Download Magnetism

Magnetism
Magnets
►
A magnet has polarity - it has a north and
a south pole; you cannot isolate the north
or the south pole (there is no magnetic
monopole)
►
Like poles repel; unlike poles attract
Magnets
►
A compass is a suspended magnet (its
north pole is attracted to a magnetic south
pole); the earth’s magnetic south pole is
within 200 miles of the earth’s geographic
north pole (that is why a compass points
"north")
Magnets
►
►
►
Some metals can be turned into temporary
magnets by bringing them close to a
magnet; magnetism is induced by aligning
areas called domains within a magnetic
field
Domains  strong coupling between
neighboring atoms of ferromagnetic
materials to form large groups of atoms
whose net spins are aligned
Unmagnetized substance  domains
randomly oriented
Magnets
► When
an external magnetic field is applied
the orientation of the magnetic fields of
each domain may change to more closely
align with the external magnetic field
► Domains
already aligned with the external
field may grow at the expense of others
Magnets
► Materials
soft
can be classified as magnetically hard or
– like iron - are easily magnetized, but lose
magnetism easily
 once an external field is removed, the
random motion of the particles in the material
changes the orientation of the domains
 the material returns to an unmagnetized state
► Soft
Magnets
►
Hard – like cobalt and nickel – difficult to
magnetize, but retain their magnetism
 domain alignment persists after an
external field is removed
 the result is a permanent magnet
Magnetic Fields
► The
concept of a field is applied to
magnetism as well as gravity and electricity.
► A magnetic field surrounds every magnet
and is also produced by a charged particle
in motion relative to some reference point.
►B =
F____
q0(v*sinq)
Magnetic Fields
► The
direction of a magnetic field, B, at
any location is defined as the direction in
which the north pole of a compass needle
points at that location
Magnetic Fields
► To
indicate direction on paper we use the
following conventions:
 Arrows show direction in the plane of
the page
X Crosses represent the tail of an arrow
and show direction into the page
. Dots represent the tips of arrows and
show direction out of the page
Magnetic Force
►A
charge moving through a magnetic field
experiences a force
Fmagnetic =qv(sinq)B
 q –magnitude of charge, in Coulombs (C)
 v –velocity of charge, in m/s and must have a
component perpendicular to the field
 B –magnetic field strength, in Teslas
(1T=Ns/Cm)
 no magnetic force acts on a stationary charge
Magnetic Force
► Use
the right-hand rule to find the direction
of the magnetic force
► Magnetic force is always perpendicular to
both v and B
► Place your fingers in the direction of B with
your thumb pointing in the direction of v
► The magnetic force on a positive charge is
directed out of the palm of your hand
► If q is negative, find the direction as if q
were positive and reverse the direction
The Circular Trajectory
► Consider
a positively charged particle
moving perpendicular to a magnetic field
► Since the magnetic force always remains
perpendicular to the velocity the magnetic
force causes the particle to move in a
circular path
► The
force according to the RHR is directed
to the center of the circular path
The Circular Trajectory
► Since
Fmag = qvB and Fc = mv2/r then
qvB = mv2/r
and
r = mv/qB
Magnetic Fields Produced by
Currents
►A
current carrying wire produces a magnetic field
of its own
► Discovered by Hans Christian Oersted in 1820
► Marked the beginning of electromagnetism
►
►
0 I
B
2r
r  radial distance
μ0  permeability of free space = 4π x 10-7 Tm/A
Magnetic Field of a Current Carrying
Wire
► The
direction of this field can be determined
using the right-hand rule.
 Grasp the wire in the right hand with
your thumb in the direction of the current
 Your fingers will curl in the direction of
the magnetic field
Magnetic Field of a Current Loop
can use the right-hand rule to
determine the field around a current
carrying loop
► Regardless of where you are on the loop the
magnetic field inside of the loop is always
the same direction - upward
► You
Magnetic Field of a Current Loop
► Solenoids
– produce strong magnetic fields
by combining several loops of wire together
 are important in many applications
because they act as a magnet when it
carries current
 magnetic field can be increased by
inserting an iron rod through the center
of the coil creating an electromagnet
Magnetic Force on a CurrentCarrying Conductor
► Current
motion
► Since
electricity is charged particles in
charged particles moving in a
magnetic field experience a force, likewise a
current-carrying wire placed in a magnetic
field also experiences a force
Magnetic Force on a CurrentCarrying Conductor
►Fmagnetic
►B
= BILsinө
 Magnetic field strength in Teslas (T)
► I  Current
► L  length of conductor within B
Magnetic Force on a CurrentCarrying Conductor
► To
find the direction of the magnetic force
on a wire we again use the right-hand rule
► You place your thumb in the direction of the
current (I) in the wire rather than the
velocity (v)
► Your fingers as before are in the direction of
the magnetic field B
► The magnetic force comes out of your palm
Magnetic Force on a CurrentCarrying Conductor
► Current-carrying
wires placed close together
exert magnetic forces on each other
 when current runs in the same direction
the wires attract one another
 when current runs in opposite directions
the wires repel one another
Magnetic Force on a CurrentCarrying Conductor
► Loudspeakers
use magnetic force to
produce sound
► Most speakers consist of a permanent
magnet, a coil of wire and a flexible cone
► A sound signal is converted to a varying
electrical signal and is sent to the coil
► The current causes a magnetic force to act
on the coil
Magnetic Force on a CurrentCarrying Conductor
► When
the current reverses direction, the
magnetic force on the coil reverses
direction, and the cone accelerates in the
opposite direction
► Alternating force on the coil results in
vibrations of the attached cone, which
produces variations in the density of air in
front of it, or sound waves