Download Lodestones Magnetic Poles

Lodestones
• Natural Magnets
• Magnetite, Fe3O4 (an oxide of iron)
• Ancient civilizations (Greek 590 BCE, Chinese 2600 BCE) realized that these stones would cling to iron tools.
• A suspended, pivoting lodestone always pointed along the North­South axis
• Magnetite crystals have been found in living organisms
Magnetic Poles
• Magnets produce a force on other objects
• Poles are regions where the magnetic force is the strongest
• Like magnetic poles repel.
• Opposite magnetic poles attract.
• Most magnets have two poles (dipole), but can have three or more!
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Have we ever seen a monopole?
• Monopole: piece of a magnet that is simply a north pole or a south pole
• Many have tried to isolate a monopole by breaking magnets in half.
• No matter how we break a magnet, the pieces are always dipoles!
• No one has ever found a monopole (but that doesn't mean that they aren't still trying)
Magnetic Field
Mapping with Test­Compass
Field Lines Form Closed Loops
Field Mapped by Iron Filings
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Magnetic Field
Every magnet establishes in the space surrounding it, a magnetic field (B­field)
Map field with a test­compass
Direction of field is direction in which the test­compass needle will point at that location.
Draw field lines so that compass always points tangent to the field lines.
Field lines point from N to S outside the magnet
Field lines point from S to N inside the magnet
Field lines form closed loops
Field lines never intersect
N
S
Earth’s Magnetism
• Magnetic field has reversed direction ~300 times in the past 170 million years
• Magnetic poles wander!
• Magnetic & geographic poles not the same.
• Magnetic declination: 11.5°
• What’s strange about this picture? à
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Electrodynamics: The Study of Electromagnetism
Magnetism is caused by charge in motion.
Charges at rest have just an electric field
But, when they move, they generate both an electric field and a magnetic field
Electrons function as a subatomic dipole
Electron “spin” is shown in energy levels
Electrons existing in pairs: B­fields cancel
Electron “orbit” around nucleus
Random “orbits” of electrons: B­fields cancel
Magnetic Domains
size­usually between 10­6 and 10­3m
• Domain: region where many atomic dipoles are aligned
• Usually aligned randomly and effects cancel
• Place ferromagnetic material in strong B­field
• Entire domains realign with applied field
• Creates "permanent" magnets (at least until domains lose overall alignment)
S
Domains are not aligned
Substance is Permanently Magnetized
Electrons in domains align with applied field
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Materials:
ferromagnetic:
domains are very magnetic, however the material will not be magnetic by itself. when placed in an external field, it will display strong magnetic properties and act like a magnet until removed from the field.
Paramagnetic:
Material will weakly become magnetic when in the presence of an external field. domains will align with magnesium, molybdenum, lithium the external field.
Diamagnetic:
Material will weakly become magnetic when in the presence of an external field. Domains will align in a direction opposite of the external field. Repelled
copper, silver, and gold Can look at individual charges or
electric current in a wire
Direction of current, or motion of positive charge, determines direction of the magnetic field.
Use right hand rules for analysis and direction.
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First Right Hand Rule: thumb points in direction of current, fingers curl in direction of magnetic field­ note compass readings. Use for current­carrying wire.
Fig 19.15b, p.678
Slide 21
Diagramming 3­D Magnetic Fields
• Not everybody is an artist.
• Use 2­D images to draw 3­D field vectors.
• If field points perpendicularly into the page or X X X X
board, use X X X X
• If field points perpendicularly out of the page • • • •
or board, use • Otherwise, draw the lines neatly.
• Don’t forget, field lines are vectors!
B
I
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Sketch the magnetic fields around each of the four sides of the current carrying loop.
I
X
X
X
I
X
X
Magnetic field of a long straight wire
B
I
•
•
•
•
•
B: magnetic field strength (Teslas ­ T)
I: current (amperes ­ A)
r: radius from wire (meters ­ m)
μo: permeability constant in a vacuum
μo = 4π x 10­7 T·m/A
• What is the shape of this magnetic field?
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The magnetic field for a loop of wire is found by the equation :
radius of the loop
A loop of wire with a radius of 5cm has a current of 4A traveling through it. What is the strength of the magnetic field at the center of the loop?
2nd Right Hand Rule­ Fingers curl in direction of current, thumb points to direction of magnetic field. Use for current­
carrying loop or solenoid coil.
B
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