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Introductory Question
Suppose you have a long, thin bar magnet with
a north pole at one end and a south pole at the
Household Magnets other. If you break this bar in half, the two
ends– bar–
new ends–one on each half of the bar–will
A. attract
B. repel
C. neither attract nor repel
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Observations about 5 Questions about
Household Magnets Household Magnets
They attract or repel, depending on orientation Why do any two magnets attract and repel?
Magnets stick only to certain metals Why must magnets be close to attract or repel?
Magnets affect compasses Why do magnets stick only to some metals?
The earth seems to be magnetic Why does a magnetic compass point north?
Some magnets use electricity to operate Why do some magnets use electricity?
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Question 1 Magnetic Pole (Part 1)
Why do any two magnets attract and repel? Objects that attract or repel magnetically carry
Why don’t they attract or repel? portions of a physical quantity called magnetic
Why do their orientations affect their forces? pole or simply “pole”
Pole comes in two types:
Poles of the same type repel
Poles of different types attract
The two types are named “north” and “south”
north pole = −south pole
net pole = north pole − south pole
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Magnetic Pole (Part 2) Magnetic Pole (Part 3)
Magnetic pole Every magnet has equal north and south poles
is a conserved quantity They have magnetic polarizations, not net poles
is analogous to electric charge A typical bar or button magnet is a magnetic dipole
There i h big difference:
Th is, however, one bi diff dipole has h l d h l
A di l h one north pole and one south pole
no isolated magnetic pole has ever been found! A fragment of a magnet
the net pole on any object is always exactly zero! has a net pole of zero
retains its original magnetic polarization
is typically a magnetic dipole
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Introductory Question (revisited) Question 2
Suppose you have a long, thin bar magnet with Why must magnets be close to attract or repel?
a north pole at one end and a south pole at the
other. If you break this bar in half, the two
ends– bar–
new ends–one on each half of the bar–will
A. attract
B. repel
C. neither attract nor repel
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Magnetic Forces (Part 1) Magnetic Forces (Part 2)
Two poles push or pull on one another Since a magnet is a dipole (or more complicated)
with forces that are exactly equal in magnitude it has both north and south poles
but exactly opposite in direction. it simultaneously attracts and repels a second magnet
These
Th magnetostatic f
i forces are th ir net forces depend n distance nd ri nt ti n
their n t f r d p nd on di t n and orientation
proportional to the amount of each charge their net forces decrease precipitously with distance
inversely proportional to (distance between charges)2 they may also experience net torques
The forces increase as the separation decreases
permeability of free space ⋅ pole1 ⋅ pole2
force =
4π ⋅ (distance between poles) 2
2
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Clicker Question Question 3
If two bar magnets are oriented so that their Why do magnets stick only to some metals?
opposite poles face one another and slowly
moved together, they will
A. switch from repelling to attracting.
B. 90°
each rotate 90° so that they are aligned parallel.
C. increase their attraction gradually.
D. increase their attraction suddenly.
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Magnetism in Atoms Magnetism in Materials
Magnetism is due primarily to electrons When atoms assemble into materials,
magnetic—
Electrons are intrinsically magnetic—they’re dipoles the electron magnetic dipoles can cancel still further
Atoms contain electrons, so atoms can be magnetic and that cancellation is complete in most materials.
Wh electrons assemble i
When l bl into atoms, M t materials r non-
nti ll non magnetic
Most m t ri l are essentially n n-m n ti
their magnetic dipoles often cancel one another Some materials don’t experience full cancellation
but this cancellation is usually incomplete Ferromagnetic materials
so most atoms are magnetic. have small domains that are magnetic dipoles
Those domains ordinarily cancel on another
An external magnet, however, can alter the domains
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Refrigerators and Magnets Soft & Hard Magnetic Materials
A refrigerator’s steel has magnetic domains, Soft magnetic materials
but they normally cancel so it appears nonmagnetic. have domains the grow or shrink easily,
When a magnetic pole is brought near the steel so they are easy to polarize or depolarize.
i causes some domains to grow and others to shrink
it d i d h hi k They quickly forget their previous m n tiz ti n
Th q i kl f r t th ir pr i magnetizations.
and the steel develops a net magnetic polarization Hard magnetic materials
so that it attracts the magnetic pole. have domains that don’t grow or shrink easily,
Magnets thus stick to steel refrigerators so they are hard to polarize or depolarize.
They can be magnetized permanently.
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Clicker Question Question 4
bar- button-
When a bar- or button-shaped permanent Why does a magnetic compass point north?
magnet is first formed out of molten ingredients,
it has
A. no magnetic poles on its ends.
B. north magnetic poles on both of its ends.
C. south magnetic poles on both of its ends.
D. a north pole and a south pole at opposite ends.
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Magnetic Fields The Earth’s Magnetic Field
A magnetic field The earth is magnetic,
is a structure in space that pushes on magnetic pole so it is surrounded by a magnetic field
is vector in character: it has magnitude and direction and that field pushes north poles northward.
may depend n position nd time
m d p nd on p iti n and tim i immersed i earth’s fi ld
A magnetic compass i d in h’ field
The magnetic field at a given position and time experiences a magnetic torque
is proportional to the force on a north test pole that aligns it so that its north
is often represented graphically by an arrow pole points northward.
but is actually located at just one point on that arrow
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Clicker Question Question 5
The earth acts like a giant bar magnet. Located Why do some magnets use electricity?
near the earth’s north geographic pole is the
earth’s
A. south magnetic pole.
B. north magnetic pole.
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Electromagnetism (Version 1) Electromagnets
Magnetic fields are produced by Electric currents are magnetic
magnetic poles (but free poles don’t seem to exist), current-
A current-carrying wire has a magnetic field
moving electric charges, A coil of wire carrying current mimics a bar magnet.
nd h n in l tri fields [f r later…].
and changing electric fi ld [for l t r ] An l l i
A electromagnet uses an electric current to
Electric fields are produced by produce its magnetic field, although that field is
electric charges, often enhanced by ferromagnetic materials.
moving magnetic poles [for later…],
and changing magnetic fields [for later…].
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Summary about
Household Magnets
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
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