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Chapter 21
Magnetism
21.1 Magnets and Magnetic Fields
Objectives
1. For given situations, predict whether
magnets will repel or attract each other
2. Describe the magnetic field around a
permanent magnet
3. Describe the orientation of Earth’s
magnetic field
Magnetic Poles
…are similar to electric charges because
like poles repel and unlike poles attract
More on Magnetic Poles
While electric charges can be separated from
each other, magnetic poles cannot. No matter
how many times you cut a permanent magnet, it
will still have a N pole and a S pole. So,
magnetic poles ALWAYS occur in pairs.
Magnetic Materials
Hard magnets: materials that are
difficult to magnetize, but once they
are they tend to retain their magnetism
(examples, cobalt and nickel)
Soft magnets: materials that are easy to
magnetize, but they also tend to lose their
magnetism easily. (example, iron)
Magnetic Field
•The magnetic field is denoted by the letter B
•The direction of the magnetic field at any
location is defined as the direction a compass
needle would point at that location (N)
•The magnitude of the magnetic field is greatest
close to the poles
Magnetic Declination
The difference between
the Earth’s geographic
N pole (true north) and
the magnetic N pole
(indicated on a
compass) is called
magnetic declination.
Earth’s Magnetic Field
•The Earth’s magnetic field is created
by interactions between the inner
core and outer core.
•The inner core is solid iron
and the outer core is liquid.
•The inner core spins at a
different rate than the outer
core, which creates a dynamo
effect, or convections and
currents within the core.
•These convections and currents
create the Earth’s magnetic field.
More on the Earth’s Magnetic Field
•The earth’s magnetic field
protects it against solar winds
from the sun. The charged
particles in the solar winds are
deflected by the field.
•Without the Earth’s magnetic
field, the solar winds would
blow away our atmosphere.
Aurora Borealis
•Caused by collisions of charged
particles from solar flares
•The colors are caused by
ionization of nitrogen and
oxygen in the Earth’s atmosphere
•Most commonly seen closer to the
poles due to longer periods of
darkness and greater magnetic
field strength
•Geomagnetic storms occur most
often in the months surrounding the equinoxes
Does the Earth’s Magnetic
Field Change?
•The North pole is
moving toward Russia
at a rate of ~40 mi/year
•On a compass, this
equates to ~0.2o/yr
Reversal of Earth’s Magnetic Poles
•Last time it occurred was ~780,000 years ago
•Takes anywhere from 1,000-10,000 years to complete
21.2 Electromagnetism and
Magnetic Domains
Objectives
1. Describe the magnetic field produced by
the current in a straight conductor and
in a solenoid
2. Explain magnetism in terms of the domain
theory of magnetism
Magnetic Field and Electric Charge
•The region of space surrounding
a moving charge contains:
An electric field (E)
A magnetic field (B)
Both fields have a defined
magnitude and direction
Magnetic Field Surrounding a
Current-Carrying Wire
•Because a wire carries a
current (I), it produces
a magnetic field
•The direction of the
magnetic field (B) can
be found using the
Right Hand Rule
Magnetic Field Surrounding a
Current-Carrying Wire
•The current produces
concentric circles of magnetic
field lines around the wire
•The magnetic field strength
(B) is directly proportional
to the current in the wire
and inversely proportional
to the distance from the wire
Magnetic Field Surrounding a
Current-Carrying Loop
•The right hand rule
applies for the direction
of the magnetic field
for a looped conductor
Looped Conductors
Solenoid – a long, helically wound coil of
insulated wire, which acts as a magnet
when carrying a current
Looped Conductors, cont.
Electromagnet – a solenoid with an iron
rod inserted into the core. The magnetic
field induced into the rod adds to the
magnetic field of the solenoid.
Electromagnetic Induction
• Discovered by Michael Faraday
• A moving electrical current creates a
magnetic field (motors)
• A moving magnetic field creates an
electrical current (generators)
Magnetic Domains
• Domain – a microscopic
magnetic region
composed of a group of
atoms whose magnetic
fields are aligned in a
common direction
• In hard magnetic
materials, the domains
persist after the external
magnetic field is
removed, resulting in a
permanent magnet.
21.3 Magnetic Force
Objectives
1. Given the force on a charge in a magnetic
field, determine the strength of the magnetic field
2. Use the right hand rule to find the direction of
the force on a charge moving through a magnetic
field
3. Determine the magnitude and direction of the
force on a wire carrying current in a magnetic field
Magnetic Force and Electric Charges
• A stationary charged particle (q) does not
interact with a constant magnetic field
• A charged particle (q) that is moving at a
velocity (v) through a magnetic field (B)
experiences a magnetic force (Fmagnetic)
Fmagnetic = qvB
Note: q is assumed to be a positive charge
Magnitude of Magnetic Force
Rearranging Fmagnetic = qvB, we get
B = Fmagnetic / qv
The units for B are Teslas (T) where
1T = 1N/(C*m/s) = N/(A*m) = (V*s)/m2
Directions of B and Fmagnetic
Using your right hand, if you point your fingers in the
direction of the magnetic field (B), and your thumb in the
direction of the charge movement (v), then the force on
the charge (Fmagnetic) will point out of your palm for a
positive charge. (Into your palm for a negative charge)
More on Directions of B and Fmagnetic
Magnetic Field (B) Directions
• When the field is INTO the page, use X
• When the field is out of the page, use
• When the field is in the plane of the
page, use
.
Question
A proton moving east at 1.0x105 m/s experiences
a force of 8.8x10-19 N upward (away from Earth).
What is the magnitude and direction of the
Earth’s magnetic field at this location?
For magnitude: B = Fmagnetic/qv
For direction: Use RHR with palm facing up for
the force and thumb facing east for the velocity.
Your fingers point in the direction of the magnetic field.
Answer: 5.5x10-5 T, north
Charge moving in a magnetic field
The magnetic field (B)
is going into the page
The velocity of the charge
(v) is perpendicular to B
The force exerted on the
charge (Fmagnetic) due to B
at location O is upward
As the charge keeps moving
it is continually deflected
inward, resulting in a
circular path
Magnetic Force on a CurrentCarrying Conductor
A current-carrying conductor
in a magnetic field experiences
a force that is perpendicular
to the direction of the current
Magnetic Force on a CurrentCarrying Conductor
Fmagnetic = BIL
B is the magnitude of the magnetic field
I is the current
L is the length of the conductor within B
Question
A wire 36m long carries a current of 22A from east to west.
Find the magnitude and direction of the force on the
wire if the magnetic field of the Earth at this location
is directed from S to N and has a magnitude of 5.0x10-5 T.
Magnitude: Fmagnetic = BIL
Direction: Use RHR with thumb facing west
(for current direction) and fingers pointing
north (for magnetic field direction)
Answer: 4.0x10-2 N downward (toward Earth)