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

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

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 16

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.

SKETCH THE MAGNETIC FIELDS AROUND THE CURRENT CARRYING WIRE.

MAGNETIC FIELD OF A LONG STRAIGHT WIRE

r

IB 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 20

Fig 19.20b, p.682

1st Right Hand Rule- Thumb points in the direction of the current, fingers curl in the direction of the created magnetic field – up through the coils and around the outside. Use for current-carrying loop or solenoid coil.

MAGNETIC FIELD OF LOOPS OF WIRE (OR A COIL) CARRYING CURRENT

r

InB 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)

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)

2ND 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

2ND 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.

2ND RIGHT HAND RULE

MAGNETIC FORCE ON A MOVING CHARGE

F = qvB·sin ΘB: 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 WIRE

F = B·I·LB: field strength in teslas (T)I: current in amperes (A)L: length of current-carrying wire in meters (m)