ENGR 1310 Lecture 11 - Resistance

Resistance, Ohm’s
Law
EGR 1301 Fall 2010
Electricity – The movement
of charge (usually electrons)
• Electricity can travel through some
materials better than others
• Good conductors : Gold, silver,
aluminum, copper…
• Poor
conductors: Glass, plastic, rubber,
Insulators:
wood…
• Semiconductors: Modern materials
designed to conduct only under certain
conditions (Transistors, LEDs, amplifiers)
Flow of electricity
through a copper wire
Conventional
Net Flow Of Electrons
Current
=I
Electrons are induced by chemical
reaction in the battery
+
BATTERY
-
How many electrons???
1 cm
Atomic wt. of Cu = 63.55 g/mol
1 cm
Cu
Density of Cu = 8.94 g/cm3
1 cm
Avogadro’s Number = 6.022 x (10)23 atoms/mol
# of Cu valence electrons
= # of Cu atoms
=1 cm3 x 8.94 g/cm3 x mol/63.55 g x 6.022 x (10)23 atoms/mol
# of Cu valence electrons = 8.5 x (10 )22
This is more than the number of grains of sand on the entire surface of the earth. (Assuming
grains of sand are 1mm in diameter, packed to 68% efficiency, 10-cm deep over the entire 200
million sqare mile surface)
Flow of electricity
through a copper wire
Conventional Current
=I
Net Flow Of Electrons
Electrons are induced by chemical
reaction in the battery
+
BATTERY
Electron
Shortage
-
Electron
Supply
Voltage & Resistance
• The difference between the strength of
the electron supply and the severity
electron shortage defines the voltage.
Voltage = V
• The scattering and destructive interaction
between atoms and electrons cause
electrical resistance.
Resistance = R
Current and Amps,
Resistance and Ohms
• Current = I = quantity of electrons passing
a point per unit time
• 1 Amp = 1 Coulomb/sec = 6.28 x (10)18
electrons/sec
• Class Problem: A battery powers a
flashlight. If the battery supplies a steady
current of 1.3 A over 8 minutes, how
many electrons leave the negative
terminal of the battery during this time
period?
Ohm’s Law
+
V
-
Conventional
Current = I
R
Ohm’s Law
Voltage = Current X Resistance
Volts = Amps X Ohms
V = I x R = Amps X Ω
Resistor
Electron Flow