Document Sample
Magnets Powered By Docstoc
• Magnetism is closely related to electricity
• Magnets are materials in which some
  electric charges are in orderly orbits
• We shall learn about how moving charges
  create magnetism
  Every magnet has two
  special ends called poles.
  We designate these as the
  north pole and the south
  pole for historical reasons.
  Like poles repel and unlike
  poles attract.
   If we saw a magnet in
   half, we create a new
   north pole and a new
   south pole.
   Poles always occur in
   pairs. No magnetic
   monopoles have ever
   been observed.
            Magnetic Fields
• When we dealt with electric charges, it was
  convenient to define the interaction between
  charges as taking place due to each charge
  creating an electric field.
• Similarly, we can define the concept of a
  magnetic field established by the two poles
  of a magnet.
            Magnetic Fields
• The field lines originate at a north pole and
  terminate at a south pole
             Magnetic Fields
• As before, the number of lines per unit area
  is proportional to the strength of the field
• The direction of the field is tangent to a line
  at any point in space.
• We will define the strength of a magnetic
  field in terms of the torque the field applies
  to a compass needle.
Magnetic Fields
           The magnetic field
           strength B is
           proportional to the
           torque applied to the
           compass needle.
           We will define it
           precisely later.
Earth’s Magnetic Field
              The field doesn’t point to
              true north. The
              difference is called the
              declination. Also, the
              field is not exactly
              tangent to the earth’s
              surface at all points. The
              deviation is called the
              angle of dip.
Earth’s Magnetic Field
              Can be lots of confusion.
              The magnetic pole in the
              Arctic is really a south
              magnetic pole. The
              magnetic pole in the
              Antarctic is really a north
              magnetic pole. Note how
              the compass needle is
           Magnetic Fields
• The simplest magnetic field occurs between
  the poles of two very large magnet pole
  faces. Near the center the field will be
  simple and straight.
      Magnetism and Currents
• It was found long ago that a stationary
  electric field and a stationary magnetic field
  do not interact.
• Oersted found that if you pass a current
  through a wire, a magnetic field is created
  around the wire!!!
• The charges have to be moving!!!
Magnetism and Currents
         The magnetic field lines are
         circles which close on
         themselves. Note that there is no
         north pole or south pole. The
         field lines simply are arranged as
         circles around the wire.
         We can find the direction by
         using the right-hand rule.
Right Hand Rule
      Point the thumb of your right
      hand in the direction of the
      conventional current (+ to -) and
      your fingers will curl in the
      direction of the magnetic field
Circular Loop of Wire
          If you have a circular loop of
          wire carrying a current, the
          magnetic field lines run
          perpendicular to the plane of the
          loop. We can use another right-
          hand rule to determine the
Circular Loop of Wire
            Definition of B
• We know that a current-carrying wire exerts
  a force on a compass needle (a magnet)
• By Newton’s Third Law, the compass
  needle must exert a force on a current-
  carrying wire
• Experiment confirms this to be true
Definition of B
         The current flows from front
         to back. The magnetic field
         goes from left to right. The
         foce is downward!!!
         Isn’t this a strange result.
         Test by reversing the
Definition of B
        Now the current goes from
        back to front, while the
        magnetic field still goes from
        left to right. Now the force is
        in the opposite direction, that
        is down to up!
        We need a rule to help us
Definition of B
            Definition of B
• Each component is perpendicular to the
• I perpendicular to B and to F
• B perpendicular to I and to F
• F perpendicular to B and to I
• WEIRD!!!
• Just remember the right hand rule
             Definition of B
• Now that we have the directions straight,
  we have to get the quantitative relationship
• As you might guess from all the direction
  business, this is a little messy
• Start with a picture
Definition of B
     Here is a current-carrying wire in a
     magnetic field. The force on the
     wire, using the right hand rule, is
     into the screen.
     Force depends directly on length of
     wire and size of current. Also
     depends on the angle between the
     current and the field. When angle is
     zero, the force is zero. When angle
     is 90o the force is a maximum.
     F = I x length x B x sine(theta)
            Definition of B

     F  I Bsin 
• The unit for B is the tesla
• :You may encounter the unit gauss
• One gauss is 10-4 tesla
Definition of B
       Can precisely measure the field
       with the loop shown here. Forces
       on the vertical portions of the
       wire cancel out. The force on the
       left side wire is to the left. The
       force on the right side wire is to
       the right. Leaves only the force
       on the horizontal wire which is
       down! Hang from a balance.