Transcript Chapter 24 Magnetism - Alejandro Garcia's Web Site

Chapter 24
Magnetism
21-Jul-15
Physics 1 (Garcia) SJSU
WARNING
Strong magnets will be
passed around the room.
Keep these magnets away
from iPods and laptops
(especially my laptop)!
21-Jul-15
Physics 1 (Garcia) SJSU
Magnetic Forces
General observations regarding magnets:
Iron (and a few other metals) are
ferromagnetic, which means they can
become magnetized.
Magnets attract ferromagnetic metals.
Two magnets can either attract or repel
each other depending on poles.
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Magnetic Poles
Cow magnets
Two types of magnetic poles:
North (N) and South (S)
N
S
As with electric charges, like poles
(N&N, S&S) repel and opposites
(N&S) attract.
Unlike electric charges, cannot have
just a North or just a South pole
Magnetic poles are not electric charges
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Magnets
N&S
on sides
Compass
needle is
magnet
Demo: Magnetic Fields
Magnetic field points
from South to North.
Iron filing act as tiny
compass needles,
outlining magnetic
field lines.
Magnet
Iron filings in clear oil
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Inside the magnet, field lines
go North to South
Ferromagnetic Metals
Ferromagnetic metals have similar atomic
structure.
Spin of the
electron in
these metals
produces a
net magnetic
field
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Iron, Cobalt, Nickel
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Demo: Magnetism & Money
Most US coins are not made of ferromagnetic materials
but many other countries use iron steel in their currency.
Some pennies
were made of
steel during
World War II
Some Euro coins
contain steel
Iron is in the ink used in US paper
currency to avoid counterfeiting.
Buffalo nickels are
25% nickel metal,
which is
ferromagnetic
Magnetic Domains
The magnetic field of an single iron atom is so
strong that interactions among adjacent atoms
cause large clusters of atoms, called magnetic
domains, to line up with one another.
A microscopic view of magnetic
domains in a crystal of iron. Each
domain consists of billions of aligned
iron atoms. The blue arrows pointing
in different directions tell us that
these domains are not aligned.
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Magnetizing Iron
Magnetic domains can be induced to align
by an external magnetic field.
S
N S
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Strong
Magnet
Strong
Magnet
N
N
Demo: Magnetizing Iron
Magnetic domains in iron nails are
induced to align by proximity of
the strong magnet
Each nail becomes itself a magnet,
which in turn magnetizes the nail
below it, forming a chain.
When the strong magnet is
removed, most of the domains
un-align and nail lose most of
their magnetization.
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Demo: Demagnetizing Iron
Magnetic domains can be scrambled by
heating the iron, striking it with great force,
or other disruptions of alignment.
Magnetized
Test tube of
un-magnetized
iron filling
SHAKE
S
S
N
N
Magnetizer
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Demagnetized
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Demo: Demagnetizing Iron
Iron nail is attracted to
the large magnet due to
alignment of domains in
the nail.
Heat the nail to a high
temperature and the
domains become
randomized so the nail
is no longer attracted to
the magnet.
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Demo: Electric Currents &
Magnetic Fields
An electric current produces a magnetic field.
Electric
Current
Magnetic
Field
Field lines point in the northward direction; never reach a “North Pole”
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Electromagnets
N
Electric current in a coil of wire creates a
magnetic field similar to a bar magnet.
S
Current
passing
through
loops of
coiled wire
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Demo: Electromagnets
Electromagnet
created by
passing current
through a coil of
wire.
Electromagnet is
stronger when
an iron bar is
inserted within
the coil.
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N
Iron Bar
Note: Do this in lab too.
Wire
Coil
S
Used in lab
Connect to
battery or
power supply
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Check Yourself
When an object is charged with static electricity, is
that object also magnetized?
But isn’t iron is magnetized when the electrons are
aligned. If iron has electrons, then why isn’t it
charged?
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Loudspeakers
Loudspeaker has a membrane
but oscillations are created by
variations in electrical current,
which cause an electromagnet to
be pulled towards and away from
a second, permanent magnet.
These oscillations cause the
membrane of the loudspeaker
to vibrate with the same
frequency as the oscillations
in the electrical current.
Headphones work essentially the
same way, they’re just smaller.
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N
The configuration of the Earth's
magnetic field resembles a
strong bar magnet located
near the center of the Earth.
The magnetic South pole is
near the geographic North
pole (so North pole of
compass attracted towards
Northern direction).
There isn’t an giant
underground magnetized
chunk of iron; Earth’s interior
is simply too hot.
S
Earth’s Magnetic Field
Origin of Earth’s Magnetic Field
Earth is an electromagnet with electric currents
deep below the surface. Moving charges, looping
around within the molten part of the Earth, create
the magnetic field.
These currents are
possibly the result of
thermal convection
rising from the central
core combined with the
rotation of the Earth
about its axis.
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Demo: Magnetic Force & Current
Moving charges in an electric current
experience a force due to magnetic field.
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Electric Meters
Since magnetic force on a wire depends on the
current, can use this effect to design a meter to
measure current (an ammeter).
Current
Needle
moves
Current
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Magnetic Force on Charges
Moving electric charges deflected by
magnetic fields.
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Demo: Crooke’s Tube
Electron beam in a Crooke’s tube is deflected
when a magnet is brought near the tube.
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Television Tube
Electron beams, deflected by magnetic
fields, are used to create TV images.
Electromagnets
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Demo: Magnets & TV sets
Picture on a TV set is distorted by presence of a
magnet since picture formed by an electron beam.
Warning: Strong
magnets can
permanently
damage a
television or a
computer.
Physics 1 (Garcia) SJSU