Magnetism Grade 8

[Darci Ziler]

Thenumber comes from an actual material property, the Maximum Energy Product of the magnet material, expressed in MGOe (Mega Gauss Oersteds). It represents the strongest point on the magnet's Demagnetization Curve, or BH curve.

You can measure the strength of a magnet by looking at the pull force or the magnetic field. There are also other ways of testing, but pull force and magnetic field are the two most common ways to measure a magnets strength. It also depends on what you mean by strength as different situations will require different specs.

Pull force is how much force you have to pull on a magnet to move it away from something, such as a steel surface or another magnet. We show this force in pounds on our site, though you could also express it in Newtons, or even kilograms. The specific way the magnet is tested can have a huge influence on the measured strength.

We show several different measures of strength, as described in our pull force FAQ answer. The number we use most is Pull Force, Case 1. It is the force required to pull a magnet directly away from a steel surface. It is a great reference for magnet strength, expressed as a single number. Even if your application doesn't pull on the magnet in the same way, this is often a good number to use to compare the strength of different magnets.

The magnetic field strength is a measurement of the magnetic field's strength and direction at a particular point near the magnet. It is expressed in Gauss or Tesla (1 Tesla = 10,000 Gauss). It depends on the size, shape and grade of the magnet, where the measurement is performed, and the presence of any other magnets or ferromagnetic materials nearby. Our Surface Fields article is a good place to learn more about this.

Many of our magnets are offered in grade N42, which is a great balance between cost, strength and performance at higher operating temperatures. You can get the same strength as an N52 magnet by using a slightly larger N42 magnet.

If you have slightly elevated temperatures, in the 140F to 176F range (60C -80C), N42 magnets might actually be stronger than N52. This is especially true if your magnet shape is very thin. See our detailed article on Temperature and Neodymium Magnets for more details.

Residual Flux Density , Br, is the magnetic induction remaining in a saturated magnetic material after the magnetizing field has been removed. Scroll down to the last section of this article for a more detailed explanation.

This number is a material property which is independent of the magnet shape. Grade N42 magnets have a Br of 13,200 Gauss, while N52 magnets can be as high as 14,800 Gauss. See our specs page for more Br values for various neodymium magnet grades.

The surface field is the strength of the magnetic field measured right at the surface of the magnet. It's the field strength you might measure if you could squish a magnetometer's sensor right up against the surface. This number depends on the magnet material, the shape of the magnet and how it's used in a magnetic circuit.

Neodymium magnets are by far the strongest type of permanent magnet available. Magnet advancements are a history of increasing coercivity. Neodymium magnets are both stronger and less apt to be demagnetized than other magnet types.

The performance of a magnet material is defined by that material's hysteresis curve, also known as a Demagnetization Curve or BH curve. The Maximum Energy Product is the point on this curve where the B value multiplied by the H value is at its maximum.

At a point on the curve, multiply the "B" value (in kilo Gauss) by the "H" value (in kilo Oersted) to get the Maximum Energy Product (in Mega Gauss Oersted, or MGOe). For example, grade N42 has a Max Energy Product of 42 MGOe.

Magnets with a bigger Maximum Energy Product will have greater strength. Specifically, the shape of the BH Curve indicates both how strong a magnet is and how strong of a magnetic field you would need to demagnetize the magnet.

Consider a neodymium magnet sitting inside a magnetizer. The magnetizer is essentially a coil of wire wrapped around the magnet, through which we will apply a very strong current to create a magnetic field.

The magnet we will start with has just been manufactured, but not yet magnetized. The magnetic field it creates is zero (B). There is no current running through the wire, so the applied field (H) is also zero. Let's represent this point with a dot at the zero location on the graph, point #1.

Now, let's briefly run a terrifically strong current through the wire, placing the magnet in a uniform magnetic field. Keep increasing the current, and the applied field increases. If we measure the magnetic field, we also see an induced magnetic field, made from the magnet.

Now, let's turn the current off. The Applied field (H) drops to zero, but there remains a magnetic field produced by the magnet, shown as point #3. This point is also called Br, Br max, the Residual Induction or the Residual Flux Density.

By applying progressively more current in this direction, we can find the shape of the normal curve in the second quadrant (the upper left hand quarter) of the BH Curve graph. Where the Induced field reaches zero, at point #4, is called the Coercive force, Hc. This is the magnet's Coercivity: the measure of the magnet's resistance to demagnetization by an external magnetic field.

The farther left on the graph this point is located, the stronger the magnetic field you need to demagnetize the magnet. Not only are neodymium magnets strong, but they have the highest coercivity values of all permanent magnet types.

The shape of the curve shows how the magnet works in actual applications. The actual operating point on the graph depends on the shape of the magnet and how it is used in a magnetic circuit (its Permeance Coefficient). For some examples of how to use this information and to find the actual operating point on this curve, see our article on Temperature and Neodymium Magnets.

A magnet is a rock or a piece of metal that can pull certain types of metal toward itself. The force of magnets, called magnetism, is a basic force of nature, like electricity and gravity. Magnetism works over a distance. This means that a magnet does not have to be touching an object to pull it.

It is possible to make a magnet by taking an existing magnet and rubbing another piece of metal with it. The new piece of metal must be rubbed continuously in the same direction. This will make the electrons in that metal start to spin in the same direction.

Electricity can also create magnets. Electricity is a flow of electrons. As electrons move through a piece of wire they have the same effect as electrons spinning around the nucleus of an atom. This is called an electromagnet.

Magnets strongly attract objects that contain iron, steel, nickel, or cobalt. Magnets also attract or repel (push away) other hard magnets. This happens because every magnet has two opposite poles, or ends: a north pole and a south pole. North poles attract the south poles of other magnets, but they repel other north poles. Likewise, south poles attract north poles, but they repel other south poles.

One of the earliest uses of magnets was in compasses. A compass is a needle-shaped magnet that is free to turn around. The planet Earth is a giant magnet. Because the south pole of a compass is attracted to the north pole of Earth, the compass needle always points north.

Today magnets are found in many places. Magnets hold papers on refrigerator doors. They also hold the doors shut. Credit cards have a magnetic strip. Automatic doors, stereo speakers, and many electric motors use electromagnets.

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Each day we get asked questions about the differences in magnetic materials. Neodymium Magnets, Samarium Cobalt (SmCo) Magnets, Ceramic Magnets, Alnico Magnets, Bonded Magnets, and Injection Molded Magnets all have various options, and choosing the correct grade for your application is critical.

This article will try to simplify how magnets are designated, and define the differences between magnet grades. We will focus specifically on the strongest rare earth magnets available, neodymium magnets (NdFeB) and samarium cobalt (SmCo) magnets.

Neodymium Magnets and Samarium Cobalt Magnets are available in many grades. There are several factors and considerations that go into design, but for this article, the areas discussed will be Magnets Strength, Magnet Coercivity, and the Considerations for Magnet Selection.

Cost

Cost considerations are usually part of the discussion. As a general rule, the higher the grade, the higher the price. There are other variables, including shape and size, but for this purpose, we will use the general rule of higher grade = higher price. And, along with this rule, the higher the letter after the grade, the higher the price. So, for example, an N48H will cost a bit more than an N48. And an N48SH will cost more than an N48H, and so on. And finally, even a lower grade material can cost more than a higher grade material if a higher letter is selected. For example, an N35SH will most likely cost more than an N38 or even an N40.

Not All Grades Are Created Equal

Use of permanent magnets may mean determining the tradeoffs. As you can see from the above information, you may need to determine if strength or coercivity is more important. Or, if neodymium magnets would be a better choice than samarium cobalt magnets, or vise versa. There is a tradeoff between strength and coercivity. As you get stronger in a magnet grade, the coercive force in a magnet many not be available. For example, if you need a magnet that can handle 180C temperature, a UH material will be required. A technical magnet person can work with you to determine the best magnet for your application. If you want to learn more about past project that the SM Magnetics Engineers have worked on with our customers jump over to our Design Assistance page.

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