# What voltage appears across the primary coil of this transormer

1. Homework Statement
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. Homework 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?

## Answers and Replies

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Dick
Homework Helper
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:
ok i get it i made a stupid mistake
so its really 120 X 20 right?
so the answer is 2400 ?

Dick