Electricity

• Current


  The flow of electric charge
• Current flow


  1. Electric current is the flow of electric charge around the circuit

  2. Current will only flow around a complete circuit if there's a potential difference

  3. A current can only flow if there's a source of potential difference
• Unit of current


  Ampere (A)
• In a single closed loop, the current has the same value throughout the circuit
• Potential difference (voltage)


  The driving force that pushes the charge around
• Resistance


  Anything in the circuit which opposes the flow of current
• Current flowing through a component

  Depends on the potential difference across it and the resistance of the component
• The greater the resistance across a component


  The smaller the current that flows through it for a given potential difference
• Total charge through a circuit

  1. Depends on current and time

  2. Charge (C) = Current (A) x Time (s)
• Charge


  Measured in coulombs (C)
• Time


  Measured in seconds (s)
• More charge passes around the circuit when a bigger current flows
• Circuit diagram symbols


  • Battery
  • Switch open
  • Switch closed
  • Filament lamp
  • Fuse
  • LED
  • Resistor
  • Variable resistor
  • Ammeter
  • Voltmeter
  • Diode
  • LDR
  • Thermistor
• A current will only flow in a closed circuit
• Calculating charge transferred


  Charge (C) = Current (A) x Time (s)
• Ohmic conductors


  Components where resistance is constant with current (e.g. wire, resistor)
• For ohmic conductors, current is directly proportional to potential difference at constant temperature
• Non-ohmic components


  • Resistance changes with current (e.g. filament lamp, diode)
• 1. V characteristic


  Graph showing how current changes as potential difference is increased
• 1. V characteristics


  • Ohmic conductor (straight line)
  • Filament lamp (curved)
  • Diode (only conducts in one direction)
• Resistance can be calculated from I-V characteristic using R=V/I
• Light-dependent resistor (LDR)


  Resistance decreases as light intensity increases
• Thermistor


  Resistance decreases as temperature increases
• Sensing circuit using LDR or thermistor

  1. LDR/thermistor and fixed resistor in parallel

  2. As resistance of LDR/thermistor changes, potential difference across fixed resistor and connected component changes
• LDRs and Thermistors


  Components that can be used in sensing circuits
• Sensing circuits


  1. Turn on or increase components depending on conditions

  2. Operate a fan in a room
• Fixed resistor and fan

  Always have the same potential difference across them
• Potential difference of power supply

  Shared between thermistor and loop of fixed resistor and fan according to their resistances
• Room gets hotter


  Resistance of thermistor decreases, it takes smaller share of potential difference, potential difference across fixed resistor and fan increases, making fan go faster
• Connecting component across variable resistor

  1. Potential difference across component is high when variable resistor resistance is high

  2. Potential difference determines energy component gets
• Sensing circuits react to changes in the surroundings
• Sensing circuits can be tricky to make sense of
• Series circuits


  Components connected in a line, end to end
• Series circuits


  If one component is removed or disconnected, the whole circuit is broken
• Series circuits


  • Used to measure quantities and test components
  • Cell potential differences add up
• More cells in series

  Bigger potential difference
• Total potential difference of supply


  Shared between components in series circuit
• Current in series circuits

  Same everywhere
• Resistance in series circuits

  Adds up
• Series circuits not used much in real world