Download Magnetism

A Powerful Attraction
or
A Class of Phenomena caused by Moving Electric
Charges
Cause of Magnetism
 The root cause of magnetism is the movement of
electrons.
 In permanent magnets, unpaired electrons within the
orbitals of atoms spin in the same direction
(synchronized spinning).
 In temporary magnets, the flow of an electric current
(usually through a wire) produces a magnetic field.
Magnetic Poles
 Magnets are polarized—the force of the magnetic field
is directional.
 The direction of force from a magnetic field is the
often referred to as a pole.
 The north pole is defined as the end that will point to
the geographic north pole of the earth.
 On a permanent magnet, the poles cannot be
separated. Breaking a magnet will create two magnets,
each with both north and south poles.
Magnetic Domains
 Individual iron atom forces cause them to form
clusters called magnetic domains.
 In unmagnetized iron, these domains point in
random directions.
 In magnetized iron, the domains are aligned.
 A magnet causes the domains in unmagnetized
iron to become aligned and attracted.
Materials in Permanent Magnets
 Iron is a common magnetic material, as are nickel and
cobalt.
 Soft magnetic materials (for example iron) are easily magnetized
but tend to lose their magnetism easily.
 Hard magnetic materials (for example nickel) tend to retain their
magnetism.
 Many permanent magnets are made of an iron-based
alloy (ALNICO) with added aluminum, nickel and
cobalt.
 Some rare earth elements make strong permanent
magnets. Examples are neodymium and gadolinium.
Magnetic Field Lines
 A magnetic field is a region in which a magnetic force
can be detected.
 Magnetic field lines show the direction and strength of
a magnetic field. The lines are drawn point away from
the north pole and towards the south pole.
 Magnetic field strength (B) is measured in Teslas.
The Earth’s Magnetic Field
 The earth is a huge magnet.
 The poles of the earth’s magnetic
field do not exactly line up with
the geographic poles.
 It is not known what causes the
earth’s magnetic field.
 It may be related to the moving
charges within the molten core of
the earth.
 It may be due to convection
currents and the earth’s rotation.
Magnetic Field around a Straight Wire
 Electric current in a wire
produces a cylindrical magnetic
field.
 Changing the direction of the
current changes the direction of
the field.
 Right Hand Rule for a Straight
Wire – point thumb in direction
of the current, and the fingers
will curve in the direction of the
magnetic field.
Magnetic Field of a Loop
The overall direction for a
magnet formed from coiled
wire can be determined using
the Right-hand Rule.
– Wrap the fingers around the
coil form in the direction of
the current flow
– the thumb will point in a
direction indicating the end
which becomes the N-pole
Charged Particles in a Magnetic Field
 A charge moving through a magnetic field will
experience a force proportional to the charge and
velocity of the particle in addition to the strength of
the magnetic field.
 The strength of the force can be calculated with the
formula:
Fmagnetic = Bqv
Fmagnetic = (magnetic field)(charge)(velocity)
Charged Particles in a Magnetic Field
 The direction of the magnetic
force on a moving charge is
always perpendicular to both
the magnetic field and the
velocity of the charge.
 A different right-hand rule can
be used to find the direction of
the magnetic force.
 A charge moving through a
magnetic field follows a circular
path.
Example Problem
Particle in a Magnetic Field
A proton moving east experiences a force of 8.8  10–19
N upward due to the Earth’s magnetic field .At this
location, the field has a magnitude of 5.5  10–5 T to the
north. Find the speed of the particle.
Forces on Objects in Magnetic
Fields
 Any current carrying wire in an external magnetic field
undergoes a magnetic force.
 This force is perpendicular to the magnetic field.
 This produces a force that can be calculated using the
formula :
Fmagnetic = BIL.
 F is force (N), B is magnetic field strength (T), I is
current (A), and L is the length of the wire (m).
Force on a Current Carrying Wire
The Magnetic Force on Two
Current-Carrying Wires
 Two parallel current-carrying wires exert a force on
one another that are equal in magnitude and
opposite in direction.
 If the currents are in the same direction, the two
wires attract one another.
 If the currents are in opposite direction, the wires
repel one another.
 Loudspeakers use magnetic force to produce
sound.
Force Between Parallel Wires
Example Problem
Force on a Current-Carrying Conductor
A wire 36 m long carries a current of 22 A from
east to west. If the magnetic force on the wire due
to Earth’s magnetic field is downward (toward
Earth) and has a magnitude of 4.0  10–2 N, find
the magnitude and direction of the magnetic field
at this location.