The second invisible force

We are all familiar with gravity, the invisible force that holds us to Earth and that accounts for all of the motion of stars planets and other bodies in the universe.

You are probably familiar with magnetism, an invisible force between two objects. You've noticed several characteristics of magnets, including:

• Magnets have two ends, called poles.
• Like poles repel each other (Note that gravity is solely an attractive force!).
• Opposite poles attract.
• The magnetic force depends on the distance between magnets. It's strong close up and weak far away.
• Magnets stick to certain kinds of materials which are not magnets, like steel, but not aluminum.

Origin of magnetism

The phenomenon of magnetism has been known of since about 600 BC, when both Greeks and Indians observed that certain stones – lodestones – attracted iron metal (the beginning of the iron age was between 1200 BC and 600 BC, depending on the region). After gravity, it was the second invisible force to be discovered.

Lodestone is a word for one of a few minerals of iron, the most common one being magnetite, Fe₃O₄, or more properly, Fe²⁺Fe₂³⁺O₄^2-, indicating that it contains both Fe(II) and Fe(III) ions. Magnetite is a crystal with a well ordered structure, depicted here from experimental data. The red spheres are oxygen atoms and the green and black spheres are Fe³⁺ and Fe²⁺, respectively.

Magnets always have two poles (or do they?)

Any magnet you can find will have a north and south pole. This figure illustrates that even if we cut a magnet in half, the two halves will exhibit the same polar properties: The new magnets will instantly develop new poles to become dipolar. A bar magnet is an example of a magnetic dipole.

It doesn't matter how small the magnet is, we still observe that all magnets have two poles. Contrast this to charges. While a positive and a negative charge form a dipole (which has an electric field similar to the magnetic field of a bar magnet), it is also possible for positive and negative charges to be isolated. In fact we can create streams or "beams" of negatively- or positively-charged particles.

Magnetic monopoles?

There doesn't seem to be any theoretical barrier to finding a magnetic monopole – an isolated north or south pole, but to date, none have been observed.

Classification of materials by magnetic properties

There exist a relatively small variety of materials, usually crystalline (which means dependable order in the structure), that are naturally magnetic. That is, when formed, unpaired electrons exist in each repeating component (unit cell) of the crystal, and the resulting unpaired spins align, each a small magnet, to form a larger permanent magnet.

There are only a few kinds of permanent magnets, from magnetite to the very strong neodymium magnets (actually Nd₂F₁₄B) now used in modern motors and other devices. These materials are called ferromagnetic. The word (ferro / ferrum = iron) originates from early observations that only iron compounds were permanently magnetic.

Here is a simple representation of a permanent magnet. The spheres stand for unpaired electron spins, each with a north and south pole – each a tiny magnet.

The arrow points toward the north pole of the magnet, the general direction of all tiny poles. In reality, the spins are never so exactly aligned, and every atom, even at very low temperatures, is always in motion. But the average orientation of the spins has a strong alignment that produces permanent magnetism.

Diamagnetism

The term diamagnetic describes materials that don't show any tendency to become magnetized. The electron spins of their constituent atoms are fully paired, and thus don't create a magnetic moment.

Summary of magnetic classifications

Permanent magnets & magnetic fields

Permanent magnets, being made mostly of metallic elements, can be formed into a variety of useful shapes, each of which produced a different field of forces on other magnetic materials. We call that field the magnetic field, or sometimes B-field.

Magnetic fields are analogous to the electric fields produced by charged particles.

The simplest permanent magnet is a bar magnet, just a rod of ferromagnetic material. All magnets have two poles, which we call north and south for how they relate to the large magnet upon which we live, planet Earth. Earth has a ferromagnetic core of molten iron, which produces a magnetic field that we can detect, and indeed use for navigation.

The magnetic field of a bar magnet looks like this:

The blue lines are the direction of the force on a compass needle, were it placed at a particular location near the bar magnet.

Magnetic field strength

The strength of a magnetic field is measured in two main units. The SI unit of field strength is the Tesla ( T ).

Another common unit (still in use because it keeps the numbers on a manageable scale) is the Gauss (G).

Earth's magnetic field

Our planet is a giant magnet. Current theory suggests that electric currents circulating in its molten core form an electromagnet. This is consistent with evidence suggesting that the north-south direction of the field has reversed a few times in Earth's history. The average strength of the field is about 50 microTeslas (μT), or about 0.5 Gauss, enough to cause a lightweight magnet like a compass needle to align with the field lines, which are roughly illustrated below.

The magnetic field of Earth is tilted about 11˚ away from the rotational axis of the planet (in the rough direction of Greenland). The rotational axis passes through what we know as the north and south poles on a map (true north and true south). Indeed, maps are usually drawn so that the vertical lines (meridians) are parallel to that axis.