractiveMagnetism is one of the most important of all physical phenomena. Far beyond merely sticking things on the refrigerator, it is so fundamental that without it our universe would be very different if it could exist at all. It is certain that we would not exist, because without magnetism there would be no light, no energy produced in the sun and no electricity.
Magnetism helps us, birds and other animals to navigate and orient. It helps us to determine the ages of layers of rock. It causes the colorful aurora. It generates electricity and drives electric motors, lifts and drives maglev trains and stores volumes of data on computer disks and magnetic tape, audio and video recordings. Oh yeah, it also sticks things to the fridge.
Magnetism is a mysterious force that can best be visualized as electrical inertia.
Magnetism and electricity are closely related in a way that really has no physical equivalent, but can be compared to gravity and matter. Gravity pulls on an object in proportion to its mass, but the object resists being moved in proportion to its inertia.
One way to visualize how magnetism is like inertia is to use a fluid such as water or air as a model. Modeling in this way does not completely explain an inherently unvisualizable phenomenon such as magnetism, but it does come in handy as a comparison. Like model airplanes, mental models resemble the real thing but do not pretend to have all of the essential parts. The map is, after all, not the territory.
If you move your hand through water in the sink, the pool or the ocean, water rushes in to fill the space where the hand displaced the water. Similarly, when something moves through the air, the wind it creates is the result of the displaced air, which eventually fills in the hole caused by its presence.
Fluids do not tolerate holes and will flow to fill them. But they cannot move instantaneously to fill a hole. Because of inertia of the fluid, it takes time to fill the hole, leaving a temporary depression, a hole in the water or lowered pressure in the air. The currents that arise as the fluid moves to fill the hole are analogous to the magnetic field caused when an electric charge is moved.
Just as a current in a fluid moves in a direction toward the hole, a magnet creates lines of magnetic force around it. We can map the shape of this field by shaking iron filings onto a piece of paper placed over a magnet, or with a compass that is moved to various locations around the magnet. The lines of force, or a map of where the compass points, show the direction of the magnetic field, which is what physicists call the distorted space around the magnet.
The ultimate cause of magnetism is in the atom. Electrons move in complex paths in their orbits around the nucleus. As they move they also spin, creating a magnetic field that is like the swirls in cake batter as the mixer spins within it. Of course the magnetic field is not really like the swirls, but it is a way to model or to visualize it.
Each electron creates a minuscule magnetic field, and the movements and spins of all the electrons in the atom are in a magnetic dance, adjusting to minimize their repulsive forces on one another (electrons are negatively charged and like charges repel).
Although the electrons within any one atom are in coordination, atoms that comprise a substance are not typically aligned any particular way. The tiny magnetic fields of each atom are too small to overcome the random motion of the atom in liquids and gasses, and the electrical forces that hold atoms in a crystal structure in solids overwhelm the weak interatomic magnetic forces. Most of the time the random arrangement of the atoms cancels out the magnetism and there is no net magnetic field associated with the substance.
In some materials, most notably iron, cobalt and nickel metals, and in the mineral magnetite -- commonly called lodestone -- the crystal structure just happens to align the atoms so that their magnetic fields also align. The effect is that the magnetic fields of all of the atoms in a crystal add up, making each crystal a tiny magnet.
Most of the time the crystals in the metals are quite small, since we make the metals that way to make them stronger. As with the atoms in a nonmagnetic substance, the magnetic fields of the randomly oriented crystals cancel out, leaving the metal with no intrinsic magnetism.
Small areas of crystals in the metal are organized into randomly oriented magnetic domains, each of which has a net magnetism. But the magnetic fields of the randomly oriented domains cancel out on the large scale, leaving the metal with no overall magnetism.
When iron or other ferromagnetic material is exposed to an external magnetic influence, the atoms in the domains rotate like a compass needle and the iron is attracted to the magnet. When the magnet is removed, the atoms snap back to their original state. If the external magnet is strong enough it can put enough torque on the atoms in all of the magnetic domains to permanently twist them all into alignment. Then the metal becomes a permanent magnet. This is how information is stored on magnetic tape and on computer disks.
These magnetic materials also can be permanently magnetized in other ways. Five hundred years ago, William Gilbert reported that a magnet could be made permanent by pounding on iron while it was hot. Today we understand that the thermal motion of the atoms causes them to move faster when they are at a higher temperature, making it easier to twist them into alignment. We also know that iron that is heated above 1,400 degrees Fahrenheit loses its permanent magnetism, but will become permanently magnetized if cooled below that temperature in the presence of a external magnetic field.
The definitive description of the magnet was published in 1600 by Gilbert, an English physician. Titled "The Magnet," Gilbert's book laid down all that was known about magnets at the time, including some things of his own observations and experiments.
Two hundred years later, in 1819 a Dutch schoolteacher, Hans Oersted accidentally discovered a relationship between electricity and magnetism when he attempted to demonstrate that there was none. He discovered that a wire carrying an electric current creates a circular magnetic disturbance around the wire, similar to the wake of a boat or the shock wave generated by an airplane in flight, which if viewed from behind would appear as circles around the airplane.
Shortly thereafter Michael Faraday discovered that a loop of wire carrying a current would spin if placed between the two poles of a U-shaped magnet, thus inventing the electric motor. Around the same time he discovered that moving a magnet through a coil of wire would cause an electric current to flow in the wire, thus inventing the electric generator.
Even given the advance of the information age, the English scientist's innovations with the electric motor and generator stand arguably as the most important inventions in history. Without the generator, there would be no electricity piped into our homes, offices and factories, and no batteries charged in our cars. Without electric motors there would be no appliances or power tools, no starters in our cars, no radio or television, no recorded music, no computers, no electric lights.
In 1873 James Maxwell, a Scottish physicist, used Faraday's observations, visualizations and relationships to formulate a set of mathematical equations that characterize electricity and magnetism in mechanical terms. These are known simply as Maxwell's equations and hold the same importance in the study of electricity and magnetism as Newton's laws do in the study of forces, inertia and motion.
Maxwell's mathematical description contained an unexpected result: An oscillating electric charge ought to produce a wave of electromagnetism that radiates outward from the charge.
Fifteen years later Heinrich Hertz, a German physicist, conducted experiments that proved that radiation (called "radio waves") emanated from an electric sparking machine. Hertz used a loop of wire with a gap to detect the waves across the room as they induced a spark in the loop.
Hertz also demonstrated that the waves were reflected and refracted like light waves, and that they traveled at the speed of light but with much longer wavelengths.
By formalizing the link between electricity and magnetism Maxwell had created the first unification of forces, but also set the stage for Hertz, whose discoveries in turn paved the way for the invention of the radio by Marconi only seven years later.
But Maxwell's unification went beyond merely linking electricity and magnetism. It also brought to light was light really was. It had been known for nearly a century that light exhibited wavelike properties. Now for the first time, light was understood as an electromagnetic phenomenon -- waves of electromagnetism.
This revelation ultimately led to the two mainstays of 21st-century physics, quantum theory and relativity. It created the awareness that forces of nature could be unified, and led to the search for the Grand Unification Theory (GUT), or the Theory of Everything (TOE), as some physicists (only half jokingingly) like to call it.
Trying to reconcile quantum theory is one of the main problems of modern astronomy and cosmology, just as searching for the GUT occupies the greatest minds of 21st-century physics.
These things may seem completely esoteric to us now, but the study of electricity was seen that way when Benjamin Franklin flew a kite in a thunderstorm (and escaped electrocution by some miracle) 250 years ago. No one then could ever have imagined the uses we find for electromagnetism today. When Franklin was asked about the usefulness of studying electricity, he allegedly replied, "Of what use is a newborn babe?"
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu