Magnetism, and Electromagnetism

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Magnetism, and Electromagnetism

• Unit 9
• Date Started March 21, 2005

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I. BASIC MAGNETISM

• A.  Effects of magnetism known as early as 800 BC
  by the Greeks. (Iron oxide)
• B.  Certain stones called "magnetite" attracted,
  pieces of iron.
• C.  Magnetism is a basic force of attraction and
  repulsion in nature that is created, by moving
  charges.
• D. A magnet is an object, which has a magnetic
  field that causes a push or pulling action.

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II. Magnetic fields and field lines

• A.  Invisible lines of force exist between
  magnetic materials.
• B.  Lines of force have a direction, which
  is either north or south seeking.
• C.  All magnets have two poles, one where
  the lines of force are directed outward,
  (NORTH pole).
• D.  And another one in which the lines of
  force are directed inward (SOUTH pole).
• E. Iron filings can be used to visualize
  these lines of force.

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Law of Poles

• F.  Law of poles is similar to the law of
  charges, unlike poles are attracted.
• G.  The end of a magnet, which is north seeking,
  is indicated as the North Pole.
• H.  Magnetic field lines flow outward from the
  North pole towards the South pole causing an
  alignment with the induced material.
• I.  Magnetism can be transferred or induced into
  other materials, this is known as Magnetic
  Induction.
• J. The induction of magnetism into a material
  can be temporary or permanent.

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Magnetic Field Graphic
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III. Magnetic Characteristics and Units

• A.  No physical contact is necessary for magnetic
  induction to occur.
• B. Magnetic lines of force are referred to as
  magnetic FLUX (F) and very strong magnets have a
  high amount of flux.
• C. The UNITS to measure flux or lines of force
  are the following
•             Maxwell- one magnetic field line
  (cgs)
•             Weber- equals 100,000,000 lines of
  force. Micro-weber equals
  100 lines or maxwells.

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Magnetic Units

• A.  The magnetic lines of force represent
  magnetic energy that can be used to do work.
• B. The amount of flux lines in a given area is
  called the FLUX DENSITY.
• B- flux density symbol
• C. Two units of FLUX DENSITY are
• 1.  GAUSS (G)- one Maxwell of force in
  one square centimeter.
• 2.  TELSA (T)- one Weber per square
  meter.
• 3.  One Telsa (T) 10,000 Gauss

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Flux Density

• A.  The flux density describes the strength of a
  magnetic field produced by a magnet in a 2D
  field.
• B. Mathematically
• B F
  A
• The greater the number of flux lines in a given
  area, the higher the flux density.

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Permeability of magnetic fields

• A.  Some materials can concentrate or focus
  magnetic field lines, this is known as
  PERMEABILITY (m).
• B. PERMEABILITY is the measure of how much
  better a given material is than air as a path for
  allowing magnetic lines of force to flow.
• 1.   Symbol is m Air has a permeability of
  1.000.2.  No units since it is a comparison to
  air (units cancel).3.   Easily magnetized
  materials have a high permeability.

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Common Permeabilites

• Silver 0.9998
• Nickel 400 - 1000
• Gold 0.9989
• Iron 6000 8000
• Permalloy 100,000
• Supermalloy 1,000,000

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IV. Magnetic Characteristics

• Magnetic lines of force flow from the North pole
  to the south pole and the space betweenthe poles
  is referred to as the AIR GAP.
• The smaller the air gap the more concentrated
  the magnetic field.
• The air gap describes the type of magnet in
  permanent magnets.
•    Bar magnet - large air gap, very
  low magnetic
  fields.
•   Horseshoe magnet- small air gap,
  high flux density.
• Toroidial or ring - no air gap, highest flux
  density within
  the loop no North or South Pole.

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Flux leakage

• Some magnetic lines of force leave the toroidial
  magnet this is called flux leakage.
• A material used to close a magnetic path is
  called a KEEPER, and prevents loss of leakage
  flux and demagnetization of a magnet.
• Magnetic keepers have a high permeability.

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Magnetic Induction Characteristics

• A. Material does not have to be in
  physical contact with magnet.
• B. Magnetic lines of force pass through
  the material causing the atoms to rearrange
  themselves.
• C.  The material can hold the magnetism
  after the magnetic field is removed or it
  can revert back to it's original state.
• D.  RETENTIVITY is the ability of a
  material to retain a magnetic field after
  the removal of a magnetic field.

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How is magnetism created?

• Electrons (or negative charges) in motion
  generate magnetic fields in two ways.
• Orbital pairs in orbit create a diamagnetic
  effect (They cancel each other out.)
• Electrons spin on their axis either cw or ccw
  each orbital can have 2 electrons, opposite in
  spin.
• Strong magnetic materials have many unpaired
  electrons with spins in the same direction,
  causing a strong magnetic field to be generated.
  (Iron, Nickel, Cobalt, Steel)

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Magnetic Domains

• All atoms have small arrangements of dipoles,
  which are very similar to the poles of a magnet.
• These small arrangements of dipoles are called
  DOMAINS, and placing an external magnetic field
  on a material can cause an alignment of these
  domains with the external field.
• The retention and the direction of the aligned
  domains determine the classification of the type
  or class of magnetic materials.

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Dipole Alignment
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Loss Of Magnetism By Magnetic Materials

• Magnets will lose their alignment if they are
  subjected to
• High temperatures
• Physical shock
• Strong AC or DC demagnetizing fields.
• Strong opposite magnetic fields.
• The temperature at which a magnetic material
  loses its alignment and magnetism is called the
  CURIE temperature.
• Magnetism in magnets can be lost if proper care
  is not taken.
• RETENTIVITY is the ability of a magnetic material
  to hold or retain a magnetic field.

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VII. Classes of magnetic materials

• There are three factors that determine the class
  of magnetic material
• The type of magnetic material is based on the
  atomic Structure of atoms.
• Electrical Charges in motion within the atom.
• Electrons in their orbits around the nucleus.
• In Chemistry and Physics, the electron
  configuration of an element is the greatest
  predictor of the class of magnetic material.

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Three classes of magnetic materials

• Paramagnetic
• Diamagnetic
• Ferromagnetic

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Defining the Classes of magnetic materials

• PARAMAGNETIC materials.
• In presence of a magnetic field, atoms WEAKLY
  align in the same direction as the applied field.
• Permeability is greater than 1.
• They are weakly magnetic materials, that are
  moderately good conductors of electricity.
• Examples aluminum and palladium.

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Defining the Classes of magnetic materials

• DIAMAGNETIC materials
• A material that is less magnetic than air,
  permeability is less than 1.
• Weak magnetization occurs but is opposite to the
  magnetic field that is applied.
• Good conductors of electricity.
• Examples are gold, silver and copper.

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Defining the Classes of magnetic materials

• FERROMAGNETIC Materials
• Atomic dipoles arrange strongly in an orderly
  manner with the magnetic field that is applied to
  the material.
• Ferro- means iron-like, and are strong magnetic
  materials..
• They have high retentivity, and permeability.
• Alignment of the magnetic dipoles is in one
  direction with the field.
• Poor conductors of electricity compared to
  copper.
• Examples are Iron, Nickel, and Cobalt. Good
  conductors of electricity.
• Examples are gold, silver and copper.

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Retention of the Magnetic Field

• There are three possible effects on a material
  that has interacted with a magnetic material.
• They are as follows
• The materials magnetic domains remain aligned
  and the material has become magnetized.
• Most of the materials magnetic domains return to
  their original positions and the material has NOT
  become magnetized.
• Some of the materials magnetic domains align
  opposite to the magnetic field applied and the
  material has NOT become magnetized.
• The way the material either retains or loses the
  magnetic field determines the class of magnetic
  material.

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Hans Oersted Electromagnetism

• Oersteds Observations
• Oersted observed that there was a magnetic field
  around an electrical conductor that had
  electricity flowing through it.
• The magnetic field around the current carrying
  wire was circular in shape and rotated either
  clockwise or counterclockwise in direction.
• The magnetic field in the current carrying wire
  was greatest closest to the wire.
• An increase in the current flowing through the
  wire caused a greater the magnetic field to be
  generated around the conductor.
• The magnetic field around the conductor was
  perpendicular or at a 90 degree angle with the
  conductor.

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Hans Oersted Electromagnetism 2

• Oersteds Observations (cont)
• There is no magnetic field observed when
  measuring magnetism along the same plane
  (parallel).
• Two wires carrying current in the same directions
  are attracted to each other, and the flux density
  between these wires is reduced.
• Two wires carrying current in the opposite
  directions are repelled from each other, and the
  flux density between these wires is increased.
• All magnetic fields generate an electrical field,
  and all electrical fields generate a magnetic
  field.
• The left hand rule could be used to predict the
  direction or circulation of the magnetic field in
  a current carrying wire.

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Oersteds Experiment
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Left Hand Rule- Field circulation
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II. Coils and Magnetic Fields

• A coil is defined as a wire that is wrapped
  around a core that has a high permeability and a
  low retentivity.
• A coil that is used to create or generate a
  magnetic field is called an electromagnet or
  solenoid.
• The number or turns of electric wire on a core
  and the current flowing through the wire produce
  a magneto-motive force (mmf) of magnetic energy.
• The unit for magneto-motive force is commonly
  called the ampere-turn (NI).

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Flux Density in a coil

• There are three factors that determine the flux
  density of a coil. They are as follows
• The number of turns of wires around a core
• The amount of electrical current flowing through
  the wire.
• The permeability of the material that the core is
  constructed.
• The polarity of an electromagnets or coils
  magnetic field is based entirely on the direction
  of electrical current flow in the wire.
• Oersted observed that one had only reverese the
  polarity of the voltage source to change the
  polarity of the electromagnet.

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Left Hand Rule for Electromagnets

• Using your left hand, your fingers indicate the
  direction of current flow. (up or down) in the
  wire.
• The palm of your hand must be in contact with the
  side of the wire that is carrying current.
• Your left thumb points to the North pole or
  direction of the electromagnet.

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Left- hand Rule Graphic
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Electromagnet Examples
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Magnetic Flux in a coil

• The flux generated in a coil depends upon
• The number of turns of wire on the coil.
• The amount of current flowing in the wire.
• The permeability or opposition to the magnetic
  path.
• Reluctance is the opposition to the flow of
  magnetic energy or lines of force through a
  material.
• Materials with a high permeability have a low
  reluctance, while materials with a low
  permeability have a high reluctance.

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Magnetomotive force- mmf
Flux mmf (NI) (F) R
mmf Magnetomotive force (NI) R
Reluctance (No units)
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Characteristics of Magnetic Circuits

• An electromagnetic field can be considered to be
  a circuit or closed loop.
• Similar to an electrical circuit, and the formula
  is similar to Ohms Law. (Reluctance has no
  units).
• Reluctance is based on three factors They are as
  follows
• Length of the conductors core
• Cross-sectional area and/or diameter
• Permeability of the core.
• If the length of the core, the area of the core,
  the number of turns, and the amount of current
  carried through the coil is known, the flux
  density can be found in a magnetic circuit.

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Reluctance Formula
R L m A
L length m permeability of the core A
cross-sectional area R Reluctance (No units)
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General Formula for finding theFlux in a Coil
Flux N I A m
L
L Length m permeability of the
core A cross-sectional area N number
of turns I current in coil
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Field Intensity and the Oersted

• Since magnetic fields are 3-dimensional, flux
  density cannot adequately describe the whole
  magnetic field of an electromagnet.
• Field Intensity is term to describe a magnetic or
  electromagnetic field at a given distance in
  space. The unit of field intensity is called the
  oersted and the symbol is the letter H.

H N I B R
L L
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Other Ways to Describe Field Intensity

• A measure of how strong the magnetic field is at
  a given distance or point in space.
• The amount of flux produced at a particular point
  away from the magnet.
• The amount of magnetomotive force (ampere-turns)
  per unit length of a coils core.

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Additional Features in Electromagnetic Circuits

• Ferromagnetic materials (cores) should have a low
  retentivity, a high permeability, and no residual
  magnetism.
• A high retentivity is undesirable because the
  magnetic field or magnetizing force will remain
  after the current in the circuit ceases or stops.
• Due to core retentivity and core saturation, a
  lag of magnetic flux compared to magnetizing
  force applied occurs in electromagnetic circuits.
  This is known as HYSTERESIS.

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Additional Features in Electromagnetic Circuits-2

• When magnetic domains do not return to their
  initial positions due to the core retentivity and
  saturation, there is a partial alignment of the
  domains.
• The hysteresis effect acts like a power loss in
  electrical circuits because magnetic energy is
  lost as heat.
• Hysteresis is similar to molecular friction
  within the core which results in heat being
  generated in the core.
• Hysteresis is noticed when there is a changing
  current, such as in AC or pulsating DC circuits.
  The greater the rate of change or frequency, the
  higher the Hysteresis loss.

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Magnetization force, Flux Density and Hysteresis.

• Field strength or intensity is often referred to
  as the magnetizing force, H in oersteds.
• Flux Density is the number of magnetic lines of
  force in a given cross-sectional area.
• Permeability is the measure of the flux produced
  by a magnetizing force or how well a material
  transfers flux to another material.
• The relationship of these three factors can be
  mathematically expressed or shown in a B-H curve.

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B-H Curves and Permeability
B m H
B flux density in gauss m permeability of
core H field intensity in oersteds

• We would expect that as the field strength
  increases, the flux density would increase
  linearly.
• This is true for diamagnetic and paramagnetic
  materials, but not true for ferromagnetic
  materials.

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B-H curves with Ferromagnetic Materials

• Ferromagnetic materials deviate from this due to
  the high retentivity of this type of material
  when subjected to a magnetizing force.
• This can be illustrated when flux density and
  field intensity are graphed using the four
  quadrants in the coordinate system.
• These are called B-H curves and are used to
  predict the amount of magnetic loss in
  ferromagnetic materials.
• B-H curves are especially useful in mixed voltage
  environments (AC and DC signals) because they
  take into consideration the core saturation
  effect from a DC power source.

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Example B-H Curve
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Electromagnetic Induction

• Faradays Observation
• In 1831, Michael Faraday discovered that when a
  magnet is moved in and out of a coil of wire a
  voltage and current was produced in the wire.
• When the magnets motion was stopped, the voltage
  and current in the wire stopped.
• When the magnet was pulled out of the coil of
  wire, the voltage was opposite in polarity to
  when the magnet entered the coil.
• He concluded that a moving magnet field could
  produce a voltage in a stationary wire.
• He used the term flux linkage to indicate that
  the magnetism was transferred to coil from the
  external magnetic field.
• Electromagnetic induction is the generation of a
  voltage in a conductor and is caused by the
  conductor cutting the magnetic lines of force.

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4 Ways To Induce A Voltage In A Wire.

• Moving a wire across a magnetic field. (The wire
  must be perpendicular to the field)
• Moving a magnet past a stationary wire. (The
  wire must be perpendicular to the field)
• Changing the field strength of the magnetic
  field, by altering the permeability of the core,
  or the current in a coil, increasing or
  decreasing the number of turns in the coil.
• When two wires are parallel to each other, and
  there is a change the current in one of the wires
  this induces a voltage of opposite polarity in
  the other wire.

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Counter-EMF in a wire
N
S
S
N
N
S

• NOTE In all methods, either the movement of a
  wire or a magnetic field causes magnetic lines of
  force to be cut or broken. This transfers the
  magnetism into the wire which induces a voltage
  or counter emf.

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Induction of an AC voltage
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Faradays Law
Where E counter-emf or voltage DB change
in flux N number of turns DT change in
time

• The voltage induced in a conductor by the
  interaction between the conductor and a magnetic
  field is directly proportional to the number of
  turns in a coil and the change in magnetic flux
  in the coil during an interval of time change.
• The faster the rate of change in the magnetic
  flux the higher the induced voltage in a coil.

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Expanding and Collapsing Magnetic Fields.

• When a magnetic field is increasing or decreasing
  in intensity in a wire, this has an effect on the
  amount of voltage induced in the wire.
• Expanding Field When the current flowing in a
  wire is increased, this causes the magnetic field
  around the wire to increase. This increased
  magnetic field causes an increase in the voltage
  of the same polarity to be induced in the wire.
• Collapsing Field When the current flowing in a
  wire is decreased or stopped, this causes the
  magnetic field around the wire to shrink or
  decrease. This decreased magnetic field causes
  the voltage induced in the wire to have an
  opposite polarity or reverse direction.
• LENZs Law The polarity of an induced
  (counter-emf) voltage is such that it tends to
  set-up a current in a conductor which in turn
  sets up a magnetic field which opposes a change
  in flux in a coil.

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Expanding and Collapsing Fields
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Lenzs Law Hyperlink

• A loop of wire is moved back and forth through a
  region of uniform magnetic field (yellow). The
  magnetic field is perpendicular to the page. An
  induced current (red) is produced in the
  directions indicated by the arrow according to
  Lenz's Law. Note that there is no emf when the
  loop is entirely within the region of uniform
  magnetic field.

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Inductance Defined

• Inductance The electrical term that expresses
  the opposition to a change in current in a coil.
• Because the induced magnetic field opposes any
  change in current in a coil, inductance has a
  similar effect that resistance has in an
  electrical circuit.
• Self-Induction is the production of a counter emf
  or voltage in a coil and an opposing change in
  current in a coil.
• An AC source or a pulsating DC source must be
  present in a coil in order to have
  self-induction.
• If there is no changing current flow provided to
  the coil, there is NO inductance in the coil of
  wire.
• Self-inductance and mutual inductance is the
  basis for stepping up an AC or pulsating DC
  signal in transformers and autotransformers.
• Electromagnetic induction plays a role in motors
  due to the effect explained in Lenzs law.

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Hyperlinks to Demos

• Link to Faradays Experiment
• Oersteds Experiment
• Magnetic Field Lines
• N-S and S-S Field Lines
• AC generator
• DC generator
• Inductance