Download Magnetism - AP Physics B

Magnetism
A Strangely Attractive Topic
History #1
 Term from the ancient Greek city of Magnesia,
Many natural magnets found
 We now refer to these natural magnets as lodestones
contain magnetite, a naturally magnetic material
Fe3O4.
 (Pliny the Elder (23-79 AD Roman) wrote of a hill near
the river Indus that was made entirely of a stone that
attracted iron.)
History #2
 121 AD Chinese scholars knew
that an iron rod which had been brought near
one of these natural magnets acquired and
retained the magnetic property
that such a rod when suspended from a string
would align itself in a north-south direction.
 Use of magnets to aid in navigation can be
traced back to at least the eleventh century.
 (1819) a connection between electrical and
magnetic phenomena is shown.
Danish scientist Hans Christian Oersted
observed that a compass needle in the vicinity of a
wire carrying electrical current was deflected!
 (1831), Michael Faraday discovered that a
momentary current existed in a circuit when
the current in a nearby circuit was started or
stopped
 Shortly after, he discovered that motion of
a magnet toward or away from a circuit could
produce the same effect.
(Let This Be a Lesson!)
 (Joseph Henry (first Director of
the Smithsonian Institution) failed to
publish what he had discovered 6-12
months before Faraday)
SUMMARY:
Oersted showed that magnetic effects
could be produced by moving electrical
charges;
Faraday and Henry showed that electric
currents could be produced by moving
magnets
*All magnetic phenomena result from forces
between electric charges in motion.
Looking in More Detail
 Andre Ampere first suggested in
1820 that magnetic properties of matter
were due to tiny atomic currents
 All atoms exhibit magnetic effects
 Medium in which charges are moving
has profound effects on observed
magnetic forces
For Every North, There is a
South
Every magnet has at least one north pole and one
south pole.
Field lines leave the North end of a magnet and
enter the South end of a magnet.
If you take a bar magnet and break it into two
pieces, each piece will again have a North pole and a
South pole. No matter how many times.
S
N
S
N S
N
Making a Magnet from a Ferromagnetic
Material
• domains in which the magnetic
fields of individual atoms align
• orientation of the magnetic fields
of the domains is random
• no net magnetic field.
• when an external magnetic field is
applied, the magnetic fields of the
individual domains line up in the
direction of the external field
• this causes the external magnetic
field to be enhanced
(No Monopoles Allowed)
(It has not been shown to be possible to end up
with a single North pole or a single South pole,
which is a monopole.
S
N
Note: Some theorists believe that magnetic monopoles may have been
made in the early Universe. So far, none have been detected.
Magnets Have Magnetic
Fields
We will say that a moving charge sets up in the
space around it a magnetic field,
and
it is the magnetic field which exerts a force on
any other charge moving through it.
Magnetic fields are vector
quantities….that is, they have a
magnitude and a direction!
Defining Magnetic Field Direction
Magnetic Field vectors are written as B
Magnitude of the B-vector is proportional to the
force acting on the moving charge, magnitude of
the moving charge, the magnitude of its velocity,
and the angle between v and the B-field. Unit is the
Tesla or the Gauss (1 T = 10,000 G).
F = qvBsin θ
Magnetic Field Lines
Magnetic field lines describe the structure of
magnetic fields in three dimensions.
If at any point on such a line we place an ideal
compass needle, free to turn in any direction
(unlike the usual compass needle, which stays in 2
dimensions) then the needle will always point
along the field line.
Field lines are closer together where
the field is the strongest, and spread
out when the field is weak.
Field Lines Around a Bar
Magnet
Field Lines of Repelling Bars
Field Lines of Attracting Bars
Showing the Direction of
Magnetic Field in a wire
FIRST RIGHT-HAND RULE
Hold wire in your right hand with your thumb
pointing in the direction of current. ( + to - )
The magnetic field of the wire wraps around it in
the direction of your fingers. (At 90 degrees to the
wire)
Finding Poles of an Electromagnet
SECOND RIGHT-HAND RULE:
Hold an insulated coil of wire in your right
hand.
Wrap your fingers in the direction of current.
Your thumb points toward the north pole of the
electromagnet.
Electromagnet
(Magnetism from
Electricity)
An electromagnet is simply a coil of wires which, when a
current is passed through, generate a magnetic field, as
below.
Force on a Current Carrying
Wire
THIRD RIGHT-HAND RULE
Point your thumb in the direction of current.
Point your pointer finger in the direction of the
magnetic field. (N to S)
Point your middle finger perpendicular to your
pointer finger.
That is the direction of the force on the wire.
Magnetic Force on Current-Carrying
Wire
Since moving charges
experience a force in a
magnetic field, a currentcarrying wire will experience
such a force, since a current
consists of moving charges.
This property is at the heart of
a number of devices.
Force on a current carrying wire.
FORCE on a current carrying wire
F = ILB Sin θ
F =(current)(Length)(strength of Field in
Tesla)
So Field strength = 1N/(1A(1m))
Force on a Charged Moving
Particle
F = qvBSin θ
A beam of electrons travels at 3 x 106 m/s through
a field of 4.0 x 10-2 T at right angles to the field.
How strong is the force on each electron?
F = 1.6 x 10-19 C(3 x 106 m/s )(4.0 x 10-2 T )
Cyclotron
 Developed in 1931 by E. O.
Lawrence and M. S. Livingston at UC
Berkeley
 Uses electric fields to accelerate
and magnetic fields to guide particles
at very high speeds
How a Cyclotron Works
 Pair of metal chambers
shaped like a pillbox cut
along one of its diameters
(cleverly referred to as “D”s)
and slightly separated
 Ds connected to
alternating current
 Ions injected near gap
 Ions are accelerated as long as they remain “in step”
with alternating electric field
Electric Motor
An electric motor, is a machine
which converts electrical energy
into mechanical (rotational or
kinetic) energy.
A current is passed through a loop
which is immersed in a magnetic
field. A force exists on the top leg of
the loop which pulls the loop out of
the paper, while a force on the
bottom leg of the loop pushes the
loop into the paper.
The net effect of these forces is
to rotate the loop.
We can build one!
Well, a simpler one…