Magnetism BasicsMagnetism BasicsMagnetism BasicsMagnetism Basics

SourceSourceSourceSource: electric currents

BipolarBipolarBipolarBipolar: all magnets have a North Pole & South Polenatural monopoles do not exist! Splitting a magnet results in bipolar mini-magnets!

Magnetic ForcesMagnetic ForcesMagnetic ForcesMagnetic Forces: like electric forces

Likes Repel

Opposites Attract

Magnetic DomainsMagnetic DomainsMagnetic DomainsMagnetic Domains: atomic regions of aligned magnetic poles

Random Alignment Ferromagnetic Alignment

Net Effect = Zero!

Net Effect = Additive!

The Earth: The Earth: The Earth: The Earth: A Natural MagnetA Natural MagnetA Natural MagnetA Natural Magnet

Earths Magnetic Poles shift with timeeven reversing direction!

Magnetic Field is 3-Dimensionaland deflects downwards at the polesa compass would be totally useless!

Magnetic FieldsMagnetic FieldsMagnetic FieldsMagnetic Fields Magnetic Fields (B)Magnetic Fields (B)Magnetic Fields (B)Magnetic Fields (B): Force Field acting on magnetic domains like Electric Fields acting on electric charges

Direction:Direction:Direction:Direction: points from North to South Poles and flows through interior of magnet or coil

Strength:Strength:Strength:Strength: like Gravity, obeys Inverse Square Law: Force ~ 1/d2. Unlike gravity, a medium (i.e. iron) can alter strength. Density of BBBB field strongest at poles.

Bv

Astronomical EffectsAstronomical EffectsAstronomical EffectsAstronomical Effects Magnetic Shielding:Magnetic Shielding:Magnetic Shielding:Magnetic Shielding:Earths B field naturally shields and bottles high-energy cosmic radiation from the sun towards the poles

Aurora Borealis: Aurora Borealis: Aurora Borealis: Aurora Borealis: (Northern Lights)Due to shielding effect, cosmic radiation interacts with the atmosphere lighting up the night sky @ poles.

Solar Flares: Solar Flares: Solar Flares: Solar Flares: ~11 years the Sun experiences large-scale fluctuations in its B field--kicking out gases and intense radiation enough to disrupt radio communications on Earth!

MotorsThe motor effect:

the magnetic force pushesOn the charges perpendicular to the wire so the entire wire moves instead of just the charges down the wire--- see p. 583 in book!

S

N

IF

Motor/generator parts

Label each:

Stator(magnet)

Brushings(contacts)

Current(wire leads)

Summary-chap.28

* The source of magnetism is moving charge* 3 types of magnets

1. permanent magnets- Fe,Co,Ni2. current in wires

( magnetic field wraps around wire)3. electromagnetic- battery, wire, iron

(iron focuses magnetic field of current)

* magnetic domain- microscopic piece of iron in which atomic magnets are aligned

* galvanometer is a wire need a magnet(when current flows in wire, it deflects so you can measure very small currents)

Summary- video &lab guide

Lodestone is a type of magnetic iron oxide

To weaken an iron magnet, make the atomic magnets point in random directions by

a. Heating b. dropping c. add impurites

Your compass measures the total magnetic field of the earth & nearby magnets

Magnetic forces enable speakers, mris, computers, credit cards, etc to work

Light is an electro-magnetic waveelectro-magnetism is one force!

Moving charge creates magnetism,Moving magnets induces current (or voltage)

AC vs. DC motors

AC motors: uses ac current so the current changes direction and the force changes direction to keep the motor turning- no special contacts

Ex: plug in tools/ motion appliance

DC motors: current doesnt reverse so do it by reversing the battery connections every cycle

Ex: battery powered toolshttp://www.walter-fendt.de/ph14e/electricmotor.htm

RightRightRightRight----Hand RulesHand RulesHand RulesHand Rules CurrentCurrentCurrentCurrent----Carrying WireCarrying WireCarrying WireCarrying Wire: Thumb: points in direction of IIII, Fingers: curl in direction of BBBB in concentric circles around wire.

LoopLoopLoopLoop: Fingers: curl in direction of current Thumb: points in direction of BBBB.

Force on Moving ChargeForce on Moving ChargeForce on Moving ChargeForce on Moving Charge: Thumb: direction of vvvvFingers: direction of BBBBPalm: pushes in direction of FFFF(or use middle finger!)

Note all 3 Quantities are at 90 degrees!

RightRightRightRight----Hand Rules Hand Rules Hand Rules Hand Rules HintsHintsHintsHints

* I is in direction of positive charges as * I is in direction of positive charges as * I is in direction of positive charges as * I is in direction of positive charges as move from + battery to move from + battery to move from + battery to move from + battery to battery endsbattery endsbattery endsbattery ends(for electrons flip or use left hand)(for electrons flip or use left hand)(for electrons flip or use left hand)(for electrons flip or use left hand)

* Force is zero if the magnetic field is * Force is zero if the magnetic field is * Force is zero if the magnetic field is * Force is zero if the magnetic field is exactly parallel to the flow of chargeexactly parallel to the flow of chargeexactly parallel to the flow of chargeexactly parallel to the flow of charge

* B magnetic field flows from N to S pole* B magnetic field flows from N to S pole* B magnetic field flows from N to S pole* B magnetic field flows from N to S pole

Example Problem

current loop in a magnetic field

HelgaHelgaHelgaHelgas Super Vacuums Super Vacuums Super Vacuums Super VacuumF = ? (magnitude & direction!)B = 200 TeslasI = 20 Ampsl= 5.8 m of copper wire in motorbrush

= into page (tail feathers of arrow) = out of page (tip of arrow)

Magnetic Induction

The Missing LinkThe Missing LinkThe Missing LinkThe Missing Link: -Oersted discovered Electric Currents produce Magnetism. Will Magnetism produce Currents?

LenzLenzLenzLenzs Laws Laws Laws Law: -A changing BBBB field through a circuit will induce an opposing B B B B field. Why opposing? Mechanical energy to change BBBB will result in electrical energy! Energy is conserved! Otherwise, infinite free energy!

GeneratorsThe generator effect:

the magnetic force pushes the charges along the wire inducing voltage since the entire wire is pushed down (by steam)

see p. 583 in book!

S

N

Imove

Magnetic Force on Moving Charges

magnetic xF qv B=v vv

magnetic xF Il B=v v v

UnitsUnitsUnitsUnits: F = Force in Newtons (N)q = charge in Coulombs (C)B = magnetic field in Teslas (T)I = current in Amperes (A)l= length in meters (m)

Definition of Magnetic Field (B)Definition of Magnetic Field (B)Definition of Magnetic Field (B)Definition of Magnetic Field (B): 1 Tesla = 1 N / Am = 1 N / (Cm/s)1 Tesla = 10,000 Gauss

magnetic magneticF FBIl qv

= =

v vv

v v

Faradays Law

FaradayFaradayFaradayFaradays Law of Magnetic Inductions Law of Magnetic Inductions Law of Magnetic Inductions Law of Magnetic Induction: emf= Electro-Motive Force (Volts)N = # of loops in circuitA = Area of loop (m2)B = magnetic fieldmagnetic fieldmagnetic fieldmagnetic field (Teslas)t = time (seconds)

Emf (voltage)= - (NBA) / t

Summary

1. Moving charge (current) produces magnetic field

-magnetic field wraps around current wire

- used in electromagnets (junk car lifter)

- direct current produces a constant B

-alternating creates an alternating B field, which then creates a counter (eddy) current(ac electromagnet- floating rings demo,

remote charging of pacemakers)

Transformers

Must use ac currents!

Power in = power out

I*V = I*V

V1/turns = V2 /turns

Emf depents on # of coils