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# Magnetism - PowerPoint PPT Presentation

Magnetism. Force of Mystery demo. Magnetism Standards. Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. (Ch 36)

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## PowerPoint Slideshow about ' Magnetism' - russell-avery

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Presentation Transcript

### Magnetism

Force of Mystery

demo

• Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. (Ch 36)

• Students know how to determine the direction of a magnetic field produced by a current flowing in a straight wire or in a coil. (Ch 36)

• Students know changing magnetic fields produce electric fields, thereby inducing currents in nearby conductors. (Ch 37)

• “The nation that controls magnetism controls the Earth”

• North and South

• Like poles repel N-N S-S

• Unlike poles attract N-S

• Single poles cannot be isolated

• Magnetic Monopoles do not exist in nature

• Break a magnet:

• Get two smaller ones

N

S

N

S N

S

• Show strong magnetic effects

• Iron

• Cobalt

• Nickel

• Neodymium

Hi tech Neodymium iron boron magnets

• Earth has field

• Lines go from North to South

• Tesla (SI Unit)

• Gauss (cgs unit)

• 1 Tesla = 104 Gauss

• Earth magnetic field about 0.5 gauss

The direction the north pole of a compass would point when placed at that location

• 1 mm length

• Each acts like tiny magnet

• Normally domain cancel

• External field aligns domains

• Strong magnet can make other ferromagnetic materials into permanent magnets

• Even permanent magnets owe strength to “currents”

• No way to divide a current and get N or S pole

• Magnetism is electrical in origin

• Very weak

• Like bar magnet

• North magnetic pole

• South magnetic pole

• Magnetic field around long straight wire

Right hand rule determines direction of magnetic field

I

• Long Straight Wire (Rule #1)

• Point thumb in direction of current

• Fingers wrapped around wire point in direction of magnetic field

• Circular loop of Wire (Rule #2)

• Curl fingers around wire with tips in field direction

• Thumb points in direction of current

• Put curled fingers in current direction around loop or loops; thumb points in field direction INSIDE loop or coil.

F = BIL sinQ

• is angle between

field and wire

I

q

Force is perpendicular to both current and field direction

• Long straight fingers in (positive) current direction (or direction of moving charged particle). Curled fingers in magnetic field direction, thumb points in direction of force on current carrying wire or positive charged particle

• If particle is negative, change answer

• F = Bqvsinq

• This force is perpendicular to the magnetic field and particle velocity vector

• Circle or helix

• F = ma

• qvB = mv2/r (centripetal acceleration)

• r = mv/qB

• Direction follows right hand rule

How can F = BIL sinQ be Used to measurea Field?

• Hint: use a rectangular loop of wire

• Demo

• F = qvB sinQ

• Force perpendicular to both particle direction and field

• B = m0I/2pr

• F = BIL

• m0 permeability of free space

• 4 p x 10-7

I

• F/l = (mo/2p) I1I2/L

• Force per unit length of wire

• L is distance between wires

• Parallel currents attract

• Antiparallel currents repel

• Similarities

• Both involve attracting and repelling

• Both decrease with distance

• Differences

• Isolated poles do not exist

• Only electrical forces can be produced by stationary charges

• Only moving charged particles experience magnetic force

• Only electrical forces can do work

• Magnetic forces on charged particles are perpendicular to field direction but electrical forces are in or opposite to electric field direction