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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, wood…

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resistance ohm s law

Resistance, Ohm’s Law

EGR 1301 Fall 2010

electricity the movement of charge usually electrons
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, wood…
  • Semiconductors: Modern materials designed to conduct only under certain conditions (Transistors, LEDs, amplifiers)

Insulators:

flow of electricity through a copper wire
Flow of electricity through a copper wire

Net Flow Of Electrons

Conventional Current

= I

Electrons are induced by chemical reaction in the battery

BATTERY

+

-

how many electrons
How many electrons???

1 cm

Atomic wt. of Cu = 63.55 g/mol

Density of Cu = 8.94 g/cm3

Avogadro’s Number = 6.022 x (10)23 atoms/mol

Cu

1 cm

1 cm

# 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 wire1
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
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 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
Ohm’s Law

Ohm’s Law

Voltage = Current X Resistance

Volts = Amps X Ohms

V = I x R = Amps X Ω

Conventional Current = I

+

R

V

-

Resistor

Electron Flow