Magnetism

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Magnetism
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Properties of magnets

• Magnets have a north pole and a south pole.
• They attract iron, steel, cobalt, nickel and some
  other ferromagnetic materials, which become
  temporary magnets.

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Contd

• The origin of the magnetic properties is a result
  of the spins of the unpaired electrons.
• Since ferromagnetic materials such as iron,
  cobalt, and nickel are all B group materials they
  have more unpaired electrons due to their complex
  electron configurations (think diagonal rule from
  chemistry.. P, S, etc orbital).

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Contd

• When a material is not magnetized, the unpaired
  electrons spin in random directions, however in a
  magnet the spins of the unpaired electrons are
  all oriented in the same direction.

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Domains, continued

• In many materials, each electron is paired with
  another having an opposite spin.
• Magnetic effects mostly cancel each other.
• As a result, these materials have extremely weak
  magnetic fields.

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Domains, continued

• Some other materials, like nickel, cobalt and
  iron, have one or more unpaired electrons.
• The unpaired electrons produce magnetic fields.

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Domains, continued
Randomly-oriented domains Aligned domains
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Contd

• Like poles repel
• Opposite poles attract.

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The field concept

• A magnetic field is a region in space in which a
  suitable detector (a ferromagnetic material) can
  feel a magnetic force of attraction.
• The magnetic field lines are drawn in the
  direction that the north pole of a compass would
  experience a force.

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Contd

• The number of magnetic field lines passing
  through a surface is called magnetic flux.
• The flux per unit area is proportional to the
  strength of the magnetic field.

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Contd

• Like electric field lines, magnetic lines of
  force cannot cross since the compass can only
  point in one direction at a time.
• The concentration of lines of force indicates the
  strength of the magnetic field.
• The lines of force are concentrated at the poles
  of the magnet so the strength is greatest near
  the poles and decreases as you get further from
  the magnet.

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Contd

• Magnetic lines of force form closed loops just
  like electric lines of force do.
• Outside of the magnet, magnetic lines of force
  leave the north pole of the magnet and go to the
  south pole then inside the magnet they travel
  back to the north pole finally closing the loop.

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Magnetic fields around permanent magnets
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Contd
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Contd
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Magnetic Field Lines

• For a Java Applet on magnetic fields around bar
  magnets go to
• http//www.smaphysics.ca/phys30s/field30s/barmagne
  ts.html

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Contd

• Ferromagnetic materials are said to be permeable
  to magnetic lines of force.
• For example, a bar of iron placed in the magnetic
  field will distort the field and the field lines
  will be more concentrated in the ferromagnetic
  material.

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Magnetic field around a current-carrying wire

• A wire that has a current running through it
  creates a magnetic field around the wire.
• The lines of force are a set of concentric
  circles having the wire as a common center.
• The direction of current in the wire determines
  the direction of the lines of force.

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Contd

• Typical sign conventions for magnetic field
  diagrams include
• 1. Vectors in the same plane as the page are
  shown with arrows
• 2. Vectors into the page are shown with X
• 3. Vectors out of the page are shown with dots

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Contd

• First Right Hand Rule (aka Fonzie rule)
• Make a fist with your hand with your thumb
  extended the way the character on Happy Days
  would have.
• Rotate your hand until
• your thumb is point in the direction of the
  current,
• your fingers are now curled in the direction of
  the magnetic field around that wire.

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Contd
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• What does the magnetic field around the following
  current carrying wire look like?

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• What does the magnetic field around this current
  carrying wire look like?

X X X X
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Magnetic induction

• The strength of the magnetic field is called
  magnetic induction, which is measured in Teslas
  (T).
• The strength of the magnetic field at any point
  around a long, straight current carrying wire is
  given by

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Contd

• B magnetic field strength or magnetic induction
  in Teslas (T)
• I current in Amperes (A)
• d distance in meters (m)

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• A magnetic field of 4 10-7 T is measured 3 m
  from a current carrying wire. What is the
  current in the wire?

Given B 4 10-7 T d 3 m I ?
Answer 6 A
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• A magnetic field of 4 10-7 T is measured 3 m
  from a current carrying. What is the current in
  the wire?

Given B 4 10-7 T d 3 m I ?
Answer 6 A
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Loops of wire

• When a current flows through a single circular
  loop of wire, a magnetic field appears around the
  loop.
• You can still use the Fonzie rule to determine
  the direction of the magnetic field at any point
  around the loop.
• All the points inside the loop will have a
  magnetic field in one direction and any point
  outside the loop will have a magnetic field in
  the opposite direction.

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Contd

• The current flowing through the coil of wire
  creates something called an electromagnet.
• Most electromagnets have the wire wrapped around
  an iron core to strengthen the magnetic field.
• The electromagnet has a north pole and a south
  pole.

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Contd

• Second Right Hand Rule
• The north pole of the electromagnet can be
  determined by
• curling your fingers in the direction of the
  current
• your thumb will point towards the north pole.

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• Which end is the north pole of the electromagnet?

North
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Contd

• A current carrying wire in an external magnetic
  field will also experience a force.

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Moving charges in a magnetic field.

• The Third Right Hand Rule (Indian How)
• Hold your hand up with the fingers extended and
  the thumb extended and palm flat the way they
  showed the Indian greeting on old western movies.
  
• Your fingers point in the direction of the
  external magnetic field
• your thumb in the direction of the current in the
  wire
• your palm will point in the direction of the
  force that is exerted on the wire.

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• The wire is moving through the magnetic field
  from left side of the slide to the right side of
  the slide, is the current induced in the wire
  toward the top of the slide or the bottom of the
  slide?
• XXXXXXXXXXXXXXXXXXXXXXXXXXXX
• XXXXXXXXXXXXXXXXXXXXXXXXXXXX
• XXXXXXXXXXXXXXXXXXXXXXXXXXXX
• XXXXXXXXXXXXXXXXXXXXXXXXXXXX

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Contd

• F force in Newtons
• B magnetic field strength or magnetic induction
  in Teslas
• L length of the wire in meters
• ? - angle between the wire and the magnetic field

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• A wire 50 cm long carrying a current of 4 A is at
  right angles to a magnetic field of 0.5 T. What
  is the force acting on this wire?

Given B 0.5T I 4 A T 90 L 0.5 m F
Answer 1 N
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Contd

• The force on charges moving in a magnetic field
  can also be measured.

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Contd

• F Force in Newtons
• B magnetic field strength or magnetic induction
  in Teslas
• q charge in Coulombs
• v speed in m/s

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• A 4 10-8 C charge is moving through a 5 10-5
  Tesla magnetic field directed to the North. This
  charge experiences an upward force of 3 10-11
  N. What is the velocity of the moving charge?

Given B 5 10-5 T q 4 10-8 C F 3
10-11 N v ?
Answer 15 m/s
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Galvanometer

• A galvanometer is a device that can be used to
  measure either current or voltage.
• It is made up of a small rectangular coil of wire
  around a soft iron core with a pointer attached,
  placed in a strong magnetic field of a permanent
  magnet.

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Contd

• When it is being used as an ammeter, it has a
  shunt (low resistance metallic strip) connected
  in parallel to the coil of the galvanometer.
• RSHUNT IMETER RMETER / (ITOTAL IMETER)

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• A galvanometer has a resistance of 10 O and a
  full scale deflection of 2.5 mA. What shunt
  resistance must be added to convert this meter
  into an ammeter reading 10 A full scale?

Answer 0.0025 Ohms
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Contd

• When the galvanometer is used as a voltmeter, it
  has a high resistance coil placed in series the
  movable coil of the galvanometer.
• RSERIES (VTOTAL / IMETER) - RMETER

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• What resistance must be added to the galvanometer
  which has a resistance of 10 Ohms and a full
  scale deflection of 2.5 mA to convert it to a
  voltmeter reading 15 V on full scale deflection?

Answer 5990 Ohms
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Electric motor

• A motor is a device that converts electrical
  potential energy into mechanical kinetic energy.
  
• A motor consists of a coil of wire (armature)
  connected to a split copper ring (commutator),
  which is in contact with the metallic brushes,
  which are connected to the battery.
• The armature is then allowed to turn in the
  magnetic field created by 2 permanent magnets.

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Contd

• A motor is a device that converts electric
  potential energy into mechanical kinetic energy.

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Contd

• A DC motor consists of
• A coil of wire, called an armature, which is free
  to turn and attached to a split copper ring, or
  commutator, that rotates with the coil.
  Continuous contact with the commutator is made by
  two stationary pieces of carbon, called brushes,
  which push gently against the rotating
  commutator. Current enters the coil through one
  brush and leaves through the other. The coil is
  placed in an external magnetic field, either a
  permanent magnet or an electromagnet.

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Motor Diagram 1
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Contd

• In diagram 1, current flows in through the bottom
  brush, into the commutator segment B, and through
  the coils, eventually entering commutator segment
  A and leaving the motor through the top brush.
  End A of the armature becomes a N - pole, using
  the right - hand rule, and is repelled by the N -
  pole of the field magnet, causing it to move away
  and rotate clockwise.

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Motor Diagram 2
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Contd

• In diagram 2, tracing the path of the current
  through the motor verifies that end A remains a N
  - pole and is, therefore, attracted toward the S
  - pole of the field magnet.

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Motor Diagram 3
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Contd

• In diagram 3, a significant change occurs. The
  bottom brush is now in contact with commutator
  segment A. Current continues to flow through the
  coils, leaving by commutator segment A and the
  top brush. End A of the armature now becomes a S
  - pole and is repelled by the S - pole of the
  field magnet, causing the clockwise motion to
  continue.

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Motor Diagram 4
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Contd

• Again, tracing the flow of current through the
  motor in diagram 4 confirms that end A of the
  armature remains a S - pole and is attracted
  toward the N - pole of the field magnet,
  completing one full rotation of the motor.

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Electric Motor

• The speed of rotation of a motor depends on
• a. The magnitude of a current flowing through it.
• b. The strength of the field magnet
• c. The number of coils on its armature.
• d. The permeability of the armature
• e. The mechanical load connected to the shaft.

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Contd

• The direction of rotation depends on
• a. The direction of the magnetic field
• b. The direction of the current flow.

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Contd

• How direction is determined?

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Motor on the Web

• An interactive motor can be found at
• http//www.sciencejoywagon.com/physicszone/lesson/
  otherpub/wfendt/electricmotor.htm