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Lesson 5. Lesson 5. Current and Resistance. Batteries Current Density Electron Drift Velocity Conductivity and Resistivity Resistance and Ohms’ Law Temperature Variation of Resistance Electrical Power and Joules Law Classical Model of Conduction in Metals. Electrical Resistance. - PowerPoint PPT Presentation
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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 MetalsMetals
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 frictionWork must be done to overcome frictionNeed driving force, hence
Electric FieldPotential Difference
Driving force for Current
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 = dUdt
dQdtV IV
Power I V AV C
s
Nm
CNm
sJs
W (Watts)
Power
For Ohmic MaterialsFor Ohmic Materials
Resistance is proportional to length of conductorResistance 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
Current Density Current per
magnitude cross sectional area
J Ia
E
E
= conductivity 1
==Current Density
Definition of Variables
Charge in Volume VQnaxq navdtqn 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 collsion
vd drift velocity
Equations II
acceleration of charge q in field E
a q
mE
average time between collisions
at each collision charge carrier forgetsforgets
drift velocity , so we can take initial drift
velocity = 0 and just before collisions
v d a q
mE q
m
E
v d J
nq q
mE
J nq2 m
E
nq2 m
Temperature EffectsTemperature Temperature
EffectsEffects
1
m
nq2
As temperature increases decreases
thus increases:
T 0
1 T T0
1
0
ddT
Temperature Coefficient of Resistivity