# PHYSICS UNIT 8: MAGNETISM

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

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=m0I/2pr B: magnetic field strength (T) I: current in wire (A) r: distance from wire (m) m0=4p×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=m0nI 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 cathode ray tube (TV & monitor picture tube)

MAGNETIC FORCES cathode 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 mass m = 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 Green wire voltage

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 m0 = 4p×10-7 Tm/A B = m0I/2pr B = m0nI
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