Electricity P2

• Circuit diagram


  Diagram that shows the components and connections in an electrical circuit
• Charge


  The flow of electrical charge, measured in coulombs (C)
• Current


  The rate of flow of electrical charge, measured in amperes (A)
• Resistance


  The measure of how a component resists the flow of electrical charge, measured in ohms (Ω)
• Potential difference


  The electrical push or voltage difference between two points in a circuit, measured in volts (V)
• Investigating factors affecting resistance

  1. Set up standard circuit

  2. Apply voltage

  3. Measure voltage and current

  4. Repeat for range of lengths

  5. Calculate resistance
• Standard circuit symbols


  • Switch
  • Bulb
  • Fuse
  • Battery
  • Diode
  • Resistor
  • Variable resistor
  • Voltmeter
  • Ammeter
  • Thermistor
  • LDR
• As resistance increases


  Current decreases
• As potential difference increases

  Current increases
• Ohmic conductor


  A resistor where current is directly proportional to potential difference at constant temperature
• Ohmic conductors


  • Resistance remains constant as current changes
  • Indicated by linear (straight line) graph
• Filament lamps


  • Resistance increases as current increases
  • Indicated by curved graph
• Diodes


  • Current only flows in one direction
  • Very high resistance in reverse direction
  • Indicated by horizontal line along x-axis
• Thermistors


  • Resistance decreases as temperature increases
  • Useful for temperature control or response
• LDRs


  • Resistance decreases as light intensity increases
  • Useful for automatic light control or detection
• In a series circuit, the total potential difference of the power supply is shared between the components
• In a parallel circuit, the potential difference across each component is the same
• Adding resistors in series increases the total resistance
• Adding resistors in parallel reduces the total resistance
• Power


  The rate of energy transfer, measured in watts (W)
• Power


  Potential difference x Current
• Power


  Current^2 x Resistance
• Direct current (DC)


  Current that always flows in the same direction
• Alternating current (AC)


  Current that alternates direction
• Mains electricity in the UK is 230V AC at 50Hz
• Three-core cable


  • Live wire (brown, 230V)
  • Neutral wire (blue, 0V)
  • Earth wire (green and yellow, 0V)
• The live wire carries the alternating potential from the supply, the neutral wire completes the circuit, and the earth wire is a safety wire
• Alternating current (AC)


  Has a potential difference that alternates from positive to negative, the current direction alternates
• Direct current (DC)


  The type of current used in mains electricity
• Mains electricity in the UK is 230V and changes direction 50 times a second, i.e. it has a frequency of 50Hz
• Wires in a mains electricity cable

  • Live wire (brown) - 230V potential
  • Neutral wire (blue) - at or close to the 0V earth potential
  • Earth wire (green and yellow stripes) - 0V potential
• Operation of mains electricity

  1. Potential difference causes current to flow through the live and neutral wires

  2. Live wire carries the alternating potential from the supply

  3. Neutral wire completes the circuit

  4. Current will only flow in the earth wire if there is a fault connecting it to a non-zero potential
• Earth wire

  A safety wire, which stops the exterior of an appliance becoming live
• Mains electricity can be very dangerous - an electric shock from a mains supply can easily be fatal
• Touching the live wire can create a large potential difference across the body and result in a large current flowing through the body
• The live wire can be dangerous even if a switch in the circuit is open
• Power


  The rate at which energy is transferred or work is done
• An energy transfer of 1J per second is equal to 1W of power
• Efficiency


  The ratio of useful energy out to total energy in
• To increase the efficiency of an energy transfer, the amount of wasted energy needs to be reduced