PHYSICS UNIT 8: MAGNETISM

MAGNETISM Magnetic Fields are always dipolar: N and S

poles law of magnetism: like poles repel, unlike poles

attract magnetic force is directly proportional to the

magnets’ strength, inversely proportional to the square of the distance between the magnets

only certain materials can be magnetized –

ferromagnetic materials (Fe,Co,Ni ores/alloys)

MAGNETISM Magnets

are always dipoles: north-seeking N pole, south-seeking S pole

MAGNETISM magnetic field: region

of magnetic force detect using a

compass (needle points in field direction)

field runs N to S outside magnet, S to N inside magnet

MAGNETISM magnetic field strength, B: strength

of the magnetic field, in Tesla, T earth’s magnetic field at the surface =

0.5 x 10-4 T field directions:

left right up down out toward you in away from you ו

MAGNETISM Earth is a

magnet north

geographic pole is an S pole

field like a bar magnet

MAGNETISM Source of Magnetism

electrons are magnetic due to “spin” paired electrons: opposite “spin”, cancel out

occurs in most materials unpaired electrons magnetic atoms

occurs in ferromagnetic materials

MAGNETISM magnetic

domains: regions of aligned magnetic atoms

magnetic object: aligned magnetic domains

ELECTROMAGNETISM Field Around A Wire:

a current (moving charge) produces a magnetic field encircling the wire right hand rule: grasp

conductor in right hand, with thumb pointing in I direction; fingers will circle wire in B direction

ELECTROMAGNETISM magnetic field strength around a

wire depends on amount of current in wire, & distance from wire B=0I/2r

B: magnetic field strength (T) I: current in wire (A)r: distance from wire (m)0=4×10-7 Tm/A

ELECTROMAGNETISM Field Around A

Solenoid (coil)

like bar magnet when carrying current

add iron core: electromagnet (much stronger field)

solenoid field strength, B=0nI n = #loops/length of coil

ELECTROMAGNETISM right hand rule for

solenoids: grasp coil in right hand, with fingers circling coil in I direction; thumb will point to N pole

core is pulled into solenoid when turned on (striker, switch)

ELECTROMAGNETISM parallel wires exert forces on each

other due to their magnetic fields currents in same direction – wires

attract

currents in opposite direction – wires repel

PHYSICS

UNIT 8: MAGNETISM

MAGNETIC FORCES cathod

e ray tube (TV & monitor picture tube)

MAGNETIC FORCES cathod

e ray tube (TV & monitor picture tube)

MAGNETIC FORCES Magnetic fields exert force on

moving charges magnetic force on a moving charge, F = qv x B (vector cross product)

F: force (N) B: field strength (T) q: particle charge (C) v: speed (m/s)

MAGNETIC FORCES F is perpendicular to v and B

can only make the particle turn

F is a centripetal force Bqv = mv2/r mass spectrometer: shoot

particles into magnetic field to find their massm = Bqr/v

MAGNETIC FORCES right hand rule for + particles, left hand rule

for – particles: point thumb in v direction, index finger in B direction (N to S); other fingers will bend in F direction

N S

proton:v

S N

electron:

v

MAGNETIC FORCES Magnetic fields exert force on

current-bearing wires magnetic force on a wire, F = BIL

B: magnetic field strength (T) I: current (A) L: length of wire in field (m) for a coil, L = length of wire in field × # of loops

MAGNETIC FORCES right hand rule for wires: point

thumb in I direction, index finger in B direction; other fingers will bend in F direction

I

× × × ×

× × × ×

× × × ×

I

• • • •

• • • •

• • • •

PHYSICS

UNIT 8: MAGNETISM

ELECTROMAGNETIC INDUCTION

Electromagnetic Induction: a voltage (and a current) is induced in a wire when it moves in a magnetic field induced voltage V = BLv

V: potential difference between ends of wire (V)

L: wire length inside field (m) v: wire speed (m/s) induced current I = V/R

ELECTROMAGNETIC INDUCTION

either the wire or the field can move

wire must cut across field lines to induce a voltage

ELECTROMAGNETIC INDUCTION

right hand rule for electromagnetic induction: point thumb in v direction, index finger in B direction; other fingers will bend

to point I direction

v× × × ×

× × × ×

× × × ×

• • • •

• • • •

• • • •

v

ELECTROMAGNETIC INDUCTION

Generators: spinning a coil in a B field induces current in the coil

Greenwirevoltage

ELECTROMAGNETIC INDUCTION

generator current is alternating current (AC) frequency: current

cycles/sec (Hz) = coil rotation rate

DC generator: coil connects to commutator, which switches external connection when current switches

ELECTROMAGNETIC INDUCTION

Lenz’s Law: an induced current generates a magnetic field which opposes the change that induced it mechanical energy must go in to get

electrical energy out ex: The Drop Zone - magnets on cars

move past metal fins; current is induced in fins which takes energy from the cars, slowing them down

PHYSICS

UNIT 8: MAGNETISM

UNIT 8 REVIEW 0 = 4×10-7 Tm/A B = 0I/2r B = 0nI n = #loops/length of

coil F = Bqv m = Bqr/v F = BIL V = BLv V = IR e = +/- 1.60x10-19 C

right hand rule for field around a wire

right hand rule for field around a solenoid

right (or left) hand rule for force on particles

right hand rule for force on wires

right hand rule for induced voltage