PHYSICS UNIT 8:
MAGNETISM
MAGNETISM
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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:
northseeking N
pole,
southseeking 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 ×
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MAGNETISM

Earth is a
magnet
 north
geographi
c pole is
an S pole
 field like a
bar
magnet
MAGNETISM
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Source of Magnetism
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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
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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
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Field Around A
Solenoid (coil)
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like bar magnet when carrying
current
add iron core: electromagnet
(much stronger field)
solenoid field strength, B=m0nI
n = #loops/length of coil
ELECTROMAGNETISM

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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
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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
2
 Bqv = mv /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
proton:
N
electron:
v
S
S
N
v
MAGNETIC FORCES

Magnetic fields exert force on currentbearing 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
×
×
×
×
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×
×
×
×
•
•
•
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×
×
×
×
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I
I
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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
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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
×
•
•
•
×
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•
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×
•
•
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•
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
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
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PHYSICS
UNIT 8: MAGNETISM
UNIT 8 REVIEW
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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
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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