Magnetic lines of force of a bar magnet shown by iron filings on paper
A magnet is an object that has a magnetic field. The word magnet comes from the Greek "magnítis líthos" (μαγνήτης λίθος), which means "magnesian stone". Magnesia is an area in Greece where deposits of magnetite have been discovered since antiquity.
In the modern sense, a magnet is any material that has a magnetic field. It can be in the form of a permanent magnet or an electromagnet. Permanent magnets do not rely upon outside influences to generate their field. Electromagnets rely upon electric current to generate a magnetic field--when the current increases, so does the field.
Physical origin of magnetism
All normal matter is composed of particles (protons, neutrons, and electrons), and all of these particles have the fundamental property of quantum mechanical spin. Spin gives each one of these particles an associated magnetic field. Because of this, and the fact that the average macroscopic piece of matter contains huge numbers of these particles, it would be expected that all matter would be magnetic. Everyday experience shows that this is not the case.
Within each atom and molecule, the spin of each of these particles is highly ordered as a result of the Pauli Exclusion Principle. However, there is no long range ordering of these spins between atoms and molecules. Without long range ordering, there is no net magnetic field because the magnetic moment of each one of the particles is canceled by the magnetic moment of other particles.
Permanent magnets are special in that long range ordering does exist. The highest degree of ordering exists within magnetic domains. These domains can be likened to microscopic neighborhoods in which there is a strong reinforcing interaction between particles, and as a result, a great deal of order. The greater the degree of ordering within and between domains, the greater the resulting field will be.
Long range ordering (and the resulting strong net magnetic field) is one of the hallmarks of a ferromagnetic material.
Electrons play the primary role in generating a magnetic field. Within an atom, electrons can exist either individually or in pairs within any given orbital. When they are paired, the individuals in that pair always have opposite spin (one up, one down). The fact that the spins have opposite orientation means that the two cancel one another. If all electrons are paired, no net magnetic field will be generated.
In some atoms, there are electrons that are unpaired. All magnets have unpaired electrons, but not all atoms with unpaired electrons are ferromagnetic. In order for the material to become ferromagnetic, not only must there be unpaired electrons present, but those unpaired electrons must interact with one another over long ranges such that they are all oriented in the same way. The specific electron configuration of the atoms (as well as the distance between atoms) is what leads to this long range ordering. The electrons find that they can exist in a lower energy state if they all have the same orientation.
An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops. This coil is known as a solenoid. When electric current flows along the coil, a magnetic field is generated around the coil. The orientation of this field can be determined via the right hand rule. The strength of the field is influenced by several factors, including:
- the number of loops
- the amount of current
- the material in the core
- the machining of the face of the electromagnet
The more loops of wire (within reason) and the greater the current, the stronger the field will be.
If the coil of wire is empty in the center, it will tend to generate a very weak field. Different ferromagnetic or paramagnetic items can be placed in the center of the core with the effect of magnifying the magnetic field (soft iron is commonly used for this purpose). The addition of these types of materials can result in a several hundred to thousand fold increase of field strength.
Magnetic fields are defined by an inverse square law. If the face of an electromagnet is machined to a high degree of precision, it will be able to get much closer to the surface it is trying to attract. Take the case of an electromagnet trying to attract an extremely smooth, flat metal plate. If the electromagnets face is extremely smooth and flat as well, it will be able to be in much closer proximity to the plate. The closer it can get, the stronger the attraction will be.
Characteristics of magnetic materials
Permanent magnets and dipoles
All magnets are dipoles, but that is not really a statement about having two poles. The poles are not a pair of things on or inside the magnet. In the accompanying image, the poles look like specific locations as a result of the fact that the highest surface intensity of the field occurs at the poles.
To understand the concept of poles, imagine a row of people who are all facing the same direction and standing in line. While there is a "face" end of the line and a "back" end of the line, there is no one place where all of the faces are and all of the backs are. The person at the front of the face end has a back; and the person at the back end has a face. If you divide the line into two shorter lines, each one of the shorter lines still has a face end and a back end. Even if you pull the line completely apart so that there are just individuals standing around, each one of the individuals still has a face and a back.
The same holds true with magnets. There is not one place where all of the north or south poles are. The poles simply represent the fact that the spin of most of the particles are oriented in such a way that there is long range ordering. If a magnet is divided in two, two magnets will result--and both magnets will have a north and a south pole. Those smaller magnets can then be divided, and all of the resulting pieces will have both a north and south pole. Even down to the molecular level, there is no point where a magnet can be divided into an individual north pole and an individual south pole.
There are theories involving the possibility of north and south magnetic monopoles, but as of January 2005, no magnetic monopole has been found.
North/South Pole designation and the Earth's magnetic field
A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet.
The Earth's current geographic north is actually its magnetic south. Unfortunately, it has been shown that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be backward at some time in the future (see Earth's magnetic field).
Fortunately, by using an electromagnet and the right hand rule, the orientation of the field of a magnet can be defined without reference to the Earth's geomagnetic field.
Explaining magnetic attraction
Although each atom in a ferromagnetic material is a magnetic dipole due to its quantum properties, it is easier to understand magnetic attraction if these dipoles are instead viewed as a current-carrying loop (which also acts like a magnetic dipole) and the magnet used as an example is a cylindrical bar magnet. In a permanent magnet, each atom's current loop flows in the same direction, as the magnetic dipoles of each atom are aligned. Because opposite sides of neighboring atoms are adjacent, these currents effectively cancel out within the magnet. This cancellation leaves only a net current around the surface of the magnet. Because this is equivalent to a current flowing around a cylinder, this produces a magnetic field equivalent to that of a solenoid.
Because of this, the attraction between opposite magnetic poles and the repulsion between similar magnetic poles (in both permanent magnets and electromagnets) can be explained in terms of attractive and repulsive forces between current carriers. Magnets end to end in the same orientation, like two elements carrying currents flowing in the same direction, create magnetic fields that attract each other. Magnets end to end in opposite orientations, like two elements carrying currents flowing in opposite directions, create magnetic fields that repel each other.
The magnetic force between current carriers is also a major concern when designing powerful electromagnets. If an electromagnet does not have sufficient structural stability, the attractive force between neighboring loops of wire will cause the electromagnet to be crushed by its own magnetic field.
Common uses for magnets
- Magnetic recording media: Common VHS tapes actually contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. This is why magnets will destroy the information in these types of tapes. Common audio cassettes also rely on magnetic tape.
Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individuals financial institution and connect with their account(s).
Loudspeakers and microphones: Loudspeakers actually rely on a combination of a permanent magnet and an electromagnet. A speaker is fundamentally a device to convert electric energy (the signal) into mechanical energy (the sound). The electromagnet carries the signal, which generates a changing magnetic field that pushes and pulls on the field generated by the permanent magnet. This pushing and pulling moves the cone, which creates sound. Not all speakers rely on this technology, but the vast majority do. Standard microphones are based upon the same concept, but run in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law ). This voltage in the wire is now an electric signal that is representative of the original sound.
Electric motors and generators: Electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.
Transformers: Transformers are devices that transfer electric energy between two devices that are electrically disconnected via magnetic coupling.
How to magnetize materials
When a ferromagnetic material is exposed to an external magnetic field and then removed from the field, the material can retain some of the field. This effect is known as hysteresis--the material retains a magnetic field. At this point, the material has been magnetized.
There are several ways to magnetize an item. If an item is placed into the center of a coil of wire, and then a large amount of direct current is sent through the wire, the item can be magnetized. The orientation of its poles will match the orientation of the field generated by the coil. A strong permanent magnet can also be used to magnetize another item. To do this, hold the magnet at one end of the item. Slowly slide the magnet along the length of the item. Then pull the magnet away so that the two items are no longer attracted to one another. Then, place the magnet back against the item at the same end it began at. Slide the magnet along the length of the item, and again remove the magnet. By repeating this procedure, the item can be magnetized.
In the case of an electromagnet, current flowing through its coil is all that is required to magnetize it.
How to demagnetize materials
Permanent magnets can be demagnetized in the following ways:
- Heat. Heating a magnet past its Curie point will destroy the long range ordering.
- Contact. Stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.
- Hammering and/or Jarring. Such activity will destroy the long range ordering within the magnet.
- Being placed in a solenoid which has an alternating current being passed through it. The alternating current will disrupt the long range ordering, in much the same way that direct current can cause ordering.
In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.
Types of permanent magnets
Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets. However, a formula exists for the simple case of the force between two magnetic poles:
F is force (SI unit: newton)
m is pole strength (SI unit: weber)
μ is the permeability of the intervening medium (SI unit: tesla meter per ampere)
- r is the separation (SI unit: meter).
Last updated: 10-17-2005 04:33:51