Modeling%20Electric%20Fields%20in%20Circuits%20%20TWENTY-EIGHTH%20STATEWIDE%20MEETING%20OF%20HIGH%20SCHOOL%20PHYSICS%20AND%20PHYSICAL%20SCIENCE%20TEACHERS - PowerPoint PPT Presentation

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Modeling%20Electric%20Fields%20in%20Circuits%20%20TWENTY-EIGHTH%20STATEWIDE%20MEETING%20OF%20HIGH%20SCHOOL%20PHYSICS%20AND%20PHYSICAL%20SCIENCE%20TEACHERS

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Modeling Electric Fields in Circuits. TWENTY-EIGHTH STATEWIDE MEETING. OF HIGH SCHOOL PHYSICS ... Electric Current = charge difference in a given time through a ... – PowerPoint PPT presentation

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Title: Modeling%20Electric%20Fields%20in%20Circuits%20%20TWENTY-EIGHTH%20STATEWIDE%20MEETING%20OF%20HIGH%20SCHOOL%20PHYSICS%20AND%20PHYSICAL%20SCIENCE%20TEACHERS


1
Modeling Electric Fields in Circuits
TWENTY-EIGHTH STATEWIDE MEETINGOF HIGH SCHOOL
PHYSICS AND PHYSICAL SCIENCE TEACHERS
  • Department of Physics and AstronomyThe
    University of Maine - Orono, MEFriday, March 14,
    2008James Vesenka, University of New
    EnglandDepartment of Chemistry and
    Physicsjvesenka_at_une.edufaculty.une.edu/cas/jvese
    nka

2
2nd Semester Physics
1st Semester Physics
The Big Picture
Particle of Mass m Constant Velocity ?F 0
Fluid Statics Density "stuff/space" Pressure P
Magnetic dipole µ Magnetic Field B Magnetic Force
FB Cross Product/RHR Induction
Source of Waves Simple Harmonic
Oscillator Linear Restoring Force SHO
Kinematics Energy Conservation
Conservation of Energy ?Etotal WQR
Fluid Dynamics Macroscopic motion
Particle of Mass m Changing Velocity ?F ? 0,
projectile motion
E -gt current I Ohms Law, Circuits I -gt Magnetic
Field B
Mechanical Waves Sound Waves Energy
Propagation Superposition Principle Doppler Shift
Impulse/Momentum ?F?t ?p ptotal mv
constant Conservation of Linear Momentum
Particle of Mass m Gravitational Field
g Gravitational Force Fg Potential Energy
Eg Potential Vg Tools
Particle of Charge q Electric Field
E Electrostatic Force FE Potential Energy
EE Potential V Tools
Light Waves Interference Diffraction/Refraction
Polarization/Colors
Central Force ?F (mv2/r) inward Rotational
Mechanics
3
Modeling Cycle
4
Discussion Flashlight Physics
  • Two identical flashlights one is connected to a
    fresh battery the other to a charged
    capacitor.
  • Predict what will happen after each flashlight is
    switched on.
  • How do you know?

5
Operational Definitions
  • Electric Current charge difference in a given
    time through a section of wire.
  • I º Dq/Dt
  • Electric field drives the current.
  • Units coulomb/second º ampere (A)
  • André Marie Ampère
  • Explained forces between current carrying wires
  • 1775 - 1836
  • French Physicist

6
Electric Field vs. Time
  • Lab Results Part A

f(t)Bings, q (t), I(t)
1.0
0.37
t (s)
10
30
f(t) f(0)exp(-0.1/s)t(s) f(t)
f(0)exp(-t/t) t decay constant time f(t)/f(0)
0.37 when t/t 1
ti
tm
tf

---

-
E, I big
E, I small
E, I zero
7
Electric Field Drives Current
  • Uniform E throughout a wire because of uniform
    charge distribution.

-

Etotal
EL
ER
-


- -
-
-
-





-
-
Etotal
Etotal
Etotal
Etotal
Etotal
-
-




EL
ER
-
-
EL
EL
ER
ER
ER
EL
ER
EL
-
-




-

-
-

- -
8
Charge Density Picture
  • Uniform E throughout a wire because of uniform
    charge density.

Conventional Current I

Etotal
Etotal
Etotal
Etotal
Etotal
-
EL
ER
EL
EL
ER
ER
ER
EL
ER
EL
-
Electron Current I-
9
Charge Density I
  • Strength of E depends on the distribution of
    charge.

E
V-
V
I
-I
10
Charge Density II
  • The longer the parallel lines, the more positive
    the charge.

E
V-
V
I
-I
11
PRS ?
  • Where is the current the largest? Where is it
    the smallest?
  1. Smallest in battery, largest in resistor
  2. Smallest in resistor, largest in battery
  3. Same in battery and resistor

Demo Student Current
Conservation of charge Electrical current can be
neither created nor destroyed.
12
Review Ohms Law
  • Lab Results

?V(V)
5.0
V
t (s)
10

---
I(A)
1.0
t (s)
Capacitor
10
?V(V)
A
E -DV/Dx
I (A)
Resistor
?V(V) (5 V/A)I(A)
13
Ohms Law
  • ?V vs I
  • ?V IR
  • R Resistance
  • Units V/A W
  • Georg Simon Ohm
  • 1787-1854
  • Representations
  • Verbal, Graphical, Diagrams
  • Motion Map, Forces, Energy and charge conservation

I(0.2 A/V)?V
?V(5 V/A)
?V (V)
I (A)
14
PRS ?
  • How do the bulb brightness (called the
    Luminance, symbol L, compare in three
    circuits containing identical batteries and
    identical bulbs drawn below?
  1. LALBLCLDLE
  2. LALDLEgtLBLC
  3. LAgtLBLCgtLDLE
  4. LBLCgtLALDLE
  5. LBLCgtLAgtLDLE

LB
DV
DV
DV
LA
LD
LE
LC
15
Ohms Law Ratio Diagram
  • I ?V/R

R
I
I
16
Electric Field Model
  • R rDx/A units Wmm/m2
  • I ?V/R E?x/R E?x/rDx/A EA/r

E I
17
PRS ?
  • Brightness depends on current which depends on
    the electric field E. Use the field concept to
    predict how the bulb brightnesses will behave.
  1. EAEBECEDEE
  2. EAEDEEgtEBEC
  3. EAgtEBECgtEDEE
  4. EBECgtEAEDEE
  5. EBECgtEAgtEDEE

Dx
EB
DV
DV
EA
EE
ED
DV
Dx
Dx
Dx
EC
18
Light Bulb Answer
  • As E decreases, so does I and brightness.

V
DVV-V-
Dx
E -DV/Dx
2Dx
V-
Energy Conservation DVin DVBDVC
Charge Conservation Iin IDIE
19
Conservation of Energy
DV (V)
  • Energy is constant
  • DVAB DVDC DVDE
  • IReff IR1 IR2 gt
  • Rseries R1 R2 SRi

x (m)
A
B
E
C
D
A
Vbat
C
D
R1
DVR1
DV(V)
t(s)
VR2
B
DVbat
VR1
I
A
E
I (A)
R2
DVR2
t(s)
E
D
20
Conservation of Charge
  • Potential difference is same in parallel ()
  • DVR1 DVR2 DVbat DV
  • Current splits I I1 I2
  • DV/R DV/R1 DV/R2
  • 1/Reff 1/R1 1/R2 gt 1/RS1/Ri

Vbat
I
DV(V)
t(s)
R2
R1
VR1,VR2
DVbat
DVR2
DVR1
I1
I2
I (A)
t(s)
21
Circuit Lab Quiz Diagram
  • What happens to the current at A?
  • The current splits I I1I2
  • At B?
  • I1I2 I

A
What is q and E through each resistor?
B

- - - - - -
22
Snap Circuit Set -up
Snap Circuits by Elenco, www.elenco.com
23
Typical Ohmic Data
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