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:

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 µo.

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

B = µ H       or

B = µo ( 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

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

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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:

Experiment 1:    NdFeB permanent magnets

Precautions:

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 Bg versus Lg.

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 Bg versus Lm (total length of magnets)

 

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 Bg versus Lg.   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 Bg versus Lm (total length of magnets).  Compare to Experiment 1.

 

Experiment3:     Electromagnet

   Flux density versus current

  1. Wedge the tip of the Gauss/Tesla meter probe 1900 midway between the pole faces using a slotted block of sponge.  With a coil current of  1 Ampere, slide the probe tip back and forth in the slot to find the maximum field reading.  Record it.
  2. Increase the coil current to 2 Amperes and repeat the procedure.
  3. Continue to increase the by increments of 1 Ampere and record the corresponding field measurements for each different gap up to the maximum current possible.
  4. Plot Bg versus  I.    This will be used as a calibration plot for your experiments on the Hall Effect.