What are the differences between the different magnet formulations you sell? NdFeB (Neodymium-Iron-Boron) -- The most powerful 'rare-earth' permanent magnet composition known to mankind, and our specialty. This formulation is relatively modern, and first became commercially available in 1984. NdFeB magnets have the highest B, Br, and BHmax of any magnet formula, and also have very high Hc (see below for definitions). They are however very brittle, hard to machine, and sensitive to corrosion and high temperatures. Useful in the home, workshop, pickup truck, laboratory, wind turbine, starship and more. We carry both new and surplus stock in many sizes and shapes.In almost all magnet applications, NdFeB are the best choice for incredible strength and coercivity at a reasonable price! In power generation applications, NdFeB magnets can be expected to give 4-5 times the power output of ceramic magnets.
Ferrite (Ceramic) -- Also known as 'hard ceramic' magnets, this material is made from Strontium or Barium Ferrite. It was developed in the 1960s as a low-cost and more powerful alternative to AlNiCo and steel magnets. Less expensive than NdFeB magnets, but still very powerful and resistant to demagnetization. Useful everywhere. We carry both new and surplus in multiple shapes and sizes. Ferrite magnets are lower in power (B, Br, BHmax) compared to other formulations, and are very brittle. However, they have very high Hc and good Tc (see below), and are quite corrosion-resistant. A very cost-effective choice.
AlNiCo (Aluminum-Nickel-Cobalt) for medium strength and excellent machinability. Developed in the 1940s and still in use today. They perform much better than plain steel, but are much weaker in strength (lower B, Br and BHmax) and must be carefully stored since they are prone to demagnetization. Contact with a NdFeB magnet can easily reverse or destroy the field of an AlNiCo magnet.
SmCo (Samarium Cobalt)-- for high power and resistance to high temperatures and corrosion. Developed in the 1970s, these were the first so-called 'rare earth' magnets. They are almost as powerful as NdFeB magnets, and far more powerful than all the others (high B and Br). They are the most expensive magnet formulation, and usually only used where resistance to high temperatures (high Tc) and corrosion are needed. Also very brittle and hard to machine.
Bonded (flexible)-- magnets are a rubberized formulation often seen on refrigerators and magnetic signs. Though they may be manufactured from any magnet formulation when powerdered and mixed with rubberizer, the result is always less powerful than a traditional sintered magnet of any formula. Used only where unusal and difficult shapes are needed.
How are your magnets measured and graded for strength, quality, etc.? Magnet Strength Measurements (B)--The units for measuring the field strength (flux density) of a magnet are Gauss or Tesla. 1 Tesla = 10,000 Gauss. The Earth's magnetic field is on the order of 1 Gauss. There are different ways to classify and measure field strength:
B (flux density): This is the measurement (in Gauss or Tesla) you get when you use a gaussmeter at the surface of a magnet. The reading is completely dependant on the distance from the surface, the shape of the magnet, the exact location measured, the thickness of the probe and of the magnet's plating. Steel behind a magnet will increase the measured 'B' significantly. Not a very good way to compare magnets, since B varies so much depending on measurement techniques.
Br (residual flux density): The maximum flux a magnet can produce, measured only in a closed magnetic circuit. Our figures for each magnet are provided to us by the magnet manufacturer. They are a good way to compare magnet strength...but keep in mind that a magnet in a closed magnetic circuit is not doing any good for anything except test measurements.
B-H Curve: Also called a "hysteresis loop," this graph shows how a magnetic material performs as it is brought to saturation, demagnetized, saturated in the opposite direction, then demagnetized again by an external field. The second quadrant of the graph is the most important in actual use--the point where the curve crosses the B axis is Br, and the point where it crosses the H axis is Hc (see below). The product of Br and Hc is BHmax. If we have these measurements available, they are provided to us by the magnet manufacturer--very complicated and expensive equipment is needed to plot a B-H curve. Magnet Quality (BHmax): The quality of magnetic materials is best stated by the Maximum Energy Product (BHmax), measured in MegaGauss Oersted (MGOe). This is because the size and shape of a magnet and the material behind it (such as iron) have a large effect on the measured field strength at the surface, as does the exact location at which it measured. All of our Nickel-plated NdFeB magnets are grade N35 (BHmax=35 MGOe) and all of our Gold-plated NdFeB magnets are grade N45 (BHmax=45 MGOe). This gives about a 5% difference in strength, and a 150% difference in cost...it is wise to balance your magnet strength needs by cost too. Other magnets are measured the same way -- a grade 8 ferrite magnet (grade C8) has BHmax=8 MGOe.
Coercivity (Hc): This measures a magnet's resistance to demagnetization. It is the external magnetic field strength required to magnetize, de-magnetize or re-magnetize a material, also measured in Gauss or Tesla.
How does temperature affect the behavior of a permanent magnet? I've read about patent problems concerning NdFeB magnets. Are your magnets licensed? Can I cut, drill or machine magnets to my own sizes and shapes? How can you ship magnets safely? Don't they affect airplane compasses? What safety issues should I be aware of when handling magnets? 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.
Magnetic fields are perhaps more easily understood in terms of magnetic field lines. Field lines, also known as lines of force, define the direction and strength of the magnetic field at any local in space. As explained later, magnetic fields have both a direction and strength (or "magnitude"). The direction of the field lines indicates the direction of the field, while the density of the field lines indicates the magnitude of the field. Thus at points where the field lines are closer together, the field is stronger. Field lines are described mathematically with a quantity known as flux.
Magnetic fields are commonly a result of magnetic dipoles. A simple example of a magnetic dipole is the bar magnet:
As you can see, the magnetic field lines always begin on the north pole of a magnet, and end on the south pole. This diagram illustrates the magnetic field lines of a typical magnetic dipole.
Magnetic dipoles always like to align themselves parallel to an external magnetic field, so the dipole's field matches the one applied to it. This is why bar magnets line up north-to-south. It also explains the behavior of a compass needle, which, being composed of Iron (a ferromagnet), behaves like a magnetic dipole.
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!
Technically, Gauss and Tesla are units of magnetic induction, also known as magnetic flux density. (This term is described in an earlier question.) Quantitatively, the force on a charged particle q moving with velocity v is given by the vector equation F = qv x B, where B is the magnetic induction.
Another common quantity of interest is the corecivity or corercive force of a magnet. Also measured in Gauss, the coercivity is the magnetic field required to demagnetize a material. For example, Neodymium magnets typically have a coercivity of about 12000 Gauss. Please note that the coercivity is the magnetic field required for de-magnetization. It is not actually a mesaure of the "strength" of the magnet, although the highly coercive magnets are usually quite strong.
The maximum energy product is used to determine the quality of magnetic materials. This is typically measured in Megagauss Oersted (MGOe - quite a mouthful!). The maximum energy product basically determines what materials make the best magnets.
Magnetic field strengths are measured with devices known as magnetometers, also called Gaussmeters.
Why are there so many odd units and terms used to describe magnetic fields? Keep in mind that magnetic fields appear any a large variety of contexts besides permanent magnets. Engineers and scientists regularly deal with magnetic fields in the study of electric circuits, motors, optics, and various other fields of technology. The power of the magnetic fields involved can vary a great deal, so many ways of measuring things have developed through the ages.
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.
Another example of the relationship between magnetism and electricity is the Lorentz force mentioned previously. Perhaps the most significant relationship between electricity and magnetism is light, which is known to physicists as an electromagnetic wave. Light waves are oscillating patterns of electric and magnetic fields, propagating through space at the speed of light (3x10^8 meters/second).
Light is the best known example, but microwaves, radio waves, X-rays, infrared and ultraviolet light are also electromagnetic waves.
Electric and Magnetic phenomena are intricately described by a collection of physical laws, known as Maxwell's equations. Fully understanding these complex equations require a thorough knowledge of calculus and differential equations. For more information, take a course in electromagnetic theory from your local university. :-)
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.
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...
