Electricity

• Electrical Charge


  • 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
• Current, potential difference and resistance

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


  • If resistance 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)

  
• How resistance changes


  1. With current (as current increases, electrons have more energy and collide more with atoms, 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)

  4. With light (LDRs - 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
• Parallel Circuits


  Branched circuit, current splits into multiple paths, total resistance is less than smallest individual resistance
• In a series circuit, potential difference is shared across the whole circuit and current is the same through all parts
• In a parallel circuit, potential difference is the same across all branches and current is shared between the branches
• Mains electricity in the UK is AC (alternating current) with a frequency of 50 Hz and voltage of 230 V
• Live, Neutral and Earth wires in a plug

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


  • Energy transferred per second, directly proportional to current and voltage
  • Power loss is proportional to resistance and square of current
• Energy transfers in appliances

  • Electrical energy transferred to other forms (kinetic, thermal, etc.)
  • Work done = energy transferred = charge x potential difference
• National Grid


  • System of cables and transformers linking power stations to consumers
  • Step-up transformers increase voltage, step-down transformers decrease voltage
• Charge


  • Property of all matter, positive and negative charges exist
  • Like charges repel, opposite charges attract
• Insulators and Conductors


  Insulators do not conduct electricity, conductors can conduct electricity
• Static Electricity


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

  2. Sparking occurs when enough charge builds up
• Electrostatic Forces


  Greater charge = greater force, closer together = greater force (inverse square of distance)
• Electric Fields


  Point in direction a positive charge would go, stronger charge = more field lines = stronger force