Electric Current

• Electric current - rate of flow of charged particles
• Charge at a point - product of the current at that point and the time for which the current flows
• Charge = Current * Time
• Conventional Current - The current direction is from positive to negative terminal.
• Charge carriers - electrons which carry the elementary charge
• Quantization of a charge - the idea that the charge can only exist in discrete packets of multiples of elementary charge. values of charge are not continuous
• Electric current - Ampere

  Charge - Coulombs
• Coulomb - charge flowing per second pass a point at which the current is 1 ampere
• Elementary charge - 1.60 x 10^-19
• Number density - number of conduction electrons per unit volume
• I=vAnq

  • I- electric current
  • v- mean drift velocity
  • A- cross-sectional area of the conductor
  • q- charge on each charge carrier
• Voltage is also known as the electric potential difference
• Potential difference - energy transferred per unit charge
• Potential difference - difference in electric potential between two points in a circuit
• Potential difference - work done per coulomb to move a charge from the lower potential point to the higher potential point
• Potential difference is measured in volts
• Volt - joule per coulomb
• Potential difference is used when charges lose energy by transferring electrical energy to other forms of energy
• Power - rate at which energy is transferred
• Power is measured in Watts
• The term potential difference is used when charge transfers energy to the component or to the surroundings
• The term electromotive force is used when describing the maximum energy per unit charge that a source can provide
• Potential difference and electromotive force both means the work done per unit charge
• Potential difference - energy transformed from electrical to other forms per unit charge

  Electromotive force - energy transformed from other forms of energy to electrical
• Electromotive force - energy supplied by the source per unit charge passing through the source
• Resistance - ratio of potential difference to the current
• Ohm - volts per ampere
• Resistors in series

  • Current same
  • Potential difference divides
• Resistors in parallel

  • same potential difference
  • current is shared
• Ammeters have a low resistance and are connected in series
• Voltmeters have a high resistance and are connected in parallel in a circuit
• Ohm's Law - the current in a component is proportional to the potential difference across it, provided physical conditions stay constant
• Ohmic conductor - component that obeys the ohm's law

  Non-ohmic conductor - component that does not obey ohm's law
• Metallic conductor

  • Ohmic
  • I/V constant
  • temperature constant
• Filament lamp

  • Non-ohmic
  • when Voltage increases, temperature increases, resistance increases
  • Vibrations of ions increases
  • collisions of ions with e- increases
• Thermistor

  • non-ohmic
  • when voltage increases, temperature increases, resistance decreases
  • released e- increase
  • uses - fire sensors | water temperature sensors | baby breathing monitors
• Semiconductor diode

  • non-ohmic
  • low resistance in one direction and infinite resistance in opposite
  • threshold voltage - the voltage at which the diode suddenly starts to conduct
  • the resistance on the diode depends on the potential difference
• Light Dependent Resistor (LDR)

  • when light intensity increases the resistance decreases
• Resistance of impure metals is greater than the resistance of pure metals
• Factors that affect the resistance

  • temperature
  • presence of impurities