Mains Electricity

Alternating Current (AC):

  • The voltage of alternating current fluctuates continually:
    • In Hong Kong the frequency of this fluctuation is 50 hertz which means 50 cycles per second.
    • Thus meaning that whenever the sine curve crosses the x axis the current is switched off and the wires are allowed to cool to prevent overheating.
      • This allows one to transport much more electricity than one would be able to with a wire carrying direct current.
      • Remember that temperature and resistance are proportional.

 

AC RMS

Graph from http://physics.stackexchange.com/questions/41779/why-do-we-use-root-mean-square-rms-values-when-talking-about-ac-voltage (Crazy Buddy)

  • It is possible to work out how much heat a transmission of AC current produces in a wire relative to the heat produced by a transmission of DC current by working out the Root Mean Square (RMS):
    • VRMS

      • Peak voltage (VP) is the maximum voltage that the sine curve reaches.
      • Take the graph above (y-axis is voltage and x-axis is time), the RMS (indicated by the dotted line) is 0.707.
        • This means that a 1V alternating current only produces the same amount of heat energy during transmission as a 0.707V direct current.
        • This might not seem like much, but at high voltages, this could mean millions of dollars worth of electricity!
  • This can be generated by generators.
  • However, since many electronic components are polarised, this current is useless, even destructive in all but the simplest components:
    • Therefore it must be converted into direct current for the electricity to be usable in circuits.
    • Lamps, though can used AC.
    • And transformers can only function on AC.

Direct Current:

  • The voltage of steady direct current does not fluctuate and so is always constant as you can see in the graph below:
    • The current direction does not change.
      • But because of this it is difficult to transmit as it generates more heat energy than AC.
  • The voltage of smooth direct current does fluctuate somewhat but the current direction remains constant:
    • It has a small variation or ripple.
  • It can be generated by:
    • PV panels:
      • Photovoltaic panels (i.e. solar panels).
    • Batteries.
    • DC generators (dynamos).

 

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