Schedule
• Today:– Check Exam I– Circuit Problems– Magnetism– Turn in Lab 6
• Thursday– Do Lab 8 (in lab book)
Exam Trouble Spots
• Difference between field, potential, force, potential energy
• Vectors (GPII)
• Conservation of Energy
Electric Field
Starts at +Ends at –
Can also go to “infinity”
Field makes force
Indicates direction of force on + charge
Magnetic Fields
Start at North Go to SouthUnlike charges Every North has a SouthMake a force – but more complicated
Magnetic Fields
• Magnetism discovered ages ago
• Fields like electrostatics
• Terminology “pole”arization
• More complicated than electric forces
• Forces Related (will see, eventually…)
Magnetic Interaction
S
Magnets have poles N & SLike Electrostatic Forces
Opposites Attract, Like Repel, & non-magnetic unaffected“Magnetic” can be polarized…
S S SN N N N
Compass
• Floating Magnetic Needle
• North end attracted to “North Pole”– Points North
• Handy outside
• Work inside?
Field of a bar magnet
• Field lines start at N and end at S– Notice no infinity!
SN
Field of a Horseshoe Magnet
• Stronger at Poles
Fridge Magnet
• Material like many horseshoe magnets• “Domains”
Fridge Magnet:is Fridge Magnetic?
• What are magnetic materials?• Why do magnets stick to “magnetic
materials”?
Magnetism: Macroscopic ViewFerromagnetic Domains
Arrows Point from North to SouthIndividual “Little Magnets”
Unpolarized Ferro-magnet Fridge, Chunk of Iron, etc…
Magnetism: Macroscopic ViewFerromagnetic Domains
Polarized MaterialBar Magnet, Compass Needle, Earth
How to Polarize : Use magnetic Field!Magnet polarizes nearby magnetic material (Think Charge) Always attractive
Non-Magnetic Materials
• Paramagnetism– Some things just don’t care about fields– Wood, Paper, Aluminum
• Diamagnetism– Actually small repulsion does occur
• (non-ferro)– In a large enough field “anti-polarization”– 16 T floats a frog…
Ferromagnetic Properties
• Iron, Nickel, Cobalt are ferromagnets
• Aluminum, silicon, argon are not
• Similar properties between 3 elements?
• Are they neutral?
• Hint: magnetism & electric forces related
• What gives rise to magnetic properties?
Electrons
• 90% of anything’s properties due to Electrons
• Determine insulator / conductor– Heat & Electricity
• Determine Magnetism
• Determine Color, etc…
Magnetism
• Created by & Acts on MOVING charges
• Magnetic Fields B, units T (Tesla)– Tesla Coil
Magnetic Materials must be Metals?
• Magnetic force acts on MOVING charges
• What moves in a metal?
• Total charge of a metal?
• Do protons move?
• Magnetic Force on electrons?
• Magnetic Force on protons?
Moving Charges Make Magnetic Fields (Straight Wire)
0
2
IB
r
0 is magnetic permittivity of free spacer is distance to wireI current in wire
Like Electrostatic Permittivity
Field Obeys R.H.R.
0
2
IB
r
Fielf From a Loop?
Current
B?
B?
CurrentCurrent
Magnetic Field from a Loop
0
2
IB
R
Like a Bar Magnet
Field at Center
Magnetic Field from Many Loops
0
2
IB
R
Field at Center for one loop
What if a small coil, say 5 loops?Hint: Superposition
Solenoids: Capacitor of Magnetism
00
NIB nI
LN
nL
L
Field Constant InsideField ~ zero outsideDirection R.H.R.
What are electrons doing in a bar magnet?
N
S
What are they doingIn a non-magnetic Metal?
Magnetic Force on Moving Charge
sinMAGF Q vB
Q = 0 F = 0
V = 0 F = 0
Force is PerpendicularEverything Perpendicular
• Force perpendicular to BOTH v & B
Magnetic Field from Moving Charge
sin
FB
Q vB
Definition of magnetic Field
Right Hand RuleWorks for wrenches too!
sinMAGF QvB
How to determine directions Quickly
Negative Charges Opposite“Left Hand Rule”
Sign Important
What is trajectory?
Straight Line?Bendy?How?
Force Perp. To v & B
Motion of Charged Particle in a Magnetic Field
• Force perpendicular to velocity & field
Velocity Perp. To B
B is pointing “out”
Circular Motion
R.H.R.
Circular Motion Review
• Centripetal Acceleration / Force
ˆ ˆˆy x x z z yv B z v B y v B x
2
C
va
r
2
C
vF m
r
F mar
If a velocity not quite perpendicular:
• Helical Motion
• Separate Components
• vP2 + vL
2 = v2
2
C
va
r
Mass Spectrometer
Simple Design2 Isotopes, IonizedSame Charge (+e)Different MassSame velocity
2
2
C
c
vF m
r
mvr
F
r1 r2
Force on a Current Carrying WireCharges moving in Wires
sin
sin sin
F qvB
x qqv q x I x
t t
F I x B ILB
Remember Current is positive charge flowElectron flow opposite (negative charges)
Work with current: everything positive & R.H.R.
Force Obeys R.H.R.
Point in current flow direction, rest same
Try Problems
• Circuits
• Magnetism problems Thursday before lab– Read over chapter & lab
Additional Examples
1. In the circuit shown below:A. Rank in order, from most to least bright, the brightness of
bulbs A–D. Explain.B. Describe what, if anything, happens to the brightness of
bulbs A, B, and D if bulb C is removed from its socket. Explain.
Slide 23-41
Additional Examples
2. In the circuit shown below, rank in order, from most to least bright, the brightness of bulbs A–E. Explain.
Slide 23-42
3. In the circuit shown below:A. How much power is dissipated by the 12 Ω resistor?B. What is the value of the potential at points a, b, c, and d?
Slide 23-43
Additional Examples
The diagram below shows a segment of a circuit. What is the current in the 200 resistor?A. 0.5 AB. 1.0 AC. 1.5 AD. 2.0 AE. There is not enough information to decide.
Slide 23-13
Clicker Question
There is a current of 1.0 A in the circuit below. What is the resistance of the unknown circuit element?
What is the current out of the battery?
Slide 23-19