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Magnetism and Electromagnetic
Induction
Chapters 21 and 22 of the Book
Magnets
• A magnet has a north pole and a south pole.
• Similar to electric charges, like poles repel and
opposite poles attract.
• Poles cannot be isolated. If you cut a magnet into
two pieces, each piece will have a north and south
pole.
• Magnetism is caused by the “spinning” of electrons.
Magnetic Domains
• In most materials, electrons pair up with spins opposite to
each other, resulting in no net magnetism. The magnetic
fields produced by ferromagnetic materials do not cancel
out completely.
• In these materials large groups of atoms form with aligned
spins. These groups are magnetic domains.
• When these domains experience an external magnetic field
they can line up in the same orientation to enhance the
overall magnetic field strength.
Permanent Magnets
• Similar to charging, one magnetic object can make
another ferromagnetic material a magnet.
• In a magnetically hard material, domains will
remain aligned after the first magnet is removed
resulting in the creation of a permanent magnet.
• In a magnetically soft material, domains become
dissociated easily.
• Domains can also become unaligned by heating or
hammering.
Magnetic Field Lines
• When we draw magnetic field lines, they leave the
north pole and enter the south pole.
• They do not have a beginning or ending. They form
continuous loops.
• Arrows on lines will indicate a field line in the plane
of the paper. Crosses show that the field is directed
into the page. Circles show that the field is directed
out of the page.
Magnetic Flux
• Magnetic flux is a quantity that refers to the
amount of field lines that cross a given area at right
angles.
Φm = ABcosƟ
• Where around a magnet is the flux greatest?
• Due to convection currents in Earth’s liquid core,
Earth acts like a giant magnet.
• The Earth’s magnetic poles switch on average about
every 300,000 years.
• The magnetic field shields Earth from solar wind.
Magnetism from Electricity
• In 1820, Hans Christian Oersted demonstrated
that a current-carrying wire caused compass
needles to be deflected.
• An induced magnetic field was created in
directions tangent to concentric circles around
the wire.
The Right Hand Rule
• Which way is the magnetic field pointing?
1. Hold your hand as if grasping the wire.
2. Point your thumb in the direction of the
current.
3. The magnetic field is in the direction that
your remaining four fingers point.
• The right hand rule can also be applied to a loop of
wire.
Solenoids
• A solenoid combines loops of wire to increase
magnetic field strength. The greater the number of
loops, the greater the field strength.
• The field of a solenoid can be increased further by
inserting a ferromagnetic rod in the center. This is
called an electromagnet.
Magnetic Force
• When a charge moves through a constant magnetic
field, it experiences a force.
F=qvB
(for a charge moving perpendicular to the field)
• The unit of a magnetic field is the Tesla (T). 1 T is equal
to the field strength that would act on 1 C of charge
moving at 1 m/s with a force of 1 N.
• A Tesla is a large quantity. The magnetic field of the
Earth is ~ 50 µT. The LHC superconducting magnets can
generate a 30 T field.
Another Right Hand Rule
• Which way does the force push the charge?
1. Put your fingers along the direction of the
field.
2. Point your thumb in the direction of the
charge’s velocity.
3. Your palm will point in the direction of the
magnetic force.
Particle in a Magnetic Field
A proton moving east experiences a force of 8.8x10-19
N upward due to the Earth’s magnetic field. At this
location, the field has a magnitude of 5.5x10-5 T to
the north. Find the veloctiy of the particle.
Magnetic Force on Wires
• The force experienced by a current-carrying wire is
the sum of all of the individual forces of the
electrons moving within it.
F = BIl
The parallel wires near each other will experience
magnetic force: towards each other if the current is
in the same direction and away if it is in opposite
directions.
• A speaker uses the force on a coil of wire to vibrate
a cone that transmits sound.
Electromagnetic Induction
• Just as a flow of current can generate a magnetic field,
a change in the flux of a magnetic field can create a
flow of current.
• As long as there is relative motion between a magnet
and a wire, there will be an induced current.
• The magnetic force pushes charges in along the wire
similar to the electric force created by a battery.
• The angle of the wire affects the current. It is
maximized if the wire is perpendicular to the field, zero
if the wire is parallel.
3 Ways of Inducing a Current
1. A circuit is moved into or out of a magnetic
field.
2. A circuit is rotated in a magnetic field.
3. The intensity or direction of a magnetic field
is varied.
Induced Current Continued
• Consider a bar magnet approaching a coil of wire.
• As the magnet approaching the coil, the magnetic
field passing through the coil increases in strength.
The increasing induced current generates an
increasing magnetic field that opposes that of the
bar magnet.
• The opposite situation occurs as the bar magnet is
moved away from the coil. However, the magnetic
field still opposes the movement of the magnet.
Lenz’s Law.
• This is Lenz’s Law.
• The magnetic field in an induced current is in a
direction that opposes the change that caused it.
Faraday’s Law of Induction
• Michael Faraday calculated the magnitude of
electromotive force (emf) caused by induced
current.
emf = -NΔφm/Δt
• emf is measured in volts. N is the number of loops
in a wire. What factors go into flux?
Induced emf
A coil with 25 turns of wire is wrapped around a
hollow tube with an area of 1.8 m2. Each turn has
the same area as the tube. A uniform magnetic
field is applied at a right angle to the plane of the
coil. If the field increases uniformly from 0.00 T to
0.55 T in 0.85 s, find the magnitude of the induced
emf in the coil. If the resistance in the coil is 2.5 Ω,
find the magnitude of the induced current.
Electric Guitars
• Electric guitars use a device called a pickup to
change mechanical energy into electrical energy.
• A pickup consists of a permanent magnet wrapped
in a copper wire. The number of wrappings
determine the current that the pickup produces.
• Guitar strings are slightly magnetic. When one is
plucked it changes the magnetic field above the
pickup, resulting in a change in the magnetic field
and therefore a current.
Generators
• A more practical way of generating electricity is to
move the wire, not the magnet.
• A generator uses mechanical energy to move a coil
of wire in a magnetic field inducing a current in the
wire.
• Generators produce alternating currents, but ac can
be changed to dc.
Motors
• A motor converts electrical energy to mechanical
energy. In essence, this is a generator in reverse.
• Keep in mind that as the coil of wire turns through
the magnetic field, it induces an emf that opposes
the current already in the wire. This is called back
emf.
• The back emf will become larger as the motor turns
faster.
Transformers
• A transformer uses an alternating current in one
coil of wire to induce an alternating current in
another coil of wire.
• The voltage of the induced current can be affected
by changing the number of loops.
Transformer Equation
V2 = V1 (N2/N1)
• N1 is the primary coil. N2 is the secondary coil.
• A step-up transformer is used on a 120 V line to
provide a potential difference of 2400 V. If the
primary has 75 turns, how many turns must the
secondary have?