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Physics 21900
General Physics II
Electricity, Magnetism and Optics
Lecture 10 – Chapter 17.1-2
Introduction to Magnetism
Fall 2015 SemesterProf. Matthew Jones
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Magnetism
William Gilbert (1600)Shen Kuo (1031-1091)
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The Sad Decline in Quality of
Scientific PublicationsWhich publication would you want to read?
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Magnetism in History
Name derived from rocks (lodestones –naturally magnetized pieces of iron ore)
found in the province of Magnesia in Greece, near Turkey.
First reports – 2500 BC
Historically, more interesting than electricity due to importance in early navigation
applications.
Originally thought to be a separate topic from electricity/electrostatics.
Today: unified subject – electromagnetism.
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Fundamental Property of Magnets• Bar magnets always two opposite poles
– The ‘N’ pole is feels an attractive force towards the
northern hemisphere
– Like poles repel, unlike poles attract
– We can describe the magnetic field the same way we
talked about electric fields.
NS
N
S
N
S
S
N
N
S
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Magnetic Monopoles
• Can you cut the end off a magnet to get just the North or South poles?
• Most elementary particles behave like little bar magnets: they have a north end and a south end.
– Although they are point-like we can still talk about which way their magnetic field is pointing
• In principle, a fundamental particle with a magnetic charge could exist, but we haven’t found any evidence that they do.
S N S NS N
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Magnetic Poles – Forces Between Magnets
The magnitude of the force depends on the “pole strength” and the separation between the poles.
Just like gravity and
the electrostatic
force, magnetic
forces are non-
contact forces – they
act on objects that
are not touching
each other.
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Dipole magnetic field/Electric field
• Key point: there are no “magnetic charges”!
• MKS units for magnetic field is the Tesla (T).
Magnetic field lines � Electric field lines �
Units: N/C or V/m.MKS units: Tesla
� is a vector. � is a vector.
� lines are continuous. � lines begin/end on
positive/negative charges.
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Visualizing magnetic fields
• A bar magnet is a permanent
magnet in the shape of a bar.
• The iron filings are small,
needle-shaped, permanent
magnets.
• The magnetic field lines can be
deduced from the pattern of the
iron filings.
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Important Points to Remember
• The MKS/SI unit for magnetic field is the Tesla (T).
• The CGS unit for magnetic field is the Gauss.
1 Tesla = 10,000 Gauss.
• The magnetic field lines go from the north pole towards the south pole.
• The magnitude of the B-field lines decreases as you move farther from a pole.
• The magnetic field lines form closed loops – a general property of ALL magnetic field lines, not just for bar magnets.
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Magnetic Field StrengthSource Typical B field (Tesla)
Interstellar magnetic field 10��
Earth’s magnetic field 5 × 10��
Fridge magnet 5 × 10��
Electromagnet 10�
Rare earth magnet 1
Magnetic Resonance Imaging
(MRI) machine
2
Superconducting magnets 10
Neutron star 10�
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The Earth acts like a large magnet
Geographic north pole
of the earth roughly
corresponds to the
geomagnetic south
pole of the earth.
The north pole of a bar
magnet on the earth’s
surface will be
attracted towards the
Northern hemisphere.
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Earth’s Magnetic FieldEarth’s geomagnetic pole
wanders over time
Magnetic declination in
Indiana is about 4° West.Tru
e N
orth
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Magnetic Declination in the US
Both the geographic and
magnetic north poles are
aligned along the black line.