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… wood…
• Semiconductors: Modern materials designed to conduct only under certain conditions (Transistors, LEDs, amplifiers)

Flow of electricity through a copper wire
= I
Electrons are induced by chemical reaction in the battery
+
BATTERY
-

How many electrons???
1 cm
1 cm Cu
1 cm
Atomic wt. of Cu = 63.55 g/mol
Density of Cu = 8.94 g/cm 3
Avogadro’s Number = 6.022 x (10) 23 atoms/mol
# of Cu valence electrons = # of Cu atoms
=1 cm 3 x 8.94 g/cm 3 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
Resistor
Ohm’s Law
Voltage = Current X Resistance
Volts = Amps X Ohms
V = I x R = Amps X Ω
Electron Flow