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Electric Field Model: Current 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, ME
Friday, March 14, 2008
James Vesenka, University of New EnglandDepartment of Chemistry and Physics
[email protected]/cas/jvesenka
Electric Field: Current Page 2
Classical Mechanics
Particle Model
Waves Oscillating Particle Model
Electricity &
Magnetism
Field Model
Particle of Mass mGravitational Field g
Gravitational Force Fg
Potential Energy Eg
Potential Vg &Tools
Particle of Charge qElectric Field E
Electrostatic Force FE
Potential Energy EE
Potential V & Tools
Magnetic dipole µMagnetic Field B
Magnetic Force FB
Cross Product/RHRInduction
Particle of Mass mConstant Velocity
F = 0
E -> current IOhms Law, Circuits
I -> Magnetic Field B
Particle of Mass mChanging VelocityF ≠ 0, projectile
motion
Conservation of EnergyEtotal = W+Q+R
Impulse/MomentumFt = p
ptotal = mv = constantConservation of Linear
Momentum
Central ForceF = (mv2/r) inwardRotational Mechanics
Source of Waves:Simple Harmonic
OscillatorLinear Restoring Force
SHO KinematicsEnergy Conservation
Mechanical Waves:Sound Waves
Energy PropagationSuperposition Principle
Doppler Shift
Light Waves & Interference
Diffraction/RefractionPolarization/Colors
Fluid StaticsDensity "stuff/space"
Pressure P
2nd Semester Physics
1st Semester Physics
Fluid DynamicsMacroscopic motion
Fluids Multiple Particle Model
The Big Picture
Electric Field: Current Page 3
Modeling CycleStart/stop
Definitions Operations
Decisions
Paradigm Lab
Pre-lab
Operational
Definitions
Multiple Representatio
ns
Graph, Math
Diagram, Verbal
Test-Works?
NoRefine
Yes
Consensus
Application
Exam
Electric Field: Current Page 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?
Electric Field: Current Page 5
Operational Definitions Electric Current = charge difference in a
given time through a section of wire.I q/”t
Electric field “drives” the current. Units: coulomb/second ampere (A) André Marie Ampère
Explained forces between current carrying wires
1775 - 1836 French Physicist
Electric Field: Current Page 6
Electric Field vs. Time Lab Results Part A f(t)=“Bings”, q (t),
I(t)
t (s)10 30
0.37
1.0
f(t) = f(0)exp[(-0.1/s)t(s)]
f(t) = f(0)exp(-t/)
= “decay constant” time
f(t)/f(0) = 0.37 when t/ = 1
+++
---
+ -
ti tm tf
E, I big E, I small
E, I zero
Electric Field: Current Page 7
Electric Field Drives Current
Uniform E throughout a wire because of uniform charge distribution.
ELEL ER ER
EL
Etotal Etotal
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+ + -
-
-
-
-
-
- -
- -
Etotal
ER
EtotalEtotal
ERELEL
ER
EL ER
Etotal
-
-
+
+
Electric Field: Current Page 8
Charge Density Picture Uniform E throughout a wire because of
uniform charge density.
ELEL ER ER
EL
Etotal Etotal -Etotal
ER
EtotalEtotal
ERELEL
ER
“Conventional” Current
“I”+
Electron Current “I-”
-
=-
-
-
-
-
-
- -
- -Low
Potential+
+
+
+
+
+
+ +
+ +
=
High Potential
Electric Field: Current Page 9
Charge Density I Strength of E depends on the
distribution of charge.
V+
V-
-I
+++++
-----
+ + + + +
- - - - ----
-
-
-
+++
+
+
+
+ + + + +
- - - - -
++
--
++
+
+
--
-
-
E
+I
Electric Field: Current Page 10
Charge Density II The longer the parallel lines, the
more positive the charge.
V+
V-
E
+I
-I
Electric Field: Current Page 11
PRS ?
A. Smallest in battery, largest in resistor
B. Smallest in resistor, largest in battery
C. Same in battery and resistor
++++
----
+ + + +
- - - - ----
-
-
+++
+
+ ++
--
++
+
--
-
+ +
- - -
Where is the current the largest? Where is it the smallest?
Conservation of charge: Electrical current can be neither created nor destroyed.
Demo: Student Current
Electric Field: Current Page 12
Review: Ohm’s Law Lab Results
∆V(V) = (5 V/A)I(A)
I (A)
∆V(V)
V
A
Capacitor
Resistor
E = -V/x
+++
---
I(A)
t (s)10
1.0
∆V(V)
t (s)10
5.0
Electric Field: Current Page 13
Ohm’s Law ∆V vs I:
∆V = IR R = Resistance Units: V/A =
Georg Simon Ohm 1787-1854
Representations Verbal, Graphical,
Diagrams Motion Map, Forces,
Energy and charge conservation
∆V (V)I (A)
I=(0.2 A/V)∆V∆V=(5 V/A)
Electric Field: Current Page 14
PRS ?
A. A=B=C=D=E
B. A=D=E>B=C
C. A>B=C>D=E
D. B=C>A=D=E
E. B=C>A>D=E
A
V VV
How do the bulb brightness (called the “Luminance”, symbol “”, compare in three circuits containing identical batteries and identical bulbs drawn below?
B
C
D E
Electric Field: Current Page 15
Ohm’s Law Ratio: Diagram I = ∆V/R
R =
consta
nt
V RI I
∆V
= co
nsta
nt
x
x
A
A
Electric Field: Current Page 16
Electric Field Model R = x/A [units: m*m/m2] I = ∆V/R = E∆x/R =E∆x/x/A =EA/
E I
E/ co
nst.
A/ co
nst.
E
A
EA
con
st.
I increases with E-Field and AreaI decreases with resistivity
Electric Field: Current Page 17
PRS ?
A. EA=EB=EC=ED=EE
B. EA=ED=EE>EB=EC
C. EA>EB=EC>ED=EE
D. EB=EC>EA=ED=EE
E. EB=EC>EA>ED=EE
EA EEED
EC
EBVV
V
xxx
x
Brightness depends on current which depends on the electric field E. Use the field concept to predict how the bulb brightnesses will behave.
Electric Field: Current Page 18
Light Bulb Answer As E decreases, so does I and brightness.
EA EEED
EC
EB
V+
V-
E = -V/x2x
xV=V+-V-
Energy Conservation: Vin
= VB+VC
Charge Conservation: Iin =
ID+IE IN
Electric Field: Current Page 19
Conservation of Energy Energy is constant:
VAB = VDC + VDE
IReff = IR1 + IR2 => Rseries = R1 + R2 = Ri
E
IN
V(V
)I (A
)
t(s)
t(s)
Vbat
VR1
R1
R2
I
VR2
VR1
Vbat VR2
Ax (m)
V (V)
B EC D A
A
B
E
C
D
D
Electric Field: Current Page 20
Conservation of Charge Potential difference is same in parallel (||):
VR1 = VR2 = Vbat = V
Current “splits”: I = I1 + I2 V/R|| = V/R1 + V/R2 1/Reff = 1/R1 + 1/R2 => 1/R||=1/Ri
R1 R2 V(V
)I (A
)
I
VR
1
t(s)
t(s)IN
Vbat
VR1,VR2
I1 I2V
R2Vbat
Electric Field: Current Page 21
Circuit Lab Quiz Diagram What happens to the current at A? The current splits: I = I1+I2 At B? I1+I2 = I
+ + + + + +- - - - - -
A
BWhat is q and E through each resistor?
II1 I2
II1 I2
E
E1 E2
+++ +++
--- ---
+++ +++
+ +
+ + + +
Electric Field: Current Page 22
Snap Circuit Set -upSnap Circuits by Elenco, www.elenco.com
Electric Field: Current Page 23
Typical Ohmic Data