Transcript Magnetism and Electromagnetic Induction

Ch. 21: Magnetism
Magnetism
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magnetic poles produce magnetic forces
poles always exist in pairs (N and S)
opposite poles attract, like poles repel
there are no magnetic “monopoles”
Magnetic Fields
magnetic field lines (B-field) always point from N to S
Big and Little Magnets
• currents within the mantle
produce the earth’s field
• in atoms, orbiting and
spinning electrons produce
tiny magnetic fields
• Fe, Ni, and Co are the most
magnetic elements
Electric Current and B-Fields
• Hans Christian Oersted (1820) first noticed that
an electric current will deflect a compass needle
• first right hand rule
Electric Current and B-Fields
• a current in a coil (or solenoid)
produces an electromagnet
• second right hand rule
B
I
Magnetic Domains
• domains are clusters of billions of
iron atoms with aligned fields
• domains will align in a B-field
• permanent magnets have been
exposed to very strong fields
Magnetic Force
• a charged particle moving perpendicular to a B-field
feels a force
• Fmagnetic  q  v  B 1 Tesla (T) = 1 N/(C · m/s) = N/(A·m)
• third right hand rule:
Cloud Chamber Images
Magnetic Force Problem
• A proton moving at 1200 km/s (in the solar wind)
runs perpendicular into the earth’s magnetic field (B
= 5.5 x 10-5 T). How much force is applied to the
proton?
Mass Spectrometer
• mass spectrometer: an instrument that measures
the mass of charged particles
• used to identify elements present in a sample
Magnetic Force on a Wire
• a current-carrying wire in a B-field will feel a force
perpendicular to the wire
Fmagnetic  q  v  B
Fmagnetic  B  I  L
• How much force is applied to a 5-cm long wire
carrying 12 A of current when it is placed in a 3 mT
magnetic field?
Chapter 22: Induction
and Alternating Current
Magnetic Fields and EMFs
• electromagnetic induction: the production of a
current caused when a conductor is moved through a
magnetic field (or the magnetic field is changed)
• emf: electromotive force; an increase in PE per
charge (voltage) that pushes charges through a
conductor; emf produces a current
• Use the 3rd right hand rule to determine direction of
current.
Lenz’s Law
• Lenz’s law: the magnetic
field of an induced
current opposes the
change in the applied
magnetic field
• energy is conserved due
to this “magnetic friction”
Faraday’s Law
[ AB cos ]
emf   N
t
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N = number of loops
A = area
B = magnetic field
t = time
Generators and Motors
• generator: converts KE to
electrical energy (current)
• spinning a coil in a B-field
causes an AC to form
• the “commutator”
determines if AC or DC
Electric Motors
• motor: a device that converts electric energy (AC or
DC) to KE
Transformers
• transformer: converts AC to
higher or lower voltage
(step up or step down)
• V2 = V1N2 / N1
• Electricity is transmitted at
high V, low I (due to “I2R
loss”) then stepped down
• 230kV to 20kV to 120V