-
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