CHAPTER 19: MAGNETISM

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CHAPTER 19MAGNETISM

• Magnets, Magnetic Fields, Magnetic Force,
  Electricity

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PROPERTIES OF MAGNETS

• A magnet has 2 poles north south
• Like poles repel, opposites attract
• If you break a magnet, the 2 new pieces will have
  a north and south pole.
• Some metals can become magnetized by being
  brought near a magnet. They are called temporary
  magnets.

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MAGNETIC FIELDS AROUND MAGNETS

• Just like electric and gravitational forces
  result from electric and gravitational fields,
  magnetic forces result from magnetic fields.
• These fields move from north to south.
• A compass always points from the North to the
  South (called the magnetic north).

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THE EARTHS MAGNETIC FIELD

• The magnetic north does not coincide with the
  geographic North Pole since the axis of the
  Earths magnetic field does not lie along its
  rotational axis.
• The Earths magnetic field fluctuates.
• On a long time scale, the poles have switched
  polarity several times in the past (most recently
  700,000 years ago.
• On a short scale, it has moved roughly 110 km (70
  mi) northward per decade from its 1904 latitude
  position!

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MAGNETIC INDUCTION

• The strength of a magnetic field is called
  magnetic induction.
• It is symbolized by B
• It is measured in Teslas (T).
• 1 T 1N/A.m
• Once the magnetic field strength is determined,
  the force on any charged particle moving at any
  speed can be found as long as the direction
  remains perpendicular to the magnetic field!

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MAGNETIC FORCE

• We can use two formulas to find magnetic force
• F qvB
• If the direction of the charged particles
  velocity is not perpendicular to the magnetic
  field then
• F qvB(sinT)
• Where B magnetic field strength
• I current q charge
• v velocity of charge

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RIGHT-HAND FORCE RULE FOR CHARGES

• Determines the direction of force in a magnetic
  field.
• Use your right hand!
• Point your fingers in the direction of the
  charged particles velocity
• Now, curl your fingers in the direction of the
  magnetic field.
• Your thumb points in the direction of the
  magnetic force.

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ANOTHER RIGHT-HAND FORCE RULE!

• An alternate is known as the palm method for
  determining the magnetic force
• Point your thumb in the direction of the charged
  particles velocity
• Point your fingers in the direction of the
  magnetic field.
• Your palm now faces the direction of the magnetic
  force.

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Magnetism from Electricity

• Hans Christian Oersted found that a compass
  needle was deflected when brought near a
  current-carrying wire.
• We can find the direction of this current using
  the right hand source rule.

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RIGHT-HAND SOURCE RULE

• Determines the direction of the magnetic field in
  a current carrying wire.
• Again, use your right hand!
• Grasp the wire so that your thumb points in the
  direction of the current.
• Your curled fingers point in the direction of the
  magnetic field.

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FORMULAS FOR MAGNETIC FIELDS

• There are several formulas to use for determining
  the strength of a magnetic field in
  current-carrying wires.
• Near a long straight wire
• B µo I
• 2?d
• where µo 4? x 10-7 Tm/A (this is called the
  magnetic permeability of free space)

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MORE FORMULAS FOR MAGNETIC FIELDS

• At the center of circular loop
• B µo I
• 2r
• At the center of a solenoid, which is a long wire
  wound in a tight coil.
• B µoNI B µonI
• L
• Where N number of loops or turns and
• n number of turns per meter or (N/L)

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MAGNETIC FORCES WIRES

• Just as a charged particle moving in a magnetic
  field experiences a force, so does a
  current-carrying wire since it is made up of many
  moving charges!
• We can use formulas and another right-hand rule
  to find this force!

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MAGNETIC FORCE ON WIRES

• We can use two formulas to find magnetic force
• F BIL
• If the wire is not perpendicular to the magnetic
  field then
• F BIL(sinT)
• Where B strength of magnetic field
• I current q charge
• L length of wire

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RIGHT-HAND FORCE RULE FOR WIRES

• Determines the direction of force in a magnetic
  field.
• Use your right hand!
• Point your fingers in the direction of the
  current.
• Now, curl them in the direction of the magnetic
  field.
• Your thumb points in the direction of the
  magnetic force on the wire.