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Alternating Current

  1. Aug 15, 2009 #1
    Can someone explain to me how power sources (e.g. power plants) transmit current/electricity in a sinusoidal (AC) behavior?

    At different parts of power lines there are transformers that drop voltages so ordinary appliances can operate correctly. Is there some kind of device that converts current into a AC behavior at power plants (and thus AC at our outlets)?

    I am just not sure how the electricity we receive in the outlets model a sinusoidal behavior?


  2. jcsd
  3. Aug 15, 2009 #2


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    It is even simpler than that; in most cases the generated current already IS an alternating current; so there is no need to convert it from DC to AC (which I guess is what you had in mind).
    The reason is that nearly all power plants generate electricity by using turbines which as you know are rotating machines; the current coming out from a normal turbine is therefore as sinusoidal alternating current.
    Generating current is -in principle- very "low-tech"; most power plants (nuclear, coal etc) drive the turbin using steam; i.e. even a nuclear power plants is nothing but a steam engine!
  4. Aug 15, 2009 #3
    In the future DC generation may come back to the fore. Think about a hydrogen economy. Most generated electricity will be fed straight into an electrolyzer. No more big ugly power lines criss-crossing the country. H2 will be the perfect energy conveyer.
  5. Aug 15, 2009 #4
    Got it. So all power plants basically have turbines that generate electricity, and by faraday's law there needs to be a magnetic flux (or change in the magnetic field) for current to be produced.

  6. Oct 14, 2009 #5
    How will we get the H2 across the country? Big ugly pipe lines.
  7. Oct 14, 2009 #6


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    No different than how we transport gasoline, petroleum, or any other hydrocarbon product.
  8. Oct 15, 2009 #7
    IMHO we will still have some power lines because there will be situations that require the distrubution of power as long as man is alive in more remote places
    The big improvement would be if the superconductor transmission system would ever go on line. The tech has existed for 10+ years as a spin off of MRI technology
    then we wouldn't need the ugly overheads, as the transmission lines would be buried.

  9. Oct 16, 2009 #8
    In a hydrogen economy, every town and state can produce and store their own. Decentralisation will make for a much safer and cheaper existence. Where transport is necessary, using human driven tankers will provide jobs, and make sabbotage difficult. Let's face it - the world is an ugly place only because of selfishness and pride.
  10. Oct 16, 2009 #9


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    This is ridiculous. How is sabotage going to be any different of a threat from the current method of transporting millions of barrels of highly flammable and explosive products? Decentralization is not the efficient, cheaper, or safer answer. It would maximize the inefficiencies of any process by reducing the economy of scale. It would place strains upon the [water supplies of the municipalities (inferred by your DC quip). If there is to be a hydrogen economy then it is probably going to evolve an infrastructure very similar to current petroleum infrastructure that we have now. We already know how to store and distribute liquid and gaseous petroleum products. The refining, processing and distribution is centralized when possible to decrease costs and improve efficiency.

    Anyway, this discussion has no bearing on the OP. Hydrogen does not have an implication in the transmission and distribution of electrical power.
  11. Oct 16, 2009 #10
    correctly put
    sorry about the thread hi-jacking

  12. Oct 17, 2009 #11
    On the original topic of this thread f95toli has covered it quite ably. Still have question myself on whether the AC turbine generators produce the current at the extremely high voltages needed to be efficient in transit over the country. My guess is not - it probably is transformed up - and then transformed down at the user end?

    Following the golden rule - I am always honored when other people "highjack" my threads - makes the forums much more interesting - (I have seen forums go dead because a few over zealous ones put the fear of God into what should be a free and easy - fire and forget environment). Like to say a few more things in defence of the hydrogen economy if thats OK?!!

    1) NOTHING needs to be the same as it is today - everything could and would be utterly different if we chose to think outside the oppressed and suppressed way we have all been programmed to think in this mainstream socio-religio-political regime. For example: - Hydrogen tankers would best be air transport rather than road - and the current style of aircraft would be totaly out-dated by new designs. Keeping them smaller rather than larger would provide more jobs and make it much harder for the enemy to block a supply.

    2) Water shortage need not be a problem to anybody on account of the fact that exhaust water can be collected and new water can easily be tankered in to the a region which has a shortfall. Again more jobs.

    3) Decentralization is ALWAYS more efficient if you value the many rather than the corrupt "elite" few. This nausiating "corporation" world we live in is only more "efficient" to a few very rich people - and when they perform poorly - they get a mighty golden handshake while you watch your super-annuation dwindle to a pittance.
  13. Oct 19, 2009 #12
    Alternating Current is generated as Alternating Current.
    It could be generated as Direct Current, but Direct Current does not transmit as easly or for as far.

    Some points needed for understanding this.

    Electrical current is generated by moving a magnetic field through a conductor. The faster the magnetic field is moved the more current, the larger the magnetic field the more current.
    Current is the amount of energy available for use. (Electical Potential)

    The earth has a negative charge, and the longer the wire is, the more the earth induces a charge into the wire.

    Almost every conductor available converts some of the electricity running through it into heat. The more efficient the conductor the less heat generated.

    With Direct Current this means more power is needed to over power the induced charge. This is done by raising the voltage on the wire. The only way to raise the voltage on Direct Current systems is to push the electicity harder, spin the generators faster or use heavier magnets. Raising the voltage also raises the amount of current running through the wire, raising the amount of heat generated in the wire. At some point, the wire cannot handle this and breaks down. This turns out to be roughly a mile or so if I remember correctly.

    Moving electric fields also generate magnetic fields, and the High Voltage electric fields generated by direct current transmission lines would generate quite a stir in todays society.

    With Alternating Current voltage can be changed by coupling the magnetic field generated by the moving electic field with another transmission line. Since the voltage on an AC line is fluxuating, it generates a fluxuating magnetic field. The fluxuating magnetic field will induce a fluxuating electric field in another conductor.

    In other words take a coil of one voltage AC and couple it with a differant coil and a current will be generated in the second coil. The ratio between initial (Primary) coil and the secondary coil determines the voltage induced in the secondary coil. For simplicities sake 10 loops on primary at 100 Volts coupled with 20 loops on the secondary coil induces a current of 200 volts in the second coil. (lower amps though)
    This principle can be used to raise the voltage in the transmission lines to any desired level. Making it easy to overpower the induced charge from earths potential. Allowing for transmission lines of great length.

    Alternating Current just shakes the charge field without really pushing electrons around a loop. The electricity you use at your house is the differance between two wave forms. The actual electrons used stay close to where they are used, but they are pushed and pulled by the electric field on the wire.

    The electric field is what travels at the speed of light, electrons travel much slower.
    Similar to a pipe with water in it. When you start pushing the water at one end it comes out the other, but the water you are pushing is still close to where it started.
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