schedule today: –check exam i –circuit problems –magnetism –turn in lab 6 thursday –do lab...
TRANSCRIPT
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Schedule
• Today:– Check Exam I– Circuit Problems– Magnetism– Turn in Lab 6
• Thursday– Do Lab 8 (in lab book)
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Exam Trouble Spots
• Difference between field, potential, force, potential energy
• Vectors (GPII)
• Conservation of Energy
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Electric Field
Starts at +Ends at –
Can also go to “infinity”
Field makes force
Indicates direction of force on + charge
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Magnetic Fields
Start at North Go to SouthUnlike charges Every North has a SouthMake a force – but more complicated
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Magnetic Fields
• Magnetism discovered ages ago
• Fields like electrostatics
• Terminology “pole”arization
• More complicated than electric forces
• Forces Related (will see, eventually…)
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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
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Compass
• Floating Magnetic Needle
• North end attracted to “North Pole”– Points North
• Handy outside
• Work inside?
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Field of a bar magnet
• Field lines start at N and end at S– Notice no infinity!
SN
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Field of a Horseshoe Magnet
• Stronger at Poles
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Fridge Magnet
• Material like many horseshoe magnets• “Domains”
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Fridge Magnet:is Fridge Magnetic?
• What are magnetic materials?• Why do magnets stick to “magnetic
materials”?
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Magnetism: Macroscopic ViewFerromagnetic Domains
Arrows Point from North to SouthIndividual “Little Magnets”
Unpolarized Ferro-magnet Fridge, Chunk of Iron, etc…
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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
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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…
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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?
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Electrons
• 90% of anything’s properties due to Electrons
• Determine insulator / conductor– Heat & Electricity
• Determine Magnetism
• Determine Color, etc…
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Magnetism
• Created by & Acts on MOVING charges
• Magnetic Fields B, units T (Tesla)– Tesla Coil
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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?
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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
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Field Obeys R.H.R.
0
2
IB
r
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Fielf From a Loop?
Current
B?
B?
CurrentCurrent
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Magnetic Field from a Loop
0
2
IB
R
Like a Bar Magnet
Field at Center
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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
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Solenoids: Capacitor of Magnetism
00
NIB nI
LN
nL
L
Field Constant InsideField ~ zero outsideDirection R.H.R.
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What are electrons doing in a bar magnet?
N
S
What are they doingIn a non-magnetic Metal?
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Magnetic Force on Moving Charge
sinMAGF Q vB
Q = 0 F = 0
V = 0 F = 0
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Force is PerpendicularEverything Perpendicular
• Force perpendicular to BOTH v & B
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Magnetic Field from Moving Charge
sin
FB
Q vB
Definition of magnetic Field
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Right Hand RuleWorks for wrenches too!
sinMAGF QvB
How to determine directions Quickly
Negative Charges Opposite“Left Hand Rule”
Sign Important
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What is trajectory?
Straight Line?Bendy?How?
Force Perp. To v & B
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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.
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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
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If a velocity not quite perpendicular:
• Helical Motion
• Separate Components
• vP2 + vL
2 = v2
2
C
va
r
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Mass Spectrometer
Simple Design2 Isotopes, IonizedSame Charge (+e)Different MassSame velocity
2
2
C
c
vF m
r
mvr
F
r1 r2
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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.
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Force Obeys R.H.R.
Point in current flow direction, rest same
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Try Problems
• Circuits
• Magnetism problems Thursday before lab– Read over chapter & lab
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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
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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
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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
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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
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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