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Homework Help: What voltage appears across the primary coil of this transormer

  1. Apr 4, 2008 #1
    1. The problem statement, all variables and given/known data
    A step down transformer produces a voltage of 6.0 V across the secondary coil when the voltage across the primary coil is 120 V. What voltage appears across the primary coil of this transormer if 120 V is applied to the secondary coil?


    2. Relevant equations

    Vp/Vs=Np/Ns=Is/Ip ................. ???? (not sure if that is relevant really)

    3. The attempt at a solution

    i know Vs/Vp = 120/6

    i think i dont really know what to do with the 120 v applied to the secondary coil
    i added 120 to the 6 = 126 that means 6 was multiplied by a factor of 21
    so i multiplied the top value of the ratio (120) by 21 and got 2520 v
    but im really not sure if thats right at all? what does "120 V is applied" in mathematical terms?
     
  2. jcsd
  3. Apr 4, 2008 #2

    Dick

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    You've got everything so right, I'm not sure how you can go wrong. Vs/Vp=120/6=20. So if Vp=6V, Vs=20*Vp=120V. Now suppose Vs=120V? Where is the 21 factor coming from? You don't add the secondary to the primary, it's just a ratio.
     
    Last edited: Apr 4, 2008
  4. Apr 4, 2008 #3
    ok i get it i made a stupid mistake
    so its really 120 X 20 right?
    so the answer is 2400 ?
     
  5. Apr 4, 2008 #4

    Dick

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    Yes.
     
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