Chapter 19
Magnetism
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
Like magnetic poles repel, and unlike poles
attract.
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.
19.4 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 right-hand
rule:
When the index finger of the right hand points in the
direction of conventional current (I) and the middle
finger points with the magnetic field, the thumb
indicates the direction of the force.
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,
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:
d = distance from center of wire
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:
Equations of Magnetism
Magnetic force on a charge moving through an electric field
Magnetic force on a wire with current flowing
Magnetic field due to current through a wire
Magnetic flux through a surface area
EMF generated in a loop of wire by magnetic flux
19.7 Magnetic Materials
Recap:
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.
Right Hand Rules
Magnetism affecting a
charge/current
Magnetism caused by a
loop/solenoid
Magnetism caused by a
current
Magnetism: Acting on a Charge
The magnetic force acting upon a
moving charge carrier:
The force is perpendicular to both the
velocity and to the field. (RHR)
Magnetism: Acting on a Wire
Magnetic force on a current carrying wire:
(due to force on the moving charge carriers)
θ is the angle
between I and B.
Magnetism: Created by a Wire
Magnetic field created by a current-carrying wire:
d = distance from center of wire
The constant μ0 is called the
“permeability of free space”.
Magnetism: Inside a Loop
The magnetic field at the center of a current loop:
Magnetism: Inside a Solenoid
The magnetic field inside a solenoid:
AP Equations of Magnetism
Magnetic force on a charge moving through an electric field
Magnetic force on a wire with current flowing
Magnetic field due to current through a wire
Magnetic flux through a surface area
EMF generated in a loop of wire by magnetic flux
Ch 20
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.