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IB Physics: Magnetism and Electromagnetic
Induction.
“Field Lines Always Point
Away from the _____
North and
Toward the South
_____.”
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Words to Inspire Us. “The
search for truth is more
precious than its possession.”
- Albert Einstein
Magnetite: From Magnesia (Greece).
Formula: Fe3O4.
Description: Dark grey, slightly shiny.
Magnetite is naturally magnetic. It is also called
Lodestone. In Middle Ages, pilots were called
lodesmen. The lodestar is the Polar star, the
leading star by which mariners are guided.
The name probably comes from Magnesia,
but there is a fable of Magnes, a Greek
shepherd, who discovered magnetite when
the nails in his shoes stuck to the ground!
Early History
600 BC - Lodestone
The magnetic properties of natural ferric ferrite (Fe3O4) stones
(lodestones) were described by Greek philosophers.
1040 - One of the earliest compasses (China)
A floating fish-shaped iron leaf, mentioned in the Wu Ching
Tsung Yao which was written around 1040. The book
describes how iron can be heated and quenched to produce
thermo-induced magnetization. The first clear account of
suspended magnetic compasses in any language was written
by Shen Kua in 1088.
1175 - First Reference to a Compass
Alexander Neckem an English monk of St. Albans describes
the workings of a compass.
1269 - First Detailed Description of a Compass
Petrus Peregrinus de Marincourt, a French Crusader,
describes a floating compass and a compass with a pivot point.
The Magnetic Field
1600 - Static Electricity (De Magnete)
William Gilbert(1544-1603) studied
magnetism and in 1600 wrote "De
magnete" which gave the first rational
explanation to the mysterious ability
of the compass needle to point northsouth: the Earth itself was magnetic.
"De Magnete" opened the era of modern physics and
astronomy and started a century marked by the great
achievements of Galileo, Kepler, Newton and others.
Gilbert recorded three ways to magnetize a steel needle:
by touch with a loadstone; by cold drawing in a NorthSouth direction; and by exposure for a long time to the
Earth's magnetic field while in a North-South orientation.
Magnets in Ancient Times
Magnetism has been known since ancient times because it
occurs naturally in loadstone, a rock rich in magnetite, a form
of iron oxide.
It was believed by some
that magnetic fields
permeated humans and
their manipulation could
affect health.
Some Chinese
cities are laid out
along the direction
of the Earth’s
magnetic field.
The first
compasses
were made
in China in
~1000 AD.
Sir William Gilbert
(1544-1603)
Magnets have two poles, which
he called north and south.
Like poles repel and opposite
poles attract.
Iron can be
magnetized.
The Earth is a
giant magnet.
Gilbert’s book, De Magnete,
was enormously popular
and influenced Kepler and
Galileo.
The Magnetic Field
The ‘Gilbert Model’
Like poles repel, and unlike poles attract.
Cut a magnet in half and you will have
two magnets.
A single pole (monopole) has never been
isolated.
Magnetic Field of a
Bar Magnet.
Field lines always
point away from the
North and toward
the South.
Filing demonstration of magnetic
field lines.
Edmond Halley, 16561742, (of comet fame)
ingeniously proposed
that the Earth
contained a number of
spherical shells, one
inside the other, each
magnetized differently,
each slowly rotating in
relation to the others.
Franz Anton Mesmer
(1734-1815)
He was born in
the German town
of Iznang. At the
age of 32, he
completed his
medical training
at the University
of Vienna with a
dissertation on
the influence of
magnetism on
human disease.
•Until 1820, the only
magnetism known was
that of iron magnets
and of "lodestones",
natural magnets of ironrich ore.
•This was changed by a
professor of Physics at
University of Copenhagen,
Hans Christian Oersted
(1777-1851).
The Magnetic Field
1820 - Electromagnetism, Current
In 1820, a physicist Hans Christian Oersted, learned that
a current flowing through a wire would move a compass
needle placed beside it. This showed that an electric
current produced a magnetic field.
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Oersted’s Compass Deflections
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•Andre Ampere – 17751836, French scientist.
•Furthered the work of
Oersted on the relationship
between electricity and
magnetism.
•The basic unit of electric
current is named after him
(Ampere or Amp).
Magnetic Domains - a cluster of
magnetically-aligned atoms.
Magnetic Domains
Not Magnetic
Magnetic
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Electron Spin in the source of an
elements magnetic property.
Michael Faraday (1791-1867)
was a British scientist who
contributed to the field of
electromagnetics.
1820 –Faraday observed Oersted’s compass needle move
and wrote, “Use magnetism to produce electricity.”
1831 - Faraday built two devices to produce what he called
electromagnetic rotation: the electric motor, t hat used
continuous circular motion from the circular magnetic force
around a wire.
1832 - The electric generator used a magnet to generate
electricity.
Earth’s Magnetic Field
Geographic
North Pole
Magnetic
North Pole
Magnetic Field of Earth
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The Magnetic Field of the Earth
Variations in
Compass
Deflections
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Van Allen Radiation Belts in
Earth’s Magnetic Field
Effect of Solar Wind on Earth’s
Magnetic Field
The Right
Hand Rule
for Wires.
B = μoI / 2πr
μo= 4πx10-7 Tm/A
The First Right-Hand-Rule
B = μoI/(2πa)
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Magnetic Field Generated by a Coil
B = μonI/L
μo= 4πx10-7 Tm/A
B magnetic field strength N/(Ampere meter)
I
current in wire
(Amperes)
n number of turns of wire
L length of coil
(meters)
Loose Coil
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A Solenoid
N
B = μonI
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MAGNETIC FIELD OF A COIL
The Toroidal Solenoid
B = μoNI/(2πr)
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The magnitude of the
magnetic force is
F=qVB,
where q is the magnitude of
the charge of the particle, V
its velocity, and B is the
magnetic field. This force can
be also considered as the
centripetal force
Fc = m v2 / R ,
where m is the particle's mass
and R is the radius of the
circular trajectory.
Force on a Charged
Particle moving in a
Magnetic Field.
Force on a Current-carrying Wire in a
Magnetic Field.
F=BIL
• B is the external
magnetic field
measured in N/Am.
• I is the current
measured in amps.
• L is the length of the
current segment inside
of the magnetic field, B.
1820 - Andre Marie Ampere showed that two parallel wires
carrying current attracted each other if the currents are in the same
direction and repelled if the currents are in opposite directions.
F/L=(μoI1I2)/(2πa)
He formulated in mathematical terms, the laws that govern the
interaction of currents with magnetic fields in a circuit and as a
result of this the unit of electric current, the amp, was derived from
his name.
Application: MAGLEV Trains.
Types of Magnetism
•Diamagnetism
–opposes the applied field due to electron cloud interaction (Lenz's Law).
Characteristic of all materials - Very weak response (k negative).
•Paramagnetism
–linear response to applied field, but no residual magnetism when field is
removed. Characteristic of Fe, Mn, Co, Ni minerals. Weak; k positive.,
•Ferromagnetism
–total alignment of magnetic moments. Usually found in ferrous metals
such as steel. Very strong.
•Ferrimagnetism
– partial alignment of magnetic moments due to arrangement of ferric and
ferrous ions in a lattice structure such as magnetite. Strong (k non-linear).
Anti-ferrimagnetism
–Similar to ferrimagnetism, but magnetization opposes applied field. Due to
defects (e.g. pyrrhotite) or distorted (canted) lattice structures (e.g.,
hematite)
Force on a Current Loop
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