lesson 5 current and resistance batteries current density electron drift velocity conductivity...
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Lesson 5Lesson 5Current and ResistanceCurrent and Resistance
BatteriesBatteriesCurrent DensityCurrent DensityElectron Drift VelocityElectron Drift VelocityConductivity and ResistivityConductivity and ResistivityResistance and Ohms’ LawResistance and Ohms’ LawTemperature Variation of ResistanceTemperature Variation of ResistanceElectrical Power and Joules LawElectrical Power and Joules LawClassical Model of Conduction in Classical Model of Conduction in Metals Metals
Lesson 5
Electrical Resistance is “friction” to the flow of electric chargeObserved in Conductors and Non ConductorsNot found in Super Conductors
Electrical Resistance
Charge Pump I
I
+-
Load Resistance
Capacitor will send current through
load resistance and loose charge
Charge Pump I
I
+-
Load Resistance
Battery will send current through load resistance and not loose charge
Charge in battery is regenerated by Chemical reactions
Current is the rate of Flow of positive
charge through whole cross sectional
area of conductor
Current Picture Definition I
Flowing charge experiences friction
Work must be done to overcome friction
Need driving force, hence needElectric FieldPotential Difference
Driving force for Current
I-V plots
I
V
I
V
slope constant = 1/R slope not constant
Ohmic Material Non Ohmic Material
V-I plots
Non Ohmic MaterialsNon Ohmic Materials
R is not Constant, but R is not Constant, but varies with current varies with current
and voltage and voltage
Resistance II
Power = rate of doing work Power = rate of doing work by applied forceby applied force
Power
Power = dUdt
dQdt
V IV
Power I V AV C
s
Nm
C Nm
sJs W ( Watts)
For Ohmic MaterialsFor Ohmic Materials
Resistance is proportional to length of conductor
Resistance is inversely proportional to the cross sectional area of the conductor
Ohmic Materials II
|V| V V El
I V
R
El
l
a
Ea
Divide by Area
Current Density magnitude = Current per cross sectional area
J I
a E
E
= conductivity 1
Current Density
Definition of Variables
Charge in Volume V
Q nA x q nAvd t q
n number of charge carriers
per unit volume
A cross sectional area
q amount of charge on
each carrier x average distance moved in
time t after collision
vd drift velocity
Equations II
acceleration of charge q in field E
a q
mE
•Let average time between collisions
•at each collision charge carrier forgetsforgets
drift velocity , so we can take initial drift
velocity = 0 and just before collisions
vd a q
mE q
m
E
v d J
nq q
m E
J nq2 m
E
nq2 m
Temperature EffectsTemperature Temperature
EffectsEffects
1
m
nq 2
As temperature increases decreases
thus increases
T 0
1 T T0
1
0
ddT
Temperature Coefficient of Resistivity
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