Download Household Magnets

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Household Magnets
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Observations about
Household Magnets
They attract or repel, depending on orientation
Magnets stick only to certain metals
Magnets affect compasses
The earth is magnetic
Some magnets require electricity
Turn off all electronic devices
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5 Questions about
Household Magnets
Why do any two magnets attract and repel?
Why must magnets be close to attract or repel?
Why do magnets stick only to some metals?
Why does a magnetic compass point north?
Why do some magnets require electricity?
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Question 1
Q: Why do any two magnets attract and repel?
A: Each magnet has both north and south poles
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Like magnetic
g
poles
p
repel,
p opposite
pp
p
poles attract
Magnetic pole is a conserved quantity
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North pole is a “positive” amount of pole.
South pole is a “negative” amount of pole.
The net pole on any object is always exactly zero!
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Magnets
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Unlike charges, free poles are never observed
A magnet always has equal north and south poles
It has magnetic polarization, but zero net pole
A typical bar or button magnet is a magnetic dipole
 A dipole has one north pole and one south pole
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A fragment of a magnet has a net pole of zero
it retains its original magnetic polarization
 it is typically a magnetic dipole
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Question 2
Q: Why must magnets be close to attract or repel?
A: Forces are weakened by distance and cancellation
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The magnetostatic
g
forces between p
poles are
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proportional to the amount of each pole
proportional to 1/distance2
force =
permeability of free space  pole1  pole 2
4π  (distance between poles)2
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Forces between Magnets
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Each magnet has both north and south poles
Magnets simultaneously attract and repel
The net forces and net torques on magnets
depend on distance and orientation
 are typically dominated by the nearest poles
 increase precipitously with decreasing distance
Question 3
Q: Why do magnets stick only to some metals?
A: Only a few metals are intrinsically magnetic.
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Electrons have intrinsic magnetic
g
dipoles
p
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In a few solids, the cancellation is incomplete
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Ferromagnetic materials have magnetic domains
Soft & Hard Magnetic Materials
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Those domains ordinarily cancel on another
 A magnet can alter those domains → magnetization
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A magnet can magnetize
i a steell refrigerator
fi
it causes some domains to grow and others to shrink
the steel develops a net magnetic polarization
 it attracts the magnetic pole that magnetized it
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Magnets thus stick to steel refrigerators
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Hard magnetic materials
have domains that don’t grow or shrink easily,
so they are hard to magnetize or demagnetize.
 They can be magnetized permanently.
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Soft magnetic materials
have domains the grow or shrink easily,
so they are easy to magnetize or demagnetize.
 They
Th qquickly
i kl fforget
r t th
their
ir pr
previous
i
m
magnetizations.
n tiz ti n
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Question 4
Q: Why does a magnetic compass point north?
A: Earth’s magnetic field twists it northward.
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The earth p
produces a magnetic
g
field that
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Iron and most steels are ferro
ferromagnetic
magnetic materials
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Refrigerators and Magnets
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In atoms, much of that magnetism remains active
In solids, that magnetism is usually cancelled perfectly
pushes north poles northward, south poles southward
exerts torques on magnetic dipoles, such as compasses
Magnetic Fields
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A magnetic field
is a structure in space and time that pushes on pole
a vector field:
field: a vector at each point in space and time
 observed
b r d using
in a n
north
rth ttestt p
pole
l att each
hp
point
int
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A compass immersed in earth’s magnetic field
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aligns it so that its north pole points northward.
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Question 5
Q: Why do some magnets require electricity?
A: Electric currents are magnetic!
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A currentcurrent-carrying
y g wire produces
p
a magnetic
g
field
A currentcurrent-carrying coil mimics a bar magnet
An electromagnet typically uses an electric current
Electromagnetism (Version 1)
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magnetic poles and subatomic particles,
moving electric charges,
 and
nd changing
h n in electric
l tri fi
fields
ld [for
[f r llater…].
t r ]
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Summary about
Household Magnets
Electric fields are produced by
electric charges and subatomic particles,
moving magnetic poles [for later…],
 and changing magnetic fields [for later…].
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to produce a magnetic field
 to magnetize a ferromagnetic material
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Magnetic fields are produced by
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They all have equal north and south poles
They polarize soft magnetic materials and stick
They are surrounded by magnetic fields
Can be made magnetic by electric currents
Electric Power
Distribution
Turn off all electronic devices
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Observations about
Electric Power Distribution
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Household electricity is alternating current (AC)
Household voltages are typically 120V or 240V
Power is distributed at much higher voltages
Power transformers are common around us
Power substations are there, but harder to find
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4 Questions about
Electric Power Distribution
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Why isn’t power transmitted via large currents?
Why isn’t power delivered via high voltages?
What is “alternating current” and why use it?
How does a transformer transfer power?
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Question 1
Q: Why isn’t power transmitted via large currents?
A: Too much power would be wasted in the wires.
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Current--carrying
Current
y g wires consume and waste power
p
power wasted = current · voltage drop in wire
 voltage drop in wire = resistance · current (Ohm’s law)
 power wasted = resistance · current2.
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Question 2
Q: Why isn’t power delivered via high voltages?
A: High voltage power is dangerous.
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a spark hazard,
a fire hazard,
 and a shock hazard.
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Large currents waste large amounts of power
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High
g voltages
g can produce
p
large
g voltage
g gradients
g
Current may flow through unintended paths
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The Voltage Hierarchy
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Electric power delivered to a consumer is
power delivered = current · voltage drop
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Large currents are too wasteful for transmission
High voltages are too dangerous for delivery
So electric power distribution uses a hierarchy:
Question 3
Q: What is “alternating current” and why use it?
A: Fluctuating current → so transformers will work
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In alternatingg current,
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the voltages of the power delivery wires alternate
and the resulting currents normally alternate, too.
high-voltage transmission circuits in the countryside
highmedium--voltage circuits in cities
medium
 low
low--voltage delivery circuits in neighborhoods
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Transformers transfer power between circuits!
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AC and Transformers
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Alternating voltage in the US
completes 60 cycles per second,
 so voltage and current reverse every 1/120 second.
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AC complicates
li
the
h ddesign
i off electronic
l
i ddevices
i
AC permits the easy use of transformers,
which can move power between circuits:
from a low
low--voltage circuit to a highhigh-voltage circuit
 from a highhigh-voltage circuit to a lowlow-voltage circuit
Question 4
Q: How does a transformer transfer power?
A: Its changing magnetic fields induce currents.
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that produces an alternating magnetic field
that produces an electric field
 that pushes current through the secondary coil
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The p
primaryy coil carries an alternatingg current
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Power moves from primary coil to secondary coil
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Electromagnetism (Version 2)
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Magnetic fields are produced by
magnetic poles and subatomic particles,
 moving electric charges,
 and
nd changing
h n in electric
l tri fi
fields
ld [more
[m r llater…].
later…]
t r ].
Electromagnetic Induction
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Electric fields are produced by
electric charges and subatomic particles,
moving magnetic poles,
poles,
 and changing magnetic fields.
fields.
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Moving poles or changing magnetic fields
produce electric fields,
which propel currents through conductors,
 which p
produce magnetic
g
fields.
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Changing magnetic effects induce currents
Induced currents produce magnetic fields
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Lenz’s Law
When a changing magnetic field induces a current
in a conductor, the magnetic field from that
current opposes the change that induced it
Transformers
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Alternating current in one circuit can induce an
alternating current in a second circuit
A transformer
uses induction
i d i
to transfer power
between its circuits
 but doesn’t
transfer any charges
between its circuits
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Current and Voltage
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Step--Down Transformer
Step
A transformer must obey energy conservation
Power arriving in its primary circuit must equal
power leaving in its secondary circuit
Si
Since
power is
i the
h product
d off voltage
l
· current,
current,
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a transformer can exchanging voltage for current
or current for voltage!
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A step
step--down transformer
has relatively few turns in its secondary coil
so charge is pushed a shorter distance
 and
nd experiences
p ri n a smaller
m ll r voltage
lt
ri
rise
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A larger current
at smaller voltage
flows in the
secondary circuit
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Step--Up Transformer
Step
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A step
step--up transformer
has relatively many turns in its secondary coil
 so charge is pushed a longer distance
 and
nd experiences
p ri n a llarger
r r voltage
lt
ri
rise
Power Distribution System
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A smaller current
at larger voltage
flows in the
secondary circuit
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Summary about
Electric Power Distribution
Inductor
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A step
step--up transformer increases the voltage
for efficient longlong-distance transmission
A step
step--down transformer decreases the voltage
for safe delivery to communities
comm nities and homes
Electric and magnetic fields both contain energy
Electromagnet has magnetic energy
Stores energy as current increases
 Releases
R l
energy as current decreases
d
 Exhibits Lenz’s law
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Current change induces opposing current
 Opposes any changes in current
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Electric power is transmitted at high voltages
Electric power is delivered at low voltages
Transformers transfer power between circuits
Transformers require AC power to operate
The power distribution system is AC
Known as an inductor
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