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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