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Properties of Alternating Current

  1. Oct 31, 2012 #1
    How alternating current was transmitted over a long distance? If it is direct current then I can imagine the constant current will flow over that long distance but how ac?
    Compare with ac, why dc decreases as it is transferred over long distances?
    Thank you.
  2. jcsd
  3. Nov 1, 2012 #2

    Simon Bridge

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    Imagine a toy train set with a loop track and carriages all along the track.... DC current has the train going round and round in one direction only... AC current has the train going back and forth.
  4. Nov 1, 2012 #3
    Current is created when you apply potential difference across a conductor. Basically you are applying pressure on electrons to move. One electron pushes another and so on.

    In DC, the pressure is constant in one direction. Electrons are forced to push the other one and keep moving in one direction.

    In AC, electrons are pushed forward and then backward, forward and backward and so on.
  5. Nov 1, 2012 #4
    I wondered.... For ac to travel over long distance to deliver electricity from a place to the other place, how is the ac transfer? If it is dc, I can imagine the current flow from one place to the others but how ac? The current will move forward and backward?
  6. Nov 1, 2012 #5
    electrons would move forward and backward. for 60Hz AC, electrons move back and forth 60 times in a second.

    in the first half cycle, when an electron is pushed forward, it pushes electrons next to it, that pushes the one next to it and so on all along the conductor. then in the 2nd half cycle, they are pushed back from the other end, because the polarity is reversed.
  7. Nov 1, 2012 #6

    Simon Bridge

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    Imagine the train set again? How does the motion of the train get from the engine or your hand pushing it to the other side of the track?

    Electrons basically travel at order cm/s down a wire - yet the light goes on right away when you flick the switch. This is because the electrons push on each other - they are linked like the carriages in the train. The electrons making the light glow were already in the filament when you flicked the switch.

    A more common analogy is water in a pipe.... water fills the pipe, so moving the water at one end of the pipe moves the other water at the other end of the pipe.
  8. Nov 1, 2012 #7
    In my country, if the power station( the place produce electricity) is producing direct current and then the direct constant , one direction current can flow all the cable besides the road and telegraph pole to supply electric to a region. Am I correct?

    But now due to the lost of power during transmission of dc over long distance, alternating current is used.
    So in power station, alternating current is produced then is also need to be transferred through cable and telegraph pole so as to deliver electric to a region , am I right?

    Then my question is Since alternating current is not a steady constant current, how does it transfer? I mean now the telegraph pole has 2 sides , and assume it is 1Hz for ac, then every one second there will be current flow out from both side of telegraph pole? Am I correct?
    Then along the wire of the telegraph pole ,the current flowing will be half a wave ?

    I just want to know why power lost over long distance for dc but not ac?
  9. Nov 1, 2012 #8

    Simon Bridge

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    It is the energy that is transferred - the power station pulls and pushes on the electrons in the wire. That makes all the electrons in the wire go back and forth. If you hook some appliance up to the wire, then it is harder for the electrons to get pushed and pulled (because they have to go through the appliance as well) and so the power company has to supply more energy into the push and pull. That's what the meter is reading - how much extra work the power company has to do to push and pull electrons through your house/flat/apartment/whatever.

    Again think of the train - the engine is in one place pushing and pulling the carriages - but the energy goes through the links to all the carriages even when all the engine does is go backwards and forwards.

    Another way: think again of water in a pipe flowing in a circle, with an appliance being, in this case, a water-wheel, somewhere in the path.
    You can see that turning on the pump will make wheels turn anywhere in the loop: if the pump alternates forward and backwards them the wheels will turn forwards and backwards. The more wheels, the harder the pump making the flow has to work.

    The power transferred in this way is a full wave.
    In some devices, like a light bulb, the direction of the current does not matter. The filament glows whether the electrons go forward or backward through it.
    For some it does matter - so the current has to be adjusted so the backwards part goes by a different path to the forwards part and the whole thing smoothed out. The process is called "rectification" which can be "half-wave" (where the backwards part is just dumped) and "full wave" (where you use both parts of the cycle).

    The details of power transmission is a big topic. Read also:
    Wikipedia on Power Transmission
    AC Circuits/Power: basics
    AC Circuits - more advanced


    ... the associated vids explain other aspects of AC that you have asked about.
    Last edited by a moderator: Sep 25, 2014
  10. Nov 1, 2012 #9


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    Hi Outrageous. Unfortunately, power is lost over long distances for both DC and AC. :frown: The cables always have resistance, and power is lost as heat.

    Electricity authorities prefer to use high voltages when transferring lots of power, because for a fixed power, a higher voltage means the current is smaller, and in transmission cables the power loss (= I²R ) is smaller if I is smaller.

    A big advantage of AC is that at the destination a transformer can be used to restore the voltage to what it should be, exactly compensating for any voltage drop in the wires. (This is not free energy, of course!) Whereas, to restore DC to its original voltage level is a process nowhere near as straight-forward.
  11. Nov 2, 2012 #10
    Why higher voltage will be lower current? In our transmission substation, if we increase the voltage, there are more joule per coulomb supplied to the wire along the substation, then V=IR, since the resistance is constant, then the current should be increased.
  12. Nov 2, 2012 #11
  13. Nov 2, 2012 #12


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    A power station generates a fixed power. To transmit this fixed amount of power, if you double the voltage the available current halves.

    With the line current halved, the I2R losses will be quartered.
  14. Nov 2, 2012 #13
    By producing fixed power, the power station must have a voltage source and a current source which the two will not affect each other by V=IR ?
  15. Nov 2, 2012 #14

    Simon Bridge

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    Yes they will ... P=IV and V=IR ... the P equation limits what I and V is available.

    For fixed resistance R, and power use P, then, from Ohms' Law,
    the current through R is: $$I=\sqrt{\frac{P}{R}}$$... and the voltage across R is $$V=\sqrt{PR}$$... if the company wants to put a higher voltage on the line, then P=IV means the max available current must be lower.

    Did you read through the links I supplied yet?
  16. Nov 2, 2012 #15


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    Outrageous, Will you please take the time to read and study this historical Wiki article? It should answer all your questions.


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