MAGNETISM
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Transcript of MAGNETISM
MAGNETISM
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 North-South axis
LODESTONES Magnetite crystals have been found in
living organismsMagnetotactic bacteria!Migratory Bird brains!!Other migratory animals: bees, fishHuman brains!!!YOU HAVE ROCKS IN YOUR HEAD!!!!!
LODESTONES
PERMANENT MAGNETS By 2nd Century AD, Chinese were able to
make permanent magnets by repeatedly stroking an iron rod or needle from end to end along a lodestone, but always in the same direction.
Retained strength of a magnet depends on chemical properties of the metal.Soft iron: loses magnetism quicklyLow-carbon soft steel (paper clips, nails):
gradual lossHard steel: retains power for a long time and
is referred to as a “permanent magnet”
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!
MONOPOLE? (NO, NOT MONOPOLY!) 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!
A monopole cannot be isolated.
Do not pass GO.Do not collect $200.
MAGNETIC FIELD Every magnet establishes in the space
surrounding it, a magnetic field (B-field) Map field with a test-compass Direction of field is direction in which the test-
compass 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 SI unit for B (magnetic field strength) is the tesla (T)
MAGNETIC FIELD LINES
MAGNETIC FIELD
Mapping with Test-Compass
Field Lines Form Closed Loops
Field Mapped by Iron Filings
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?
MAGNETIC DOMAINS Domain: region where many atomic
dipoles are aligned Usually aligned randomly and effects
cancel BUT…
Place ferromagnetic material in strong B-field
Entire domains realign with applied fieldSize & shape of domains remains the sameCauses irreversible re-orientation of
domainsCreates permanent magnets
REORIENTATION OF DOMAINS
Domains are not aligned
Electrons in domains align
with applied field
Substance is Permanently Magnetized
MAGNETISM ON AN ATOMIC LEVEL Charge in motion (electric current)
produces magnetic force Electrons function as a subatomic dipole
Electron “spin” (Much More) Electrons existing in pairs: B-fields cancel
Electron “orbit” around nucleus (Very Little) Random “orbits” of electrons: B-fields cancel
DIAMAGNETISM Even “non magnetic” materials
respond to an applied B-fieldApplied B-field changes orbital motion of
electronsProduces a field that opposes applied fieldRepelled by applied field
Diamagnetic materials have no permanent atomic dipoles
Occurs for all substances, but may be swamped by other magnetic effects
PARAMAGNETISM Paramagnetic materials are attracted
when placed in a strong B-field. Composed of atoms with permanent
atomic dipolesAtomic dipoles do not interact w/ one
anotherAtomic dipoles oriented randomlyMaterial has no dipole as a whole
A strong B-field re-orients these atomic dipoles in same direction as applied field
FERROMAGNETISM Naturally “magnetic”: magnetite, iron,
nickel, cobalt, steel, Alnico, other alloysStrongly attracted to poles of a magnetEasily magnetized
Atomic dipoles interact strongly with dipoles of adjacent atoms
Dipoles align spontaneously, w/o an applied field
Many atomic dipoles cooperatively align Creates regions of parallel orientations
(domains)
ELECTRODYNAMICS: THE STUDY OF ELECTROMAGNETISM Magnetism is caused by charge in
motion.Charges at rest have just an electric fieldBut, when they move, they generate both
an electric field and a magnetic fieldCan look at individual charges or electric
current in a wire Direction of current determines direction
of the magnetic field. Use right hand rules for analysis.
Slide 19
Fig 19.15b, p.678
First Right Hand Rule: thumb points in direction of current, fingers curl in direction of magnetic field- note compass readings. Use for current-carrying wire.
DIAGRAMMING 3-D MAGNETIC FIELDS Not everybody is an artist. Use 2-D images to draw 3-D field
vectors. If field points perpendicularly into the
page or board, use If field points perpendicularly out of the
page or board, use Otherwise, draw the lines neatly. Don’t forget, field lines are vectors!
X
SKETCH THE MAGNETIC FIELDS AROUND THE CURRENT CARRYING WIRE.
MAGNETIC FIELD OF A LONG STRAIGHT WIRE
rIB o
2
B: magnetic field strength (teslas) I: current (amperes) r: radius from wire (meters) μo: permeability constant in a vacuum μo = 4π x 10-7 T·m/A What is the shape of this magnetic field?
SKETCH THE MAGNETIC FIELDS AROUND EACH OF THE FOUR SIDES OF THE CURRENT CARRYING LOOP.
Slide 24
Fig 19.20b, p.682
2nd Right Hand Rule- Fingers curl in the direction of the current, Thumb points in the direction of the created magnetic field.
Use for current-carrying loop or solenoid coil.
MAGNETIC FIELD OF LOOPS OF WIRE (OR A COIL) CARRYING CURRENT
rInB o
2
How is this equation different from the mag field of a straight wire?
The strength of the field is more in a loop than in the straight wire and a single loop.
where n is the NUMBER of loops (in this example n=8)
3RD RIGHT HAND RULE Gives the direction of the FORCE exerted
on a current (or charge) by an external magnetic field
Point thumb of RH in direction of current (or motion of positive charge)
Point fingers through in direction of magnetic field
Palm pushes in direction of force
3RD RIGHT HAND RULE
Thumb points to v, which is direction of velocity of positive charge
Fingers point to B, the direction of magnetic field lines.
Deflecting force is shown by direction of palm pushing.
3RD RIGHT HAND RULE
MAGNETIC FORCE ON A MOVING CHARGEF = qvB·sin Θ
B: field strength in teslas (T)q: charge in coulombs (C)v: charge velocity in m/sΘ: angle between v & B
FIND THE RADIUS OF THE CIRCULAR ORBIT OF THIS ELECTRON IN THE MAGNETIC FIELD
MAGNETIC FORCE ON A CURRENT-CARRYING WIREF = B·I·L
B: field strength in teslas (T)I: current in amperes (A)L: length of current-carrying wire in meters (m)