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
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