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Units of Chapter 19
Magnets, Magnetic Poles, and Magnetic Field
Direction
Magnetic Field Strength and Magnetic Force
Applications: Charged Particles in Magnetic
Fields
Magnetic Forces on Current-Carrying Wires
Applications: Current-Carrying Wires in
Magnetic Fields
Electromagnetism: The Source of Magnetic
Fields
Units of Chapter 19
Magnetic Materials
Geomagnetism: The Earth’s Magnetic Field
19.1 Magnets, Magnetic Poles, and
Magnetic Field Direction
Magnets have two
distinct types of poles;
we refer to them as
north and south.
19.1 Magnets, Magnetic Poles, and
Magnetic Field Direction
Two magnetic poles of opposite kind form a
magnetic dipole. All known magnets are
dipoles (or higher poles); magnetic monopoles
could exist but have never been observed.
A magnet creates a magnetic field:
The direction of a magnetic field (B) at any location is
the direction that the north pole of a compass would
point if placed at that location.
19.1 Magnets, Magnetic Poles, and
Magnetic Field Direction
North magnetic poles are attracted by south
magnetic poles, so the magnetic field points
from north poles to south poles.
The magnetic field may be represented by
magnetic field lines.
The closer together (that is, the denser) the B field
lines, the stronger the magnetic field. At any location,
the direction of the magnetic field is tangent to the
field line, or equivalently, the way the north end of a
compass points.
19.2 Magnetic Field Strength and
Magnetic Force
A magnetic field can exert a
force on a moving charged
particle.
19.2 Magnetic Field Strength and
Magnetic Force
The magnitude of the force is proportional
to the charge and to the speed:
SI unit of magnetic field: the tesla, T
19.2 Magnetic Field Strength and
Magnetic Force
In general, if the particle is moving at an
angle to the field,
The force is perpendicular to both the
velocity and to the field.
19.2 Magnetic Field Strength and
Magnetic Force
A right-hand rule gives the
direction of the force.
19.3 Applications: Charged Particles
in Magnetic Fields
A cathode-ray tube, such as a television or
computer monitor, uses a magnet to direct a
beam of electrons to different spots on a
fluorescent screen, creating an image.
19.3 Applications: Charged Particles
in Magnetic Fields
A velocity selector consists of an electric
and magnetic field at right angles to each
other. Ions entering the selector will
experience an electric
force:
and a magnetic force:
These two forces will be
perpendicular to each
other.
19.3 Applications: Charged Particles
in Magnetic Fields
Given the values of the electric and magnetic
fields, there will be a single velocity that will
allow ions to pass through the selector
undeflected.
19.3 Applications: Charged Particles
in Magnetic Fields
A mass spectrometer can be used to measure
the masses of ions with equal charges and
velocities.
After passing through a velocity selector, ions
enter a magnetic field. They will move in a
circle of radius:
19.3 Applications: Charged Particles
in Magnetic Fields
Then the mass of the ion is:
19.4 Magnetic Forces on CurrentCarrying Wires
The magnetic force on a current-carrying wire
is a consequence of the forces on the
charges. The force on an infinitely long wire
would be infinite; the force on a length L of
wire is:
θ is the angle
between I
and B.
19.4 Magnetic Forces on CurrentCarrying Wires
The direction of the force is given by a righthand rule:
When the fingers of the right hand are pointed in the
direction of the conventional current I and then curled
toward the vector B, the extended thumb points in the
direction of the magnetic force on the wire.
19.4 Magnetic Forces on CurrentCarrying Wires
A current loop in a magnetic
field will experience a torque:
If there are multiple loops in
a coil,
The magnetic moment:
19.5 Applications: Current-Carrying
Wires in Magnetic Fields
A galvanometer has a coil
in a magnetic field. When
current flows in the coil, the
deflection is proportional to
the current.
19.5 Applications: Current-Carrying
Wires in Magnetic Fields
An electric motor converts electric energy into
mechanical energy, using the torque on a
current loop.
19.5 Applications: Current-Carrying
Wires in Magnetic Fields
An electronic balance uses magnetic force to
balance an unknown mass. The amount of
current required is proportional to the mass.
19.6 Electromagnetism: The Source
of Magnetic Fields
Experimentally, we observe that a
current-carrying wire creates a
magnetic field.
19.6 Electromagnetism: The Source
of Magnetic Fields
The magnitude of the field is given by:
The constant μ0 is called the permeability of
free space.
19.6 Electromagnetism: The Source
of Magnetic Fields
The field lines form circles around the
wire; the direction is given by a right-hand
rule.
19.6 Electromagnetism: The Source
of Magnetic Fields
The magnetic field at the center of a current
loop:
19.6 Electromagnetism: The Source
of Magnetic Fields
A solenoid is a wire coiled into a long cylinder.
The magnetic field inside is given by:
19.7 Magnetic Materials
Atomic electrons have a property called “spin”
that gives them a small magnetic moment. In
multielectron atoms, the electrons are usually
paired with an electron of the opposite spin,
leaving no net magnetic moment.
However, this is not always the case, and
some atoms do have a permanent magnetic
moment. They will experience a torque in a
magnetic field, and will tend to align with it.
19.7 Magnetic Materials
In ferromagnetic materials, the forces between
neighboring atoms are strong enough that they
tend to align in clusters called domains. These
domains are macroscopic in size.
19.7 Magnetic Materials
When a ferromagnet is placed in a magnetic
field, the domains tend to align with it.
19.7 Magnetic Materials
When the external magnetic field is removed,
the domains tend to stay aligned, creating a
permanent magnet.
The most common ferromagnetic materials
are iron, nickel, and cobalt. Some rare earth
alloys are also ferromagnetic.
19.7 Magnetic Materials
Ferromagnetic materials can be used to form
electromagnets. Putting this material within a
solenoid greatly enhances the magnetic field:
Here, κm is the magnetic permeability of the
material; for ferromagnets, κm is typically
several thousand.
19.7 Magnetic Materials
For commercially
useful ferromagnets,
a type of iron is used
that does not retain
its magnetization
when the current is
turned off (why?).
19.7 Magnetic Materials
A “permanent” magnet can lose its
magnetization through impact or heating.
Every ferromagnetic material has a Curie
temperature, above which the thermal motion
immediately destroys any magnetic
alignment.
Lava flows “freeze” a record of the Earth’s
magnetic field at the point where they cooled
below the Curie temperature. In this way,
historical values of the Earth’s field may be
determined.
19.8 Geomagnetism: The Earth’s
Magnetic Field
The Earth’s magnetic
field is similar to that of
a bar magnet, although
its origin must be in the
currents of molten rock
at its core.
Its magnitude is
approximately 10–5 to
10–4 T.
19.8 Geomagnetism: The Earth’s
Magnetic Field
The magnetic poles are
not in exactly the same
place as the geographic
poles; when navigating
with a compass, you
need to know the angle
between them, called
the declination, at your
position.
19.8 Geomagnetism: The Earth’s
Magnetic Field
Charged particles can become trapped around
magnetic field lines. Such trapping of solar wind
particles has resulted in bands of charged
particles around the Earth called Van Allen belts.
Review of Chapter 19
Opposite magnetic poles attract; like poles
repel.
Magnetic force on a charged particle:
Magnetic force on a current-carrying
wire:
Force directions are determined using a
right-hand rule.
Review of Chapter 19
Torque on a current loop:
Magnetic field produced by a long straight
wire:
The field forms circles around the wire.
Review of Chapter 19
Magnetic field at the center of a current loop:
Magnetic field at the center of a solenoid:
Right-hand rules determine the directions
of the fields.
Review of Chapter 19
Ferromagnetic materials spontaneously align
into domains. The domains then align with an
external magnetic field.
When the external field is removed, the
ferromagnet may retain its magnetism.