Download Magnetic Fields - Eleanor Roosevelt High School

Lesson 20
Magnetism
Eleanor Roosevelt High School
Chin-Sung Lin
History of Magnetism
History
The lodestone, which
contains iron ore, was found
more than 2000 years ago in
the region of Magnesia in
Greece
History
The earliest Chinese literature
reference to magnetism lies in
the 4th century BC writings
Guiguzi (鬼谷子): "The
lodestone attracts iron”
History
Zheng He used the Chinese
compass as a navigational
aid in his voyage between
1405 and 1433
History
In the 18th century, the French
physicist Charles Coulomb
studied the force between
lodestones
History
In 1820 Danish physicist and
chemist Hans Christian Ørsted
who discovered that electric
currents create magnetic fields
Magnetic Poles
Magnetic Poles
Magnets attract and repel
without touching
The interaction depends
on the distance
Magnetic poles produce
magnetic forces
Magnetic Poles
Magnet can act as a compass
The end that points
northward is called north
pole, and the end that points
south is call the south pole
Magnetic Poles
All magnets have north
and south poles
They can never be
separated from each other
If you break the magnet in
half, what will happen?
Magnetic Poles
Each half will become a
complete magnet
Unlike electric charge, you
cannot have north or south
pole alone
Magnetic Poles
Like poles repels; opposite poles attract
Magnetic Fields
Magnetic Fields
The space around the magnet
is filled with a magnetic field
Magnetic Fields
The magnetic field lines
spread from the north pole to
the south pole
Where the lines are closer (at
the poles), the field strength is
stronger
Magnetic Fields
The magnetic field unit:
Units: tesla (T) or gauss (G)
1 tesla = 10,000 gauss
Magnetic Fields
What will happen If we place a compass in the
field?
Magnetic Fields
A magnet or small compass in the field will line
up with the field
Magnetic Fields
Electric charge is surrounded by an electric
filed
The same charge is surrounded by a magnetic
field if it is moving
Which types of electron motion exist in
magnetic materials?
Magnetic Fields
Electrons are in constant
motion about atomic nuclei
This moving charge
constitutes a tiny current
and produces a magnetic
field
Magnetic Fields
Electrons spinning
about their own axes
constitute a charge in
motion and thus
creates another
magnetic field
Every spinning electron
is a tiny magnet
Magnetic Fields
Electrons spinning in the
same direction makes up
a stronger magnet
Spinning in opposite
directions cancels out
The field due to spinning
is larger than the one due
to orbital motion
Magnetic Fields
For ferromagnetic
elements: iron, nickel, and
cobalt, the fields do not
cancel one another
entirely
Each iron atom is a tiny
magnet
Magnetic Domain
Magnetic Domain
Interactions among iron
atoms cause large clusters
of them to line up with
one another
These cluster of aligned
atoms are called magnetic
domains
Magnetic Domain
There are many magnetic
domains in a crystal iron
The difference between a
piece of ordinary iron and
an iron magnet is the
alignment of domains
Magnetic Domain
Iron in a magnetic field:
A growth in the size of the
domains that is oriented in
the direction of the
magnetic field
A rotation of domains as
they are brought into
alignment
Magnetic Domain
Permanent magnets:
Place pieces of iron or
certain iron alloys in strong
magnetic fields
Stroke a piece of iron with
a magnet
Electric Currents &
Magnetic Fields
Electric Currents &
Magnetic Fields
Current-Carrying Wire:
A moving electron produces a
magnetic field
Electric current also produces
magnetic field
A current-carrying conductor is
surrounded by a magnetic
field
Electric Currents &
Magnetic Fields
Right-hand rule:
Grasp a current-carrying wire
with your right hand
Your thumb pointing to the
direction of the current
Your fingers would curl around
the wire in the direction of the
magnetic field (from N to S)
Electric Currents &
Magnetic Fields
What will happen to the compasses if the current is upward?
Electric Currents &
Magnetic Fields
The current-carrying wire deflects a magnetic compass
Electric Currents &
Magnetic Fields
Current-Carrying Loop:
A wire loop with current produces a magnetic field
Electric Currents &
Magnetic Fields
Current-Carrying Loop:
A wire loop with current
produces a magnetic field
Electric Currents &
Magnetic Fields
Coiled wire— Solenoid:
A solenoid can be made of many wire loops
Electric Currents &
Magnetic Fields
Coiled wire— Solenoid:
A current-carrying coil of wire with many loops
The magnetic field lines bunch inside the loop
Electric Currents &
Magnetic Fields
Coiled wire— Solenoid:
A coil wound into a tightly
packed helix which produces a
magnetic field when an electric
current is passed through it
Solenoids can create controlled
magnetic fields and can be used
as electromagnets
Electric Currents &
Magnetic Fields
Intensity of Magnetic Field of Electromagnet (B):
Increased as the number of loops increased (B ~ N)
Increased as the Current increased (B ~ I)
Intensity is enhanced by the iron core (B ~ μ)
N
B
I
Electric Currents &
Magnetic Fields
Permeability:
The measure of the ability of a material to support the
formation of a magnetic field within itself. Magnetic
permeability is typically represented by the Greek letter μ
B
μ
Electric Currents &
Magnetic Fields
Permeability:
Permeability
μ [H/m]
Medium
Mu-metal
(nickel-iron alloy)
Ferrite
(nickel zinc)
Steel
Vacuum
Water
Superconductors
Relative Permeability
μ/μ0
2.5×10−2
20,000
2.0×10−5 – 8.0×10−4
16 – 640
8.75×10−4
100
1.2566371×10−6 (μ0)
1
1.2566270×10−6
0.999992
0
0
Electric Currents &
Magnetic Fields
Direction of magnetic field of electromagnet follows the
Right-hand Rule:
Your fingers indicate the direction of the current (I)
your thumb points the direction of the field (B)
B
I
Magnetic Forces on
Moving Charged Particles
Magnetic Forces on
Moving Charged Particles
When a charged particle moves in a magnetic field, it will
experience a deflecting force (FB)
+
I
Magnetic Forces on
Moving Charged Particles
When a charged particle moves in a magnetic field, it will
experience a deflecting force (FB)
FB = qvB
FB
q
v
B
magnetic force [N]
electric charge [C]
velocity perpendicular to the field [m/s]
I strength [T, Teslas]
magnetic field
Magnetic Forces on
Moving Charged Particles
The magnetic field unit:
Units: tesla (T) or gauss (G)
1 tesla = 10,000 gauss
tesla = (newton ×
second)/(coulomb × meter)
T = Ns / (Cm)
Magnetic Forces on
Moving Charged Particles
Direction of the magnetic force (FB) follows the
Fleming’s Left Hand Motor Rule
I
Magnetic Forces on
Moving Charged Particles
What will happen to the
positively charged particle?
+
Magnetic Forces on
Moving Charged Particles
The positively charged
particle will experience a
force always perpendicular
to the motion
The particle will have a
circular motion
Magnetic Forces on
Moving Charged Particles
The magnetic field has
been used to detect
particles in the cloud
chamber
What will happen to
the different radiation?
Magnetic Forces on
Moving Charged Particles
The magnetic field has
been used to detect
particles in the cloud
chamber
α He2+ helium nucleus (+)
β e– electron (–)
γ uncharged EM ray
Magnetic Forces on
Moving Charged Particles
The magnetic field has
been used to detect
particles in the cloud
chamber
α He2+ helium nucleus (+)
β e– electron (–)
γ uncharged EM ray
Magnetic Forces on
Moving Charged Particles
The magnetic field has been used to deflect the electron
beam. Where will the electron beam hit the screen?
magnet
C
electron beam
S
A
N
B
D
screen
Magnetic Forces on
Moving Charged Particles
Mass spectrometry:
To determine masses of particles, for determining the
elemental composition of a molecule
Magnetic Forces on
Moving Charged Particles
Mass spectrometry:
magnetic force = centripetal force
FB = FC
qvB = mv2/r
r = (mv)/(qB)
Magnetic Forces on
Moving Charged Particles
Mass spectrometry:
r = (mv)/(qB)
• the faster it is travelling the bigger the circles
• the bigger its mass is the bigger the circles
• the bigger its momentum the larger the circles
• the stronger the magnetic field the smaller the circles
• the larger the charge the smaller the circles
Magnetic Forces on
Moving Charged Particles
A positively charged particle moving along a spiral
path inside a uniform magnetic field
Magnetic Force on
Current-Carrying Wires
Magnetic Force on
Current-Carrying Wires
What will happen to the current carrying wires?
I
I
Magnetic Force on
Current-Carrying Wires
The current-carrying wire also follows Fleming’s left
hand motor rule
Magnetic Force on
Current-Carrying Wires
The current-carrying wire deflects a magnetic compass
and a magnet deflects a current-carrying wire are
different effect of the same phenomena
Magnetic Force on
Current-Carrying Wires
Magnetic Force Between Wires:
What will happen to the parallel
wires if both current are in the
same direction?
I1
I2
Magnetic Force on
Current-Carrying Wires
Magnetic Force Between Wires:
Parallel wires carrying currents
will exert forces on each other
When the current goes the
same way in the two wires, the
force is attractive
When the currents go opposite
ways, the force is repulsive
Magnetic Force on
Current-Carrying Wires
Magnetic Force Between Wires:
What will happen to the parallel
wires if the current are in the
opposite direction?
I1
I2
Galvanometers
& Motors
Galvanometer
A sensitive current-indicating instrument
The coil turns against a spring, so the greater the
current, the greater its deflection
Galvanometer
A galvanometer may be calibrated to measure
current— an ammeter
A galvanometer may be calibrated to measure
voltage— a voltmeter
Motor
Converts electrical energy into
mechanical energy
Motors operate through
interacting magnetic fields and
current-carrying conductors to
generate force
DC Motor
The current-carrying
wire of the motor coil
follows Fleming’s left
hand motor rule
DC Motor
AC Motor
AC Motor
Earth’s Magnetic Field
Earth’s Magnetic Field
Earth itself is a huge
magnet
The magnetic poles
of Earth do not
coincide with the
geographic North
pole – magnetic
declination
Earth’s Magnetic Field
Magnetic Pole Shift:
The magnetic poles of Earth
keep changing
The pole kept going north at
an average speed of 10 km
per year, lately accelerating
to 40 km per year
Earth’s Magnetic Field
Magnetic Pole Weakening:
The strength of the magnetic field of Earth keep decreasing
The magnetic field has weakened 10% since the 19th
century
Earth's Magnetic Field Trends
59,000.00
58,000.00
57,000.00
56,000.00
55,000.00
54,000.00
53,000.00
52,000.00
19
45
19
50
19
55
19
60
19
65
19
70
19
75
19
80
19
85
19
90
19
95
20
00
20
05
20
10
Total Intensity (nT)
60,000.00
Year
Earth’s Magnetic Field
A geomagnetic reversal is a change in the Earth's magnetic
field such that the positions of magnetic north and
magnetic south are interchanged
Magnetic Forces on
Moving Charged Particles
A positively charged particle moving along a spiral
path inside a uniform magnetic field
Earth’s Magnetic Field
Earth’s magnetic field will deflect the charged
particles from outer space to reduce the cosmic
rays striking Earth’s surface
Earth’s Magnetic Field
Van Allen radiation belt: is a torus of energetic
charged particles around Earth, which is held in
place by Earth's magnetic field
Earth’s Magnetic Field
Van Allen radiation belt: energetic electrons
forming the outer belt and a combination of
protons and electrons creating the inner belt
Earth’s Magnetic Field
Aurora: a natural light display in the sky, particularly in
the polar regions, caused by the collision of charged
particles directed by the Earth's magnetic field
The End