Lesson 20 Magnetism Eleanor Roosevelt High School Chin-Sung Lin.
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Transcript of Lesson 20 Magnetism Eleanor Roosevelt High School Chin-Sung Lin.
Lesson 20
Magnetism
Eleanor Roosevelt High SchoolChin-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 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 loopsThe 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)
BN
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:
Medium Permeability μ [H/m]
Relative Permeability μ/μ0
Mu-metal (nickel-iron alloy) 2.5×10−2 20,000
Ferrite (nickel zinc) 2.0×10−5 – 8.0×10−4 16 – 640
Steel 8.75×10−4 100
Vacuum 1.2566371×10−6 (μ0) 1
Water 1.2566270×10−6 0.999992
Superconductors 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 magnetic force [N]
q electric charge [C]v velocity perpendicular to the field
[m/s]B magnetic field strength [T, Teslas]
I
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 = Ns / (Cm)
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?
electron beam S
N screen
magnet
A
BC
D
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?
I1I2
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?
I1I2
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 decreasingThe magnetic field has weakened 10% since the 19th century Earth's Magnetic Field Trends
52,000.00
53,000.00
54,000.00
55,000.00
56,000.00
57,000.00
58,000.00
59,000.00
60,000.00
Year
Tota
l Int
ensi
ty (
nT)
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