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How is current direction in transformer determined and why?

  1. Jan 15, 2016 #1

    Please see the attached picture first.

    For typical battery (nothing but capacitor), current direction is clear as electrons can't not flow across the two plates in battery so they only choose wire as a path from cathode to anode.

    But in transformer (+ and - sign represents voltage at instant time), I guess electrons can choose coil as a flow path since coil itself is connected conductor. Maybe it is more favorable as it is short path from cathode to anode.

    The Ohm's law on resistor forces me to think that current path should be the same to case of the battery but It doesn't physically convince me.

    How we can determine current direction and why?

    Attached Files:

  2. jcsd
  3. Jan 15, 2016 #2


    Staff: Mentor

    Transformers are AC devices. Current flows alternately in both directions, switching directions 50 or 60 times per second.
  4. Jan 15, 2016 #3
    I know. That's why I mentioned + and - signs are at instant time. At certain time, upper is higher voltage than lower. At this time, I'm asking how current direction is determined and why.
  5. Jan 15, 2016 #4


    User Avatar

    Staff: Mentor

    Current flows through the external circuit ("load") from + to in accord with Ohm's Law.

    There is also a dot convention applying to transformer phasing, you will see it on schematics where phasing is important. When current flows into the dotted end of the primary, it flows out of the dotted end of each of the secondaries. This is akin to indicating which ends of the secondaries are all + together, so if you need to connect some in series so their voltages add you can do it correctly.
  6. Jan 15, 2016 #5


    Staff: Mentor

    If you really want to understand transformers, or any AC circuit, consider both voltage and current and the phase angle between them. You won't get far using DC reasoning. There is no phase angle in DC and it is so important in AC.
  7. Jan 15, 2016 #6
    Thanks to give me feedback. I was thinking this issue seriously and got clear answer. The attached picture made it briefly clear.


    In this case, I was confused by + and - notation in secondary wining. I instantaneously thought electric field direction was + through coil to - like capacitor. It was wrong. The induced field direction is from - through coil to +!

    If voltage is developed from space charge separation like capacitor, my original imagination is right. However here, + and - notation really doesn't relate to true field direction! They're just indicating current direction in conventional way.

    I think it is clear answer:)
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