the National grid

• Alternating current


  Electricity can flow as alternating current
• National Grid


  The network that connects all of the power stations in the country to make sure that everywhere has access to electricity
• National Grid


  • Distributes electricity across the country
  • Connects power stations to homes, workplaces and public buildings
• Generator


  Device that converts kinetic energy into electrical energy
• Transformer


  Electrical device that increases, or decreases, the potential difference (voltage) of an alternating current
• Transformer function


  1. Step-up transformer increases voltage and reduces current

  2. Step-down transformer decreases voltage and increases current
• Increasing voltage decreases current, decreasing voltage increases current
• In the National Grid, a step-up transformer is used to increase the voltage from 25,000 V to 400,000 V
• Before reaching the end user, a step-down transformer reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use
• Pylons are used to support transmission lines above the ground to keep people safe from the high voltage wires
• As electric current flows through the thick cables

  The cables get hotter and dissipate energy to the surroundings
• Power


  The energy transferred each second, measured in watts (W)
• Power dissipated depends on current and resistance: power = current^2 x resistance
• To ensure minimum power loss, the transmission cables are thick to have low resistance, and high voltages are used to reduce the current
• Transformer


  An electrical device that increases, or decreases, the potential difference (voltage) of an alternating current
• Potential difference


  The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them.
• Voltage


  The potential difference across a cell, electrical supply or electrical component. It is measured in volts (V).
• Transformer


  • Made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output
  • Coils are not electrically connected, instead they are wound around an iron core
  • Iron core is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil
• How a transformer works

  1. Primary voltage drives an alternating current through the primary coil

  2. Primary coil current produces a magnetic field, which changes as the current changes

  3. Iron core increases the strength of the magnetic field

  4. Changing magnetic field induces a changing potential difference in the secondary coil

  5. Induced potential difference produces an alternating current in the external circuit
• Transformers can only work with alternating current
• Step-up transformer


  Increases the voltage
• Step-down transformer


  Reduces the voltage