Electricity

• Current


  flow of charge per unit time
• Potential difference


  the energy transferred between two points per unit charge
• Ohms law

  for an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions, e.g. temperature, are kept constant
• Semiconductor diode


  forward bias of a diode allows current to flow easily past the threshold voltage, which is the smallest voltage needed to allow current to flow.

  • in the reverse direction, the resistance of the diode is very high which means only a very small current can flow
• Filament lamp


  • length of metal wire inside component will heat up as current increases, therefore resistance also increases

  
• Ammeters and voltmeters


  • ammeters can be assumed to have zero resistance
  • all voltmeters can be assumed to have infinite resistance, which means no current can flow through them
• Resistivity


  a measure of how easily a material conducts electricity
• Temp + resistance


  when the temperature of a metal conductor increases, its resistance will increase because atoms of the metal gain kinetic energy and move more which causes electrons to collide with atoms more frequently, causing them to slow down so as current decreases, resistance will increase
• Thermistors


  as the temp increases, resistance decreases

  • increasing temp causes electrons to be emitted from atoms, therefore the number of charge carriers increases and so current increases, causing resistance to decrease
  • can be used in sensor circuits

  
• Superconductor


  • below a certain temperature, has zero resistivity
  • the critical temp depends on material
  • resistance also drops to zero, so applications include power cables, whic reduce energy loss through heating to zero and strong magnetic fields, e.g. MAGLEV trains

  
• Series circuit


  • current is same everywhere
  • p.d. is shared
• Parallel circuit


  • current shared across branches
  • p.d. is same
• Kirchoff's laws


  1. The total current flowing into a junction = total current flowing out of that junction. (no charge lost)
  2. The sum of all the voltages in a series circuit is the battery voltage. (no energy lost)
• Potential dividers


  a circuit with several resistors in series connected across a voltage source
• Potential divider uses


  • if light intensity falls, resistance across R1 will increase so current decreases and resistance across R2 decreases and pd out decreases

  
• Electromotive force


  • the energy transferred by a cell per coulomb of charge
• Internal resistance


  • caused be electrons colliding with atoms inside the battery