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MAGNETISM !!
(Ch. 24)
PH 104 w/ dr. g
Lec 28
COMPASS =
magnet
Works because
EARTH = magnet !!
Magnetism
Magnetic Force:
y Can either attract or repel
y Two kinds of poles: N = north(seeking); S = south(seeking)
y All magnets are DIPOLES: always have both poles
y POLES: Likes repel, opposites attract
y Magnetic (force)field:
y Direction to which a small magnetic dipole would align
y Traced out by iron filings
S is up north,
N is down south!!
Magnetism
Magnetism
Magnetic Force on electric currents
Magnetic Force on electric currents
y Charged particles: deflected by magnetic field
y Force direction: perpendicular to velocity AND field
y Charged particles: deflected by magnetic field
y Aurorae: charges trapped in Earth’s field (at poles)
y Outward field: charges entering vertically deflect left or right
y Mass Spectrometer – mass determined from orbit radius in field
Field (outward) ON
Magnetism
Magnetism
Electromagnets: Currents produce magnetic fields.
Magnetic Force on electric currents
y Current loop twists (like compass) in magnetic field!
y Electromagnet: coil current is a dipole: N and S ends!
y Each segment of electric current: circular magnetic field
N
N
y There is magnetic force on current
because current acts like a magnet
(compass)
y Application: Current-carrying
loop in field: galvanometer
y Coil current behaves like a
magnet:
y = torque on the coil: twists
– indicates current!
N
y Coil with electric current: behaves like a magnet
S
Field (outward) OFF
S
S
1
Magnetism
Magnetism
Magnetic Force on electric currents
What makes some materials naturally magnetic
y Application: Loop current
changes direction: DC motor
N
y As loop twists, contacts
switch to opposite
terminals
y Current reverses:
electromagnet polarity
reverses
y Electric currents in atom: produce magnetic field (spin)
y SO: magnetic field threads through loops of electric current
y Knowing this, how can “N” be isolated from “S”?
y “Poles”: like “pivots” for a closed-loop field line!
S
N
Applied current:
electromagnet
N
Aligned atomic
currents:
Permanent
magnet
Both can
produce
identical
magnetic
fields!
y rotation is continued, loop
spins continuously!
S
S
Magnetism
Magnetism
What makes some materials naturally magnetic
What makes some materials naturally magnetic
y Electric currents in atom: produce magnetic field (spin)
y Most materials: non-magnetic
y Atomic spins in random directions, normally “cancel out”
y Iron, Nickel, Cobalt, Neodymium, others
y Magnetic materials: atomic spins align
y Domains: regions in which spins align
y Normally: domains randomly oriented in material
Magnetism
What makes some materials naturally magnetic
y Magnetized: domains align
y Can be aligned (magnetized) by field of an electromagnet
y Electromagnet usually has magnetic core inside coil
y Natural magnets: probably aligned by currents in LIGHTNING
y Magnetic media: domains align in two directions (binary: “0” or “1”)
•Frog “floating”
floating” in warm bore of 2525-T magnet
•All materials have some trace of DIAMAGNETISM
•Frog is magnetized opposite the external field: floats
•No evidence of medical effects of magnetic field.
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