Electricity

• Electrical Charge to flow

  • Circuit must be closed (no open switches)
  • There must be a source of potential difference (battery/cell)
• Electrical Current

  • Flow of electrical charge
  • Greater the rate of flow of charge, greater current
  • Q = It (Charge = Current x Time)
• In a single closed loop, the current has the same value at any point
• What equation links Current, potential difference and resistance?

  V = IR (Potential Difference = Current x Resistance)
• Resistors

  • If resistors temperature is constant, current is directly proportional to potential difference (linear graph)
  • If resistance changes with current, graph is non-linear (e.g. lamps, diodes, thermistors, LDRs)
  • For example, Filament lamps, filament temperature increases so current decreases making the graph non linear.
• How resistance changes

  1. With current (as current increases, electrons have more energy and collide more with metal ions, increasing resistance)

  2. With temperature (normal wires - atoms vibrate more when hot, thermistors - resistance decreases in hotter temperatures)

  3. With length (greater length means more resistance because they are colliding with more metal ions)

  4. With light (LDR - greater light intensity, lower resistance)

  5. With voltage (diodes - allow current in one direction only)
• Series Circuits

  Closed circuit, current follows single path, total resistance is sum of individual resistances, current is same everywhere
• Parallel Circuits

  Branched circuit, current splits into multiple paths, total resistance is less than smallest individual resistance
• Mains electricity in the UK is AC (alternating current) at 50 Hz and 230 V
• Wires in a plug

  • Live wire (brown, 230 V)
  • Neutral wire (blue, 0 V)
  • Earth wire (green/yellow, 0 V, safety wire)
• Power


  • Energy transferred per second, proportional to current and voltage
  • Power loss proportional to resistance and square of current
• Charges


  • Property of all matter, positive and negative charges exist
  • Like charges repel, opposite charges attract
• Static Electricity


  1. Rubbing insulators transfers electrons, forming positive and negative charges

  2. Sparking occurs when enough charge builds up and jumps between objects