Magnetic Materials Help (page 2)
Some substances cause magnetic lines of flux to bunch closer together than they are in the air; other materials cause the lines of flux to spread farther apart. The first kind of material is ferromagnetic. Substances of this type are, as we have discussed already, “magnetizable.” The other kind of material is called diamagnetic . Wax, dry wood, bismuth, and silver are examples of substances that decrease magnetic flux density. No diamagnetic material reduces the strength of a magnetic field by anywhere near the factor that ferromagnetic substances can increase it.
The magnetic characteristics of a substance or medium can be quantified in two important but independent ways: permeability and retentivity .
Permeability, symbolized by the lowercase Greek mu (μ), is measured on a scale relative to a vacuum, or free space. A perfect vacuum is assigned, by convention, a permeability figure of exactly 1. If current is forced through a wire loop or coil in air, then the flux density in and around the coil is about the same as it would be in a vacuum. Therefore, the permeability of pure air is about equal to 1. If you place an iron core in the coil, the flux density increases by a factor ranging from a few dozen to several thousand times, depending on the purity of the iron. The permeability of iron can be as low as about 60 (impure) to as high as about 8,000 (highly refined).
If you use special metallic alloys called permalloys as the core material in electromagnets, you can increase the flux density, and therefore the local strength of the field, by as much as 1 million (10 6 ) times. Such substances thus have permeability as great as 10 6 .
If, for some reason, you feel compelled to make an electromagnet that is as weak as possible, you can use dry wood or wax for the core material. Usually, however, diamagnetic substances are used to keep magnetic objects apart while minimizing the interaction between them.
Certain ferromagnetic materials stay magnetized better than others. When a substance such as iron is subjected to a magnetic field as intense as it can handle, say, by enclosing it in a wire coil carrying a high current, there will be some residual magnetism left when the current stops flowing in the coil. Retentivity, also sometimes called remanence , is a measure of how well a substance can “memorize” a magnetic field imposed on it and thereby become a permanent magnet.
Retentivity is expressed as a percentage. If the maximum possible flux density in a material is x teslas or gauss and then goes down to y teslas or gauss when the current is removed, the retentivity B r of that material is given by the following formula:
B r = 100 y/x
What is meant by maximum possible flux density in the foregoing definition? This is an astute question. In the real world, if you make an electromagnet with a core material, there is a limit to the flux density that can be generated in that core. As the current in the coil increases, the flux density inside the core goes up in proportion—for awhile. Beyond a certain point, however, the flux density levels off, and further increases in current do not produce any further increase in the flux density. This condition is called core saturation . When we determine retentivity for a material, we are referring to the ratio of the flux density when it is saturated and the flux density when there is no magnetomotive force acting on it.
As an example, suppose that a metal rod can be magnetized to 135 G when it is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. For any substance, there is always such a maximum; further increasing the current in the wire will not make the rod any more magnetic. Now suppose that the current is shut off and that 19 G remain in the rod. Then the retentivity B r is
B r = 100 × 19/135 = 100 × 0.14 = 14 percent
Certain ferromagnetic substances have good retentivity and are excellent for making permanent magnets. Other ferromagnetic materials have poor retentivity. They can work well as the cores of electromagnets, but they do not make good permanent magnets. Sometimes it is desirable to have a substance with good ferromagnetic properties but poor retentivity. This is the case when you want to have an electromagnet that will operate from dc so that it maintains a constant polarity but that will lose its magnetism when the current is shut off.
If a ferromagnetic substance has poor retentivity, it’s easy to make it work as the core for an ac electromagnet because the polarity is easy to switch. However, if the retentivity is high, the material is “magnetically sluggish” and has trouble following the current reversals in the coil. This sort of stuff doesn’t function well as the core of an ac electromagnet.
Retentivity Practice Problem
Suppose that a metal rod is surrounded by a coil and that the magnetic flux density can be made as great as 0.500 T; further increases in current cause no further increase in the flux density inside the core. Then the current is removed; the flux density drops to 500 G. What is the retentivity of this core material?
First, convert both flux density figures to the same units. Remember that 1 T = 10 4 G. Thus the flux density is 0.500×10 4 =5,000 G with the current and 500 G without the current. “Plugging in” these numbers gives us this:
B r = 100 × 500/5,000 = 100 × 0.100 = 10.0 percent
Any ferromagnetic material, or substance whose atoms can be aligned permanently, can be made into a permanent magnet. These are the magnets you played with as a child (and maybe still play with when you use them to stick notes to your refrigerator door). Some alloys can be made into stronger permanent magnets than others.
One alloy that is especially suited to making strong permanent magnets is known by the trade name Alnico . This word derives from the chemical symbols of the metals that comprise it: aluminum (Al), nickel (Ni), and cobalt (Co). Other elements are sometimes added, including copper and titanium. However, any piece of iron or steel can be magnetized to some extent. Many technicians use screwdrivers that are slightly magnetized so that they can hold onto screws when installing or removing them from hard-to-reach places.
Permanent magnets are best made from materials with high retentivity. They are made by using the material as the core of an electromagnet for an extended period of time. If you want to magnetize a screwdriver a little bit so that it will hold onto screws, stroke the shaft of the screwdriver with the end of a bar magnet several dozen times. However, take note: Once you have magnetized a tool, it is practically impossible to completely demagnetize it.
Flux Density Inside A Long Coil
Suppose that you have a long coil of wire, commonly known as a solenoid , with n turns and whose length in meters is s . Suppose that this coil carries a direct current of I amperes and has a core whose permeability is μ. The flux density B in teslas inside the core, assuming that it is not in a state of saturation, can be found using this formula:
B = 4π × 10 −7 (μ nI/s )
A good approximation is
B = 1.2566 × 10 −6 (μ nI/s )
Flux Density Practice Problem
Consider a dc electromagnet that carries a certain current. It measures 20 cm long and has 100 turns of wire. The flux density in the core, which is known not to be in a state of saturation, is 20 G. The permeability of the core material is 100. What is the current in the wire?
As always, start by making sure that all units are correct for the formula that will be used. The length s is 20 cm, that is, 0.20 m. The flux density B is 20 G, which is 0.0020 T. Rearrange the preceding formula so it solves for I :
B = 1.2566 × 10 −6 (μ nI/s )
B/I = 1.2566 × 10 −6 (μ n/s )
I −1 = 1.2566 × 10 −6 (μ n/sB
I = 7.9580 × 10 5 ( sB /μ n )
This is an exercise, but it is straightforward. Derivations such as this are subject to the constraint that we not divide by any quantity that can attain a value of zero in a practical situation. (This is not a problem here. We aren’t concerned with scenarios involving zero current, zero turns of wire, permeability of zero, or coils having zero length.) Let’s “plug in the numbers”:
I = 7.9580 × 10 5 (0.20 × 0.0020)/(100 × 100)
= 7.9580 × 10 5 × 4.0 × 10 −8
= 0.031832 A = 31.832 mA
This must be rounded off to 32 mA because we are only entitled to claim two significant figures.
Practice problems of these concepts can be found at: Magnetism Practice Test
Today on Education.com
- Coats and Car Seats: A Lethal Combination?
- Kindergarten Sight Words List
- Child Development Theories
- Signs Your Child Might Have Asperger's Syndrome
- 10 Fun Activities for Children with Autism
- Why is Play Important? Social and Emotional Development, Physical Development, Creative Development
- First Grade Sight Words List
- Social Cognitive Theory
- The Homework Debate
- GED Math Practice Test 1