Lesson 5 Lesson 5 Current and Resistance Current and Resistance Batteries Batteries Current Density Current Density Electron Drift Velocity Electron Drift Velocity Conductivity and Resistivity Conductivity and Resistivity Resistance and Ohms’ Law Resistance and Ohms’ Law Temperature Variation of Resistance Temperature Variation of Resistance Electrical Power and Joules Law Electrical Power and Joules Law Classical Model of Conduction in Classical Model of Conduction in Metals Metals Lesson 5
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Lesson 5 Current and Resistance Batteries Current Density Electron Drift Velocity Conductivity and Resistivity Resistance and Ohms’ Law Temperature.
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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
Flow of Charge
I
Current Picture
Current is the rate of Flow of positive
charge through whole cross sectional
area of conductor
Current Picture Definition I
I dQ
dt
I Q T
C
sA (Ampere)
Current Picture Definition II
Current is Conserved
I1
I2
I1+I2
Conservation of Current
Flowing charge experiences friction
Work must be done to overcome friction
Need driving force, hence needElectric FieldPotential Difference
Driving force for Current
Electrical Resistance = Potential Difference
Current
R V
I
R V I
V
A (Ohm )
SI units
I-V plots
I
V
I
V
slope constant = 1/R slope not constant
Ohmic Material Non Ohmic Material
V-I plots
Ohmic MaterialsOhmic MaterialsResistance I
V RI Ohms Law
RV
I constant
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 Materials
IV I 2R V 2
R
Ohmic Materials I
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
R l
R la
R l
a resistivity of conductor
m
Resistivity
Picture
I
V+ V-
l
E
a
|V| V V El
I V
R
El
l
a
Ea
Divide by Area
Current Density magnitude = Current per cross sectional area