Introduction to Electromagnetism

Before diving into the field of magnetic recording, it is necessary to have a basic understanding of the interaction between electricity and magnetism: what we call electromagnetism. Most physics students can probably skip this page entirely unless they want a bit of a review, but I wanted this website to be accessible to people both with and without a strong physics background: so, on to the basics.

Moving Charges Create Magnetic Fields

Hans Christian Oersted, a professor at the University of Copenhagen, had a strong metaphysical belief that all forces were related to each other. In particular, he suspected that magnetism might be a form of electricity. This is not a surprising conjecture, since magnetism and electricity have several similar features on the surface. Electricity is the interaction between positive and negative charges; magnets have north and south poles. Like charges repulse each other and opposite charges attract; similarly, the north pole of a magnet is attracted to the south pole of another magnet, but two north or south poles repulse each other.

Oersted, while preparing for a lecture, tried placing a current-carrying wire above a compass such that the wire was perpendicular to the needle in the hopes of being able to demonstrate a connection between electricity and magnetism. Nothing happened; however, after class, he tried orienting the wire parallel to the compass -- and the needle swung away from the current.

It turns out that a current-carrying wire produces a magnetic field. The magnetic field lines around such a wire (conventionally drawn as arrows leaving the north pole of a magnet and entering the south pole) form concentric circles around the wire; there are no obvious north or south poles as in a natural magnet. The direction of the field lines can be found using the right-hand rule: if your right thumb points in the direction of the current flow (the direction a positive charge would be moving in the wire), your fingers will curl in the direction of the magnetic field. Reversing the direction of the current reverses the direction of the field lines, as shown in Figure 1.

If a current-carrying wire is made into a loop instead of a straight line, its magnetic field will be as shown in Figure 2; this is easily understood if the loop is considered to be made up of many small, straight segments, each with a magnetic field as was described above. The interesting feature of this effect is that the loop now behaves like a bar magnet: it appears to have a north and south pole, with magnetic field lines leaving one and circling around to enter the other.

An even stronger electromagnet can be produced if the wire is bent into a helical coil, or solenoid. The strength of the magnetic field around a coil is proportional to the amount of current running through it and the number of turns (coils) in the wire. This provides us with a simple way to create magnets that are highly variable and easy to control.

What actually generates the magnetic field is not the wire, but the moving charges within it. What Oersted demonstrated was the combined magnetic fields of all the electrons traveling through the wire. An electron orbiting an atom is also a moving charge; thus, atoms themselves are tiny magnetic dipoles.

Magnetic Induction

In some materials, like iron, the atoms tend to form magnetic domains, small regions within the metal where all the atomic magnets line up in the same direction. Generally speaking, the domains in a material will be randomly oriented and cancel each other out so that there is no net magnetic field.

If a permanent magnet is brought close to the iron, the magnetic domains will begin to line themselves up with the magnetic field of the permanent magnet, due to the interaction between north and south poles. This is called induced magnetism. When the magnet is removed, the domains will become randomized again to a certain extent, but will retain some of their alignment; what remains is called the residual magnetism.

It is now possible to understand how heat can damage magnetic storage media. Heat randomizes the domains by "jogging" everything about inside the material; this destroys any information saved via magnetic alignment.

Changing Magnetic Fields Induce Currents

The relationship between electricity and magnetism is not a one-way street; in the same way that a moving charge creates a magnetic field, a changing magnetic field induces a voltage in a coil placed near it. The induced voltage is proportional to the rate of change of the magnetic field, the field's strength, and the number of turns in the coil. Lenz's Law, fundamentally an expression of the conservation of energy, states than an induced current will oppose the change that creates it; in other words, the direction of the voltage will be such that the resulting current would produce an electromagnet that opposes the original magnetic field.

An important result of Lenz's law is that a magnet oscillating at a certain frequency will induce an AC current with the same frequency, just as an AC current creates a magnetic field oscillating at the frequency of the current. These two effects lay the groundwork for magnetic storage of information.