MODERN PHYSICS (PHY 320) LAB

Magnetic Materials and Electromagnets

OBJECT:

        To investigate some magnetic properties of matter.
        To determine some properties of the magnetic fields of various permanent magnets.
        To measure the  magnetic field variations of an electromagnet as a function of the coil current.

APPARATUS:

1. A set of four Neodymium-Iron-Boron disk magnets with several 1 mm plastic spacers.

2. A set of four Ceramic permanent magnets with several 1 mm plastic spacers.

3. A large Cenco electromagnet with variable gap and coils to handle currents up to 15 amps.,
   associated power supply and current regulator.

4. F.W.Bell Gauss/Tesla Meter model 4048 for precision magnetic field measurements.

5. Pasco Gauss Meter and Science Workshop interface.

BACKGROUND:

In the study of magnetism and the magnetic effects associated with the motion of electric charges, one usually designates   symbols B, H, M, and µ to describe magnetic properties.

B =  the quantity known as the magnetic induction, or the magnetic flux density.  It is this property of magnetic fields that determines the force that is exerted upon a current or a moving charge.

H =  the term referred to as magnetic intensity, or field strength.  More descriptively,  it has become regarded as a
magnetizing force which acts on elementary magnets within certain material samples placed in magnetic fields.

M =  the term named the magnetic polarization or magnetization and defined as the magnetic moment per unit volume within any magnetic material.

All of the above are vector quantities.

µ = a magnetic property of matter called the permeability.  In isotropic media, it is a scalar quantity, and in many materials, like free space, it is a constant µ_o.

For most materials, the relationship between these fields can be expressed in either of two ways:

B = µ H       or

B = µ_o ( H + M)

Reference is frequently made to the example of a solenoid with some material used as the core of the windings to relate these terms.  Recall that inside the solenoid with n turns per unit length and a current I in the coil, we have approximately

B = µnI

and        H = B/µ = nI     independent of the core material.

Thus, a current in the coil produces the magnetizing force H throughout the core, and this sets up a flux density B which consists of a contribution from both the coil's field H and the material's magnetization M.  

The most easily observed of all magnetic effects is ferromagnetism, so called because of its occurrence in metallic iron and a number of iron compounds.  The experimental facts about ferromagnetic materials include that

• In these materials, very large magnetizations can be obtained.
• In these materials, M is not usually proportional to H as it is in most other materials. i.e., the value of M depends not only on the value of the applied field but also on the previous history of the sample.
• In some cases, a sample may may retain its magnetization even in the absence of of an external applied field.
• These very same materials that show such large permanent magnetization can also exist in a state with little or no net magnetization.
• In these materials, there is a strong tendency for the material to break up into many magnetic domains (regions where all dipoles are aligned), each with a different direction of magnetization, so that the macroscopic effect is to give zero magnetization.

The behavior of magnetic material during magnetization is conventionally presented as a B-H plot.

• The origin of coordinates 0, represents the unmagnetized condition of a ferromagnetic sample.  When a magnetizing force is applied, the sample proceeds along the black line to point a as the magnetic polarization increases.  This line is known as the magnetization curve.

• When the magnetizing force at point (a) is reduced, the flux density does not reduce back along the magnetization curve, but displays higher values.  When H has been reduced to zero, B still has a positive value indicated by point (b) which is known as the remanence, a measure of its retentivity. This phenomenon is known as hysteresis.

• In order to bring B back to zero, the magnetizing force must be made negative, to a value indicated by point (c) where H has a value known as the coercive force, a measure of its coercivity.

• When H is varied periodically about the origin the closed contour is known as a hysteresis loop.

• The portion of the loop which lies in the second quadrant is known as the demagnetization curve.   This is the portion of interest in a discussion of permanent magnets.   In  general, it is desirable that permanent magnets have a large remanence to retain a great portion of the magnetization, and  a large coercive force in order that the magnet will not easily be demagnetized. The best single figure of merit of a permanent magnet is the maximum value of the product BH along the demagnetization curve.  Since the product BH has the dimensions of energy density, it is referred to as the energy  product and the maximum value as the maximum energy product  BH_max.

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Dexter Magnetic Technologies supplies a great deal of helpful magnet background material online.  From their home page you can check out their information under selections:

• Introduction to Magnetics

• The Science of Magnets

• Material

• Magnetic Design

• Articles on:   Domain Theory
                      Flux density in a Dipole
                      Sponges and Magnets

Experiment 1:    NdFeB permanent magnets

Precautions:

• These magnets should never be brought close to cassette tapes, floppy disks, bank cards, telephone cards or any other magnetic card.
• Handle carefully to prevent having a finger caught between two magnets.  This force may be great enough to cause a blood blister or an abrasion.
• When bringing two magnets together, be sure to do it slowly. Otherwise the magnets will forcibly strike each other, and chipping  of the surface may result.
• If it is difficult to separate magnets , press one against the edge of a desk and slide the other one free.

A.   Flux density versus gap length

1. Use a slotted plastic spacer to separate two magnet disks. Insert the tip of the Gauss/Tesla meter probe 1900 into the slot. Slide the probe tip along the length of the slot to find the maximum field reading.  Record it.
2. Add a second spacer to separate the disks and repeat the procedure.
3. Continue to add spacers one at a time and record the corresponding measurements for each different gap up to as many spacers as you have.
4. Plot B_g versus L_g.  At the Dexter Magnetic Technologies website, find the appropriate relation and fit your data to it.

B.    Flux density versus magnet volume (length)

1. Use a slotted plastic spacer to separate two magnet disks. Insert the tip of the Gauss/Tesla meter probe 1900 into the slot. Slide the probe tip along the length of the slot to find the maximum field reading.  Record it.
2. Add a third magnet disk  on one side of the pair and repeat the procedure.
3. Continue to add magnet disks one at a time and record the corresponding measurements for each different magnet length.  It may also be helpful to record the field just outside the face of a single magnet disk.
4. Plot B_g versus L_m (total length of magnets).  At the Dexter Magnetic Technologies website, find the appropriate relation and fit your data to it.

Experiment 2:    Ceramic permanent magnets

A.   Flux density versus gap length

1. Use a slotted plastic spacer to separate two magnet disks. Insert the tip of the Gauss/Tesla meter probe 1900 into the slot. Slide the probe tip along the length of the slot to find the maximum field reading.  Record it.
2. Add a second spacer to separate the disks and repeat the procedure.
3. Continue to add spacers one at a time and record the corresponding measurements for each different gap up to as many spacers as you have.
4. Plot B_g versus L_g.   Compare to Experiment 1.

B.    Flux density versus magnet volume (length)

1. Use a slotted plastic spacer to separate two magnet disks. Insert the tip of the Gauss/Tesla meter probe 1900 into the slot. Slide the probe tip along the length of the slot to find the maximum field reading.  Record it.
2. Add a third magnet disk  on one side of the pair and repeat the procedure.
3. Continue to add magnet disks one at a time and record the corresponding measurements for each different magnet length.  It may also be helpful to record the field just outside the face of a single magnet disk.
4. Plot B_g versus L_m (total length of magnets).  Compare to Experiment 1.

Experiment3:     Electromagnet

   Flux density versus current:  Hysteresis

1. Zero the Pasco Gauss Meter probe in the transverse configuration away from the electromagnet.
2. Place the tip of the Gauss Meter in the center of the field of the electromagnet and record the magnitude and sign of the residual field.
3. Measure the current necessary to zero the magnetic field, a measure of the coercivity.
4. Increase the coil current in 0.5A steps and record the meter reading until the maximum current is reached.
5. Reduce the coil current in 0.5A steps and record the meter reading.  Measure the new residual field.
6. Turn off the power supplies and move the electrical connection on the Current Control box to Reverse.
7. Repeat steps 3, 4 and 5.
8. Plot B_g versus  I, the hysteresis curve for this magnet.  Identify the retentivity and coercivity.