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 only follows a single path, current is the same everywhere, total resistance = sum of individual resistances
• Parallel Circuits


  Branched circuit, current splits into multiple paths, potential difference is the same across each branch, total resistance is less than the smallest resistance
• Differences between Series and Parallel Circuits

  • Series: Components connected end to end, all current flows through all components, can only switch all off at once, potential difference shared across whole circuit, current same through all parts
  • Parallel: Components connected separately to power supply, current flows through each one separately, can switch each component off individually, potential difference same across all branches, current shared between branches
• Mains Electricity


  AC supply, 50 Hz frequency, 230 V in UK
• AC vs DC


  AC - alternating current, current continuously varies from positive to negative

  DC - direct current, charge moves in one direction only
• Plug Wires


  • 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

  E = Pt (Energy = Power x Time)

  E = QV (Energy = Charge x Potential Difference)
• Energy Transfers in Appliances

  • Kinetic energy, thermal energy

  Work done = energy transferred (ideally)

  Power rating shows power used in Watts
• National Grid


  System of cables and transformers linking power stations to consumers

  Step-up transformers increase potential difference from power station to grid (reducing current and energy loss)

  Step-down transformers decrease potential difference from grid to consumers (for safety)
• Charge


  • Positive and negative charges exist, like charges repel, opposite charges attract
• Insulators vs Conductors


  Insulators - electrons fixed, cannot flow

  Conductors - electrons can flow, are delocalised
• Static Electricity


  Rubbing insulators transfers electrons, forming positive and negative charges

  Sparking occurs when enough charge builds up and jumps between objects
• Electrostatic Forces


  Greater charge = greater force

  Closer together = greater force (inverse square of distance)

  Noncontact force
• Electric Fields


  Point away from positive charges, towards negative charges

  Stronger charge = more field lines = stronger force