What is a magnetic field? What are magnetic field lines?

Magnetic fields are historically described in terms of their effect on electric charges. A moving electric charge, such as an electron, will accelerate in the presence of a magnetic field, causing it to change velocity and its direction of travel. This is, for example, the principle used in televisions, computer monitors, and other devices with CRTs (cathode-ray tubes). In a CRT, electrons are emitted from a hot filament. A voltage difference pulls these electrons from the filament to the picture screen. Electromagnets surrounding the tube cause these electrons to change direction, so they hit different locations on the screen.

This is how it works: An electrically charged particle moving in a magnetic field will experience a force (known as the Lorentz force) pushing it in a direction perpendicular to the magnetic field and the direction of motion:

As a result of this force, the charged particle accelerates in the direction of the force (this is Newton's second law). In the diagram above the particle's trajectory will curve upward.

What is the north pole of a magnet and how can I identify it?

The north pole of a magnet is the pole that aligns itself with geographic north. As a result, the geographic north pole of the earth is actually very near the earth's magnetic south pole:

This is sometimes an issue of confusion, but we are stuck with it. What we call "magnetic north" is really magnetic south.

To identify the north pole of a magnet, you can make a compass out of it. Either hang it on a string or float it on water. The pole that faces geographic north is the north pole. Once you have one magnet with poles identified, it is easy to label others, as like poles repel and opposite poles attract.

How strong is the magnetic field at a given distance from the magnet?

Elementary physics states that the magnetic field of a magnetic dipole is approximately proportional to the inverse cube of the distance from the dipole. Therefore, if you double the distance from the magnet, the magnetic field strength will be reduced (roughly) by a factor of 8.

How are magnetic fields measured?

The strength of a magnetic field is measured in units of Gauss (G), or alternatively, in Tesla (T). In the MKS (metric) system of units, 1 T = 1 kilogram*ampere/second^2 = 10^4 G.

For comparison, the magnetic field of the earth at the surface is on the order of 1 Gauss, where that of a Neodymium magnet is on the order of 10^4 Gauss. This means that Neodymium magnets produce magnetic fields tens of thousands of times stronger than those of the earth!

What was the first discovered magnetic material?

The first known magnetic material is a naturally-occurring mineral deposit known as a lodestone. It was found that small rods constructed out of this material would always point north if permitted to swing freely (i.e., when suspended from a string.) This discovery was useful for early navigational systems on ships.

How are electricity and magnetism related?

Magnetism and electricity are closely related phenomena. Electric charge is a fundamental property of matter. Matter is made up of electrons, neutrons, and protons. Electrons have a negative electric charge, while protons have a positive electric charge; neutrons have no electric charge. These tiny particles are the building blocks of atoms. An atom has a net positive electric charge when it loses one of its electrons, and a net negative electric charge when it gains an extra electron. On the other hand, magnetic charges do not exist - Magnetic fields are generated solely by moving electric charges.

An example of the relationship between electricity and magnetism is the motor. In a motor, a voltage is applied across the terminals of a coil of wire. The voltage causes the electrons in the wire to move, which in turn generates a current. This current results in a magnetic field, which interacts with permanent magnets attached to the core of the motor, causing it to move.

Why do magnets have poles?

This is a physical property of magnetic fields. Magnetic fields are vector quantities, meaning they have both a magnitude and a direction. Many measurable physical phenomena are described in terms of scalar quantities, which have only a magnitude. An example of a scalar quantity is temperature; temperature has a magnitude (which you can measure with a thermometer), but no direction.

On the other hand, magnetic fields are directional. In a magnet, the magnetic field vector always points from the north pole to the south pole. In the space around the magnet, the vectors vary in both direction and magnitude. This is the behavior you see when you dump iron filings around a bar magnet, for example. Vector quantities which vary in space are known as fields; thus we have the term magnetic field for the vector field surrounding a magnet.

How does a permanent magnet work?

Some materials have a feature known as ferromagnetism. The prefix "ferro" refers to Iron, which is one such material. Ferromagnetic materials have the ability to "remember" the magnetic fields they have been subjected to.

An atom consists of a number of negatively charged electrons, orbiting around a positively charged nucleus. These electrons also possess a quantity known as spin, which is roughly analogous to a spinning top. The combination of orbital and spin motions is called the angular momentum of the electron. Angular momentum is perhaps most easily understood in the case of the Earth: The earth spins about a central axis, which means it at has an angular momentum around that axis. The planets also have an angular momentum as they revolve about the sun.

Now, the angular momentum of an electron is a vector quantity, meaning it has direction. The motion of the electron produces a current, which in turn generates a tiny magnetic field in the direction given by the angular momentum. Thus an atom can behave like a dipole, meaning "two poles". The direction of the orbital and spin angular momentum of the electron determine the direction of the magnetic field for the electron and the entire atom, thus giving it "north" and "south" poles. Different atoms have different arrangements of electrons into their orbits, and thus have different angular momenta and dipolar properties.

A ferromagnetic material is composed of many microscopic magnets known as domains. Each domain is a region of the magnet, consisting of numerous atomic dipoles, all pointing in the same direction. A strong magnetic field will align the domains of a ferromagnet, or in other words, magnetize it. Once the magnetic field is removed, the domains will remain aligned, resulting in a permanent magnet. This effect is known as hysteresis.

Few materials are actually ferromagnetic; however, all substances have a diamagnetic nature. Diamagnetism means that the molecules within a substance will align themselves to an external magnetic field. The external magnetic field induces currents within the material, which in turn result in an internal magnetic field in the opposite direction. This effect is usually quite small and disappears when the external magnetic field is removed.

Some materials are paramagnetic. This is the case when the orbital and spin motions of the electrons in a material do not fully cancel each other, so that the individual atoms act like magnetic dipoles. These dipoles are randomly oriented, but will align themselves to an external magnetic field. However, when the field is removed, the material is no longer magnetized. Again, this effect is typically small. Neither diamagnetic nor paramagnetic materials exhibit magnetic domains.

The atomic behavior of magnetic materials is actually considerably more complicated than this, as it relies on the theory of quantum mechanics. Quantum mechanics is the theory of physics used to describe the behavior of tiny particles such as electrons; like electromagnetic theory, it is complex and involves advanced mathematics.

How can I make a magnet levitate?

Click on the links below to check out our experiments with diamagnetic materials and superconductors :

Are your magnets graded for quality?

All of of our new, Nickel-plated magnets have a grade of N35, which means a maximum energy product of 35 MGOe. Our new gold plated magnets are all grade N-45. We do not have ratings for any of our surplus magnets.

Why won't my perpetual motion machine work?

Perpetual motion machines are one of the true holy grails of physics; unfortunately, perpetual motion machines do NOT EXIST according to the currently accepted theories. They are denied by the Three Laws of Thermodynamics, which are proven by statistical mechanics, which is derived from quantum mechanics, which has been verified experimentally enough that it would be silly to deny it!

The First Law of Thermodynamics states that Energy is Conserved. This means that the total energy of a closed system must remain constant. The universe, considered as a closed system, thus has constant energy.

Energy exists in many forms: as heat, kinetic (motion) energy, and potential (gravitational, electric, and magnetic) energy, to name a few. The energy can change from one form to another. It can also pass from one system to another. Still, the total energy of a closed system will always be constant. You can neither create nor destroy energy; it always moves elsewhere.

The Second Law of Thermodynamics states that The entropy of a closed system must always increase. The entropy is a measure of the disorder of a system. If you consider the universe itself as a closed system, then the entropy of the universe must always increase. Therefore, the universe is continually moving towards a state of greater disorder!

Consider a glass of water, sitting initially on a table. You decide to push it off the edge and it shatters into fragments. It is now in a more disordered state, so it has greater entropy. The process can never reverse itself; you can't reassemble the glass and put the water back in it.