Mains Electricity

• Electrical Terms

  • Electricity: The flow of charged particles
  • Negatively charged electrons flow through a metal wire
  • Rate of electricty flow is called current
  • Measured in Amperes / amps (A)
  • Current is driven around the circuit by voltage
  • potential difference, type of electical pressure
  • Measured in volts (V)
  • The flow of electricity is opposed by resistance
  • Slows the flow of electricity around a circuit
  • Measured in ohms (Ω)

  A) Voltage (V)

  B) Electron flow (opposite to current flow)

  C) Current always flows in the same direction

  
• Electrical Safety

  • 50 volts is sufficient to drive a potentially lethal current through the body
  • High current also generate lots of heat (can lead to electrical fires)
  • Most electrical appliances are insulated
  • Electrical wires (usually made out of copper) are excellent conductors
  • Covered in insulated material, such as plastic or rubber

  
• Electrical Safety: Wiring

  • All electrical appliances are earthed
  • Standard UK plug has 3 wires - live, neutral and earth
  • The live wire carries a voltage of about 230V
  • Electricity flows into the appliance through the live wire
  • The neutral wire carries a voltage of 0V
  • Electricity flows out of the appliance through the neutral wire
  • The earth wire does not usually carry a voltage and, together with a fuse, provides an additional safety layer
  • Earthing helps to prevent electric shocks and fires
  • If a fault develops in an electrical appliance, the metal casing can become a live conductor
  • If this happens, a large current (electrical surge) will flow through the earth wire, melting the fuse and cutting off the supply of electricity

  A) Earth Wire

  B) Neutral Wire

  C) Cable Grip

  D) Outer Insulator

  E) Live Wire

  F) Fuse

  
• Electrical Safety: Double Insulation

  • Layer 1: Plastic or rubber casing around the copper wiring
  • Layer 2: Casing of the appliance itself

  A) Symbol for double insulation

  
• Fuses

  • Fuses and circuit breakers are designed to cut off the flow of electricity to the appliance
  • Occurs if a surge of electricity flows through the appliance
  • If a surge of current flows through a fuse, it will melt and the circuit will be broken
  • It then needs to be replaced

  A) Metal end

  B) Glass tube

  C) Metal end

  D) Low melting point wire

  
• Circuit Breakers

  • Can switch of electricity to the whole house
  • Detect surges in current
  • A switch is opened, immediately breaking the circuit
  • Can be reset by flicking the switch back to its original position

  A) Main circuit breaker

  B) Branch circuit breakers

  
• Resistors

  • Electrical component that resists the flow of electric current
  • Used to limit the amount of current that flows through the circuit
  • As the resistance of the circuit increases, the current will decrease
  • Referred to as Ohm's Law
  • Ω's Law - Resistance up, Temp up, Current down
  • High currents produce lots of heat
• Thermistors

  • Sensitive to temp changes
  • Inverse of traditional resistors
  • If temp is high, the resistance in the thermistor will decrease
  • Allows more current to flow through the circuit
  • Practical applications:
  • As fire alarms
  • As thermostats
  • In car engines

  A) Resistance (Ω)

  B) Temp (c°)