Magnetism Notes

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Bell Ringer
• Give an example of something that you
observed over break which involved
physics.
• Be sure to thoroughly explain your answer.
Magnetism
Magnetism
• Force of attraction or repulsion cause by the
movement of charged participles
– It is not charge
– It is caused by the movement of charge
Magnetic Poles
• Magnetic Poles: A region on a magnet
which produces magnetic forces
• The poles of a suspended magnet will
align themselves to the poles of the
Earth
• Fundamental Rule: Like poles repel;
opposite poles attract
Magnetic Poles
• Magnetic poles behave similarly to
electric charges EXCEPT:
– Electric charges can
be isolated
– Magnetic poles
cannot
Magnetic Fields
• Magnetic Field: The space around a magnet
in which a magnetic force is exerted
– The shape of a magnetic field is revealed by
magnetic field lines
• Directed away from north poles and toward
south poles
• The strength of magnetic fields are
measured in units of Tesla (T)
Magnetic Fields
Magnetic Fields
Magnetic Fields
• Magnetic fields are caused by the motion of
electric charges
– Since motion is relative, so are magnetic
fields
• Magnets at rest consist of charges in motion
– Every spinning electron is a tiny magnet
– Electrons spinning in the same direction
produce a stronger magnet
Magnetic Domains
• Magnetic Domain: Clusters of aligned
atoms
Magnetic Domains
Bell Ringer
• 1.) Draw the magnetic field lines for a bar
magnet with its North Pole on the bottom
and its south pole on top.
• 2.) How are the domains arranged in a
permanent magnet?
Magnetic Domains
• If the magnetic domains of a material can
align, this material is called
ferromagnetic.
Magnetic Domains
• Permanent magnets are made by placing
materials in strong magnetic fields
–Aligns the domains
• It is possible to weaken a magnet by
dropping or heating it
–Pushes the domains out of alignment
Earth’s Magnetic Field
• Currents in the molten part of Earth
beneath the crust create the Earth’s
magnetic field
– Moving charges loop within the Earth
• Earth’s magnetic field is not stable
– Magnetic pole and geographic pole are
offset
Earth’s Magnetic Field
• Magnetic poles are located:
– North: Hudson Bay region of northern
Canada (1800 km from geographic pole)
– South: Just south of Australia
• Magnetic Declination: Difference
between geographic and magnetic poles
Bell Ringer 5/13
• From what was discussed
in yesterday’s class…what
do you believe the true
name of our school should
be?
Earth’s Magnetic Field
Earth’s Magnetic Field
• The magnetic field of Earth reverses on an
irregular cycle
• 1 million years ago, the field completely
“shut off” for 10,000-20,000 years
• Its strength has decreased by 5% over the last
100 years
– Indicates a reversal possibly in the next 200
years
Earth’s Magnetic Field
Earth’s Magnetic Field
Courtesy of NASA, JPL and GSFC
Electric Current
• Recall: Electric current is
the movement of electric
charges
– Current produces a magnetic
field
• When current is carried
through a conductor, the
magnetic field produces a
pattern of concentric circles
Electric Current
• This concept is
known as Oersted’s
Principle
• If the current
switches direction,
the compass needles
will turn 180o
Electromagnets
• If a current carrying wire is bent into a
loop:
–Field lines become bunched inside the
loop
• Two overlapping loops:
–Twice the concentration of magnetic
field lines
Electromagnets
Electromagnets
Electromagnets
• Electromagnet: A current-carrying coil
of wire with many loops
–The magnetic domains of a piece of
iron placed in the coil will be aligned
–The will intensify the magnetic field
–The strongest electromagnets use
superconductors as a core
Electromagnet Applications
• An alarm bell uses an electromagnet
with a changing magnetic field to move
a plunger
Junkyardpickerupper
Bell Ringer
How does a
speaker work?
Moving Particles
• A moving charged particle could be
deflected by a magnetic field
• For particles moving parallel to the
magnetic field, no deflection occurs
• For particles moving perpendicular to
the magnetic field, maximum deflection
occurs
Moving Particles
Moving Particles
• A current carrying wire will also be
deflected when it encounters a magnetic
field
–The particles trapped inside the wire are
deflected, therefore pushing the wire itself
• If the direction of the current is
reversed, the deflecting force acts in the
opposite direction
Moving Particles
Applications
• The follow items use the idea
• TVs
• MRIs
• Aurora Borealis
Bell Ringer
• When will a charged particle experience the
maximum amount of force when entering a
magnetic field?
Motors
• A simple DC motor uses a permanent
magnet to establish a field where an
electromagnet is made to rotate
– When connected to a battery, the electromagnet will rotate one half turn
– Its south pole will align with the north pole
of the permanent magnet
Motors
Motors
Brushes
Commutator
• These two devices change
the direction of the current
at exactly the correct
moment so that the poles
of the electromagnet
reverse
• This causes the
electromagnet to keep
spinning
Motors
• As the armature passes through the horizontal
position, the poles of the electromagnet
reverse due to the commutator
Bell Ringer
• Where the motors that you made last week
operating on alternating or direct current?
• What would happen with your motor from
yesterday if you took off all the insulation
off of each end?
Meters
• Galvanometer: A sensitive instrument used to
detect electric current
– Consists of a magnetic needle centered
under loops of insulated wire
– Electric current will cause the needle to
pivot, detecting even very small currents
– May be calibrated to measure current
(ammeter) or voltage (voltmeter)
Meters
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