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IB Physics: Magnetism and Electromagnetic
Induction.
“Magnetic Field Lines Always
Point Away from the North
_____
and Toward the South
_____.”
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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!
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.
•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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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 Right
Hand Rule
for Wires.
B = μoI / 2πr
μo= 4πx10-7 Tm/A
Single Loop
B = μoI / 2r
Examples:
1. A long, straight wire carries current from West to East. What is
the direction of the magnetic field directly
Above the wire.
(b) Below the wire.
2. If the current in the wire from #1 is 2.5 A, find the magnitude of
the magnetic field at a distance of 1.5 cm from the wire.
3. A circular loop of copper wire with current 8.4 A determines a
magnetic field. The area of the circle is 2.0 m2. Find the magnitude
of the magnetic field.
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)
A Solenoid
N
B = μoNI, N=n/L
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The Toroidal Solenoid
B = μoNI/(2πr)
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The magnitude of the
magnetic force is
F = q v B sin θ,
where q is the charge
v, its velocity
B, magnetic field
θ, angle between v and B
Force is centripetal and RHR
Fc = m v2 / R ,
M, particle mass
R, radius of the circular path
Force on a Charged
Particle moving in a
Magnetic Field.
Force on a Current-carrying Wire
in a Magnetic Field.
F = B I L sin θ
• 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
• θ, angle between L and B
• F, direction by RHR
1820 - Andre Marie Ampere showed that two parallel wires
carrying current could attract or repel.
F/L=(μoI1I2)/(2πa)
ATTRACT – current going in SAME direction.
REPEL – current in OPPOSITE direction.
IB Physics Topic 5: E&M Equations and Constants. Quick Reference.
F = k(q1∙q2 /r2)
k = 8.99 x 109 N∙m2/C2
1 C = 6.25 x 1018 e
E = F/q
E = k·Q/r2 k = 1/(4πε0)
ε0 = 8.85x10-12 C2/Nm2
W = qEd
V = W/q
W = U = qV
V = Ed
I = Δq/Δt I = n∙A∙vd∙q UA + KA = UB + KB K = ½mv2
V = IR (series) RTOT = R1+ R2+…+ Rn V = V1+ V2+…+Vn I (same)
(parallel) 1/RTOT = 1/R1+ 1/R2+…+ 1/Rn I = I1+ I2+…+In V (same)
P = VI = I2R = V2/R 1 kWh = 3.6x106 J
Mass
Charge
Proton
1.67 x 10-27 kg
1.6 x 10-19 C
Electron
9.11 x 10-31 kg
-1.6 x 10-19 C
Alpha particle 4(1.67 x 10-27 kg) 2(1.6 x 10-19 C)
B = μoI /(2πr)
μo= 4πx10-7 Tm/A
B = μoI/(2r)
B = μoNI, N=n/L
FB = qvB sin(θ) FC = mv2/r
c = 10-2
m = 10-3
μ = 10-6
1T = 1 N/(Am)
B = μoNI/(2πr)
(the Right Hand Rules!)
F = BIL sin(θ) F/L=(μoI1I2)/(2πa)
n = 10-9
p = 10-12
k = 103
M = 106
G = 109