Magnetism FAQs

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.

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.

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!

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.

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.

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.

Actually, the ferromagnetic properties of a material can change for temperatures well below the Curie temperature. Neodymium magnets, for example, begin to lose their magnetization at temperatures as low as 300 F.

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.

Now a perpetual motion machine has, by definition, moving parts. The motion results in the transfer of heat through friction and air resistance. This results in a loss of energy by the device; the First Law implies that the total energy of the universe is conserved, so the energy is actually being transferred elsewhere.

Assuming you don't add energy to this system, its energy will continually decrease and its entropy will likewise increase. Eventually it will slow and stop. Therefore, there is no such thing as a perpetual motion machine.

However, you're welcome to try...