Electrical Energy

• Hydroelectric power station


  1. Water stored behind a dam has gravitational potential energy

  2. Dam is opened, flowing water provides kinetic energy to rotate turbine

  3. Turbines coupled with magnet, relative rotation induces current in coil, kinetic energy transformed to electrical energy

  
• Role of power station


  Generate electrical energy to all places over long distance
• Efficiency of energy transmission through cables

  Increases at very high voltage
• Increasing voltage from 20 to 400 kV


  Step up transformer used
• Distributing to local users

  Series of transformers (substations) step voltage down gradually to 220V
• Thermal power station


  • Produces heat energy by burning fossil fuels or coal, or from fission of uranium nuclei
  • Water heated, turns to steam, drives steam turbine which drives electrical generator
  • Steam-driven turbine converts heat to mechanical power as intermediate to electrical power
• Thermal power station


  • Not too expensive, but causes air pollution and uses non-renewable fossil fuels
• Transformer


  • Electric device that transfers electric energy from one side of circuit to other by electromagnetic induction
  • Enables transforming voltage or current to higher or smaller value

  
• Step-up transformer


  Changes voltage to higher value, current to lower value
• Step-down transformer


  Changes voltage to lower value, current to higher value
• Transformer operation


  1. Primary coil receives electric energy from mains

  2. Variable current in primary coil creates variable magnetic field that induces variable voltage in secondary coil

  
• Transformation ratio "k"


  k = U2/U1 = n2/n1, where n1 and n2 are number of turns in primary and secondary coils, U1 and U2 are effective voltages across primary and secondary coils
• Ideal transformer


  • Output power to secondary coil exactly equal to input power of primary coil
  • Efficiency 100%, impossible in practice but useful model
• Ideal transformer ratio


  k = U2/U1 = n2/n1 = I1/I2
• Step-up transformer


  k > 1, U2 > U1
• Step-down transformer


  k < 1, U2 < U1
• Transformer reversibility


  Primary and secondary coils can be reversed, transformation ratio becomes k' = 1/k
• Transformer efficiency "r"


  r = P2/P1 x 100%, ideal transformer r = 100%
• Frequency of AC generator is the same across primary and secondary coils
• Power lost in wires due to Joule's effect is P = R I^2, decreasing current I decreases power loss, step-up transformer increases voltage and decreases current