1. PHYSICS UNIT 8: MAGNETISM

2. 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)

3. MAGNETISM
Magnets are always dipoles: north-seeking N pole, south-seeking S pole

4. 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

5. 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 • ×

6. MAGNETISM Earth is a magnet north geographic pole is an S pole
field like a bar magnet

7. 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

8. MAGNETISM magnetic domains: regions of aligned magnetic atoms
magnetic object: aligned magnetic domains

9. 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

10. 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

11. 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

12. 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)

13. 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

14. PHYSICS
UNIT 8: MAGNETISM

15. MAGNETIC FORCES
cathode ray tube (TV & monitor picture tube)

16. MAGNETIC FORCES
cathode ray tube (TV & monitor picture tube)

17. 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)

18. 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

19. 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

20. 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

21. 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
• • • •

22. PHYSICS
UNIT 8: MAGNETISM

23. 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

24. ELECTROMAGNETIC INDUCTION
either the wire or the field can move
wire must cut across field lines to induce a voltage

25. 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

26. ELECTROMAGNETIC INDUCTION
Generators: spinning a coil in a B field induces current in the coil
Green
wire
voltage

27. 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

28. 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

29. PHYSICS
UNIT 8: MAGNETISM

30. 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