MAGNETISM

Adapted from Mr. Dellibovi

• 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

• Magnetite crystals have been found in living organisms– Magnetotactic bacteria!– Migratory Bird brains!!– Other migratory animals: bees, fish– Human brains!!!– YOU HAVE ROCKS IN YOUR HEAD!!!!!

Magnets- History and Interesting Facts

History of Permanent Magnets

• By 2nd Century AD, Chinese were able to make permanent magnets by repeatedly __________________________________________________________________________________________________________________.

• Retained strength of a magnet depends on ___________________________________.– ________: loses magnetism quickly– _________________ (paper clips, nails): gradual loss– ________: 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 _______________.• Opposite magnetic poles _____________.• Most magnets have ______ 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!

• ________________________________________

• 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 _____________________• Direction of field is direction in which the test-compass

needle will point at that location.• Draw field lines so that compass always points _______

to the field lines.• Field lines point from N to S __________ the magnet• Field lines point from S to N __________ the magnet• Field lines form closed loops• Field lines never _____________________• SI unit for B (magnetic field strength) is the tesla (T)

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? ____________________________________________________________

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!

Magnetism on an Atomic Level

• Charge ________________ (or electric __________) produces magnetic force

• Electrons function as a subatomic dipole– Electron “spin”

• Electrons existing in pairs: B-fields cancel

– Electron “orbit” around nucleus• Random “orbits” of electrons: B-fields cancel

Diamagnetism

• Even “non magnetic” materials respond to an applied B-field– Applied B-field changes orbital motion of electrons– Produces a field that _____________ applied field– ______________ by applied field

• Diamagnetic materials have no _____________ atomic dipoles

• Occurs for ______ substances, but may be swamped by other magnetic effects

Paramagnetism

• Paramagnetic materials are ___________ when placed in a strong B-field.

• Composed of atoms with ____________ atomic dipoles– Atomic dipoles do not interact w/ one another– Atomic dipoles oriented randomly– Material has no dipole as a whole

• A strong B-field re-orients these atomic dipoles in _____________ as applied field

Ferromagnetism• Naturally “magnetic”: magnetite, iron, nickel,

cobalt, steel, Alnico, other alloys– Strongly attracted to poles of a magnet– Easily magnetized

• Atomic dipoles ____________ with dipoles of adjacent atoms

• Dipoles align spontaneously, w/o an applied field• Many atomic dipoles cooperatively align• Creates regions of __________ orientations

(_____________)

Magnetic Domains

• Domain: region where many atomic dipoles _________

• Usually aligned randomly and effects cancel• BUT…

– Place ferromagnetic material in strong B-field– Entire domains realign with applied field– Size & shape of domains remains the same– Causes irreversible re-orientation of domains– Creates permanent magnets

Reorientation of DomainsDomains are not

aligned

Electrons in domains align

with applied field

Substance is Permanently Magnetized

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– Can 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.

First Right Hand Rule: thumb points in direction of _________, fingers curl in direction of ____________ . Note compass readings. Use for: ___________________

Magnetic field of a long straight wire

• B: magnetic field strength (teslas)• I: current (amperes)• r: radius from wire• μo: ____________________________________• μo =

• The shape of this magnetic field is: ______________________________________________________________

r

IB o

2

Slide 19

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 ________________________________________________

Magnetic field of loops of wire (or a coil) carrying current

r

InB o

2

where n is the NUMBER of loops (in this example n=8)

•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.

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 ___________

2ND Right Hand RuleFingers point to B, the direction of magnetic field lines.

Thumb points to v, which is direction of velocity of positive charge

Deflecting force is shown by direction of palm pushing.

2ND Right Hand Rule

Magnetic Force on a Moving Charge

F = Bqv·sin ΘB: field strength in _____________

q: charge in _________________

v: charge velocity in ____________

Θ: angle between __________

Magnetic Force on a Current-Carrying Wire

F = B·I·LB: field strength in _______________

I: current in ______________

L: length of current-carrying wire in __________