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Voltage and current AC help!

  1. Feb 29, 2012 #1
    Here is my problem I am having: I am having trouble seeing how current is step down in a transformer when the voltage increase. This is how I see it. In a battery ( I know d.c just using as example. ) The voltage is basically an electric field created by the internal circuit in the battery, so the work done on the electrons to give them the energy to move from a low to a high potential is done by the battery, so a 12V energy is given to each Colombo of charge so the current would flow at X amount, but if you wanted to increase current flow you would increase the voltage so more energy is given to each Colombo of charge mean they move fast so to speak, I do realize that electrons don't move that quick, it the effect of the electric field throughout the conductor (hope I am right saying that). But when I look at a A.C transformer e.g step up the voltage has increased but the current has decreased, why is this? could someone please explain.

    I do apologize if some of the information I put is incorrect, I am self taught, so I can only quote what I have read so far.
  2. jcsd
  3. Feb 29, 2012 #2


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    Gold Member

    Forget about the stuff you just said....

    Simple as this....power in = power out. (minus some minor losses)


    Change the voltage....the current changes proportionally to make the same power....or watts.

    Also, a trasformer will not work in DC. A transformer in DC is just a dead short. Reactance of a inductor is JWL. W=0 in DC. Therefore zero resistance....dead short.

    To add a little more....there is a turns ratio in the transformer. To go from 120 volts to say 12 volts.....you could have 1000 windings of coil on the primary side and 100 windings of coil on the secondary side. The primary coil creates a rotating magnetic field that induces a current into the secondary side also creating a rotating magnetic field. Some of the other gurus on here will have to give the the details down to the electrons and coloumbs and so forth.
    Last edited: Feb 29, 2012
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