electrical polarization - yeshiva university · pdf file• like charges repel, opposite...
TRANSCRIPT
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Electrical polarization
Figure 19-5 [1]
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Properties of Charge
• Two types: positive and negative • Like charges repel, opposite charges attract • Charge is conserved • Fundamental particles with charge: electron (negative) and proton (positive) • Magnitude of charge on electron: e=1.60 x 10-19 C (SI unit coulomb, C) • Charge is quantized: ±ne, where n=0,1,2,3… (what about quarks?)
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Any two charges exert a force on each other whose magnitude is directly proportional to the product of the magnitude their charges and inversely proportional to the square of the distance between them.
Coulomb’s Law (I)
Fig 19-7 [1]
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Coulomb’s Law (II)
1 22
0
14
q qF
rπε= (for charges at rest
in a vacuum)
•Permittivity constant (of vacuum/free space): ε0=8.85 x 10-12 C2/ (N m2)
•Principle of superposition applies (vector addition of forces) •Compare with Newton’s law of gravitation: similar form, but gravity is always attractive
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Electrostatic force field
Figure 19-9 [1]
Magnitude of force is proportional to test charge q0
Define electric field as force per unit charge
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Summary
• Charge (properties) • Conductors and Insulators • Coulomb’s law (electrostatic forces) • Electric field (force per unit charge)
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Figure 19-14 [1]
Figure 19-17 [1]
Field is zero at midpoint
Field is not zero here
Electric Field Lines
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Field lines for a conductor
Figure 19-20 [1]
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Figure 19-23 [1]
Electric flux
cosEA θΦ =Units: N m2/C
Φ = ⋅E A
dΦ = ⋅∫ E A
More generally, A
A is a vector normal to surface; magnitude is area of surface
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Gauss’s Law
If a net charge q is enclosed by an arbitrary surface, the net electric flux Φ through the surface is
0
qε
Φ =
Use this to calculate electric field. Most useful when system has some symmetry: Sphere, plane, cylinder
Figure 23-8 [2]
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Summary
• Electric field: point charges • Electric field lines • Conductors (properties) • Electric flux • Gauss’s Law: calculate electric fields
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Electric Potential and Electric Potential Energy
Figure 20-2 [1]
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Electron Microscope
Taken from http://www-outreach.phy.cam.ac.uk/paw2004/exhibitor/microscope_facility.htm
Electrons accelerated by electric field
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Summary
• Electric Potential: V • Electric Potential Energy: U=qV (cf. F=qE) • Electric Field: E= -∆V/∆s
, 24 40 0
q qV Er rπε πε
= =
For a point charge q: For two point charges q1 and q2:
1 2 1 22
0 0
, 4 4q q q qU F
r rπε πε= =
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Figures 20-5a and 20-6 [1]
Electric potential
Positive point charge placed at origin
Equipotential surfaces
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Charged spherical shell: radius 1m, charge on surface 1.0 µC
Figure 24-18 [2]
20 0
, 4 4
q qV Er rπε πε
= =
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Conductors of arbitrary shape
Thin conducting wire; connects two conducting spheres of radii R and r (with R > r)
Model real conductor in (b) as simplified system in (a)
Adapted from figure 20-10 [1]
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Parallel-Plate Capacitor
Connected to battery (not shown) which applies voltage difference V between plates
0
=
QCV
ACd
ε
= general
parallel-plate capacitor
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Dielectrics
Figure 20-15 [1]
0
=
EE
ACd
κκε
=
κ (“kappa”) is the dielectric constant; κ=1 in a vacuum, very close to 1 for air, about 80 for water
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Summary
• Equipotential surfaces and conductors
• Capacitors and dielectrics
• Parallel-Plate Capacitor
• Electrical energy storage (in electric field)
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Summary
• Current • Direct-Current (DC) Circuits • Resistance: Ohm’s Law • Energy and Power in Electric Circuits
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Resistors in series
1 2 3eqR R R R= + +
Figure 21-6 [1]
More generally, equivalent resistance is sum of individual resistances
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Resistors in parallel
Figure 21-8 [1]
1 2 3
1 1 1 1
eqR R R R= + +
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Kirchhoff’s Rules
C
D
E
F
Junction rule: algebraic sum of all currents at any junction must be zero. Loop rule: algebraic sum of all potential differences around any closed loop must be zero.
Figure 21-14 [1]
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RC circuits
Figures 21-18, 19, 20 [1]
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Summary
• Resistors in series (sum) • Resistors in parallel (harmonic mean) • Kirchhoff’s rules • Capacitors in circuits: series, parallel, RC ([RC]=[time])
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Magnetic Force on Moving Charges
F = q v × B
Figures 22-7 and 22-8 [1]
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Lorentz Force Law
F = q (E+ v × B)
Figure 22-10 [1]
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http://hepweb.rl.ac.uk/ppUKpics/ images/POW/1998/980318.jpg
Bubble Chamber: electron-positron shower
Can deduce charge (sign) and mass of particle
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Mass spectrometer (electrospray):
Can calculate mass of protein to within 0.02%
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http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/
Hall effect
Velocity selector
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Magnetic Force on Current-Carrying Wire
F = I L × B
L is in the direction of the current
Figure 22-15 [1]
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Summary
• Magnetic field: properties • Magnetic force on moving charges:
different types of motion • Magnetic force due to currents
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Top view
Side view
Magnetic Torque
Figures 22-16 and 22-17 [1]
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Ampère’s Law
μ0 is the permeability of free space. Its value is 4π x10-7 Units: T m/A
0 encd Iµ⋅ =∫ B l
More generally,
l is a vector along the closed path
0 encB l Iµ∆ =∑
Figure 22-22 [1]
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Figure 22-24 [1]
Forces between current-carrying wires
Parallel wires attract; Opposite currents repel
d
L
2
0 1 0 1 22
2 2
F I LBI I II L Ld d
µ µπ π
=
= =
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Current Loop
0 (center)2
IBR
µ=
R is radius of loop
Figure 22-25 [1]
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Solenoid
0 (inside)B nIµ=
n=number of turns/unit length Figure 22-27 and 22-28 [1]
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Summary
• Loops of current and torque • Ampère’s Law • Current Loops and Solenoids
You should be able to derive magnetic field for
simple cases using Ampère’s Law
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cosBA θΦ =Units: T m2 = 1 weber (wb)
Φ = ⋅B A
compare with electric flux
A is a vector normal to surface; magnitude is area of surface
Figure 23-3 [1]
Magnetic Flux (I)
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Magnetic Flux (II)
Which loop has a magnetic flux that changes with time?
Conceptual Checkpoint 23-1 [1]
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Faraday’s Law of Induction: Electric Guitar
pickups
Cost: $2,449
http://www.gibsoncustom.com; Figure 23-5 [1]
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Lenz’s Law
An induced current always flows in a direction so as to oppose the change that caused it.
Figure 23-11 [1]
Example: rod in frictionless contact with two vertical wires
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Summary
• Faraday’s Law of Induction:
dNdtΦ
= −E
E = induced electromotive force N = number of (tightly wound) turns Φ = magnetic flux minus sign from Lenz’s Law
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Direct current generator (DC dynamo)
brushes are fixed in space; commutator rings move with wire
From “Ordinary Level Physics” 4th ed., A.F. Abbott p. 496-497
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Alternating current generator
brushes are fixed in space; slip ring always in contact with same brush From “Ordinary Level Physics”
4th ed., A.F. Abbott p. 495
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Inductance and Induced EMF
Figure 30-17 [2]
dILdt
= −E
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RL circuits Figures 23-19 and 20 [1]
LR
τ =compare RC circuits: charge and τ =RC
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Transformers
Figure 23-22 from the text
s s
p p
V NV N
=
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Summary
• Inductance • RL Circuit ([L/R]=time) • Energy Stored in a Magnetic Field • Transformers, Motors, Generators
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Alternating Current (I)
Figures 24-1 and 2 [1]
max sinV V tω=
max sinI I tω=
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rms max 2I I=
Alternating Current (II)
2 2 2max
2 2av max av
22max
av rms
sin
sin
2
P VI I R I R t
P I R t
IP R I R
ω
ω
= = =
= < >
= =
Power dissipated in a resistor:
Figure 24-4 [1]
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Alternating Current: Power
e.g. resistor e.g. capacitor, inductor
av 0P =2av rmsP I R=
av avP VI=< >Figure 24-12 [1]
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Electricity in the world (I)
Some countries have issues: e.g. Brazil (110 V, 115 V, 127 V, 130 V, 220 V or 240 V) e.g. Japan (East: 50Hz; West 60Hz)
from http://en.wikipedia.org
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Electricity in the world (II)
A
B
C G M
from http://en.wikipedia.org
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LC circuits
Oscillations with angular frequency: 1LC
ω =
Figure 31-1 [2]
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RLC circuits Figure 24-20 [1] Figure 31-5 [2]
Start with charge on capacitor: damped harmonic oscillator Resistor is source of damping (can be underdamped, overdamped or critically damped)
Drive oscillations with ac power supply Resonance occurs when the frequency of the power supply is same as the natural frequency of circuit (1/√LC)
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Summary
• Alternating voltage and current • Root mean square values • Power in ac circuits • LC and RLC circuits (oscillations and resonance)
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Maxwell’s Equations (I)
ρ0
t
t
∇ ⋅ =∇ ⋅
∂∇× −
∂∂
∇×∂
DB =
BE =
DH = J +
Gauss’s Law
No magnetic monopoles
Faraday and Lenz’s Laws
Ampère’s Law (modified)
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Maxwell’s Equations (II)---Genesis 1: 3
http://www.cafepress.com/; http://www.thegiftedchildlearning.com/ http://www.gaftee.com/
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The Electromagnetic Spectrum
Figure 25-8 [1] c f λ=
from http://www.pa.msu.edu/courses/2000spring/PHY232/lectures/emwaves/visible.html
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References
• [1] J. S. Walker, Physics, 2nd ed (Pearson/Prentice Hall, Upper Saddle River, 2004).
• [2]: D. Halliday, R. Resnick and J. Walker, Fundamentals
of Physics, 7th ed; extended (Wiley, New York, 2005).