In an ideal 1:8 step-down transformer, the primary power is 10 kW and the secondary current is 25 A.The primary voltage is:
A)25,600 V
B)3200 V
C)400 V
D)50 V
E)6.25 V

Question

Quizplus · Accepted Answer

We know that power in the primary and secondary is equal. So, 
Primary power = Secondary power
10 kW = (25 A)^2 x R
R = (10,000 W) / (625 A^2) = 16 ohms

Also, we know that:
Vp / Vs = Np / Ns
Np / Ns = 1 / 8

So,
Vs / Vp = 8
Vs = 8 Vp

Now, using Ohm's Law in the secondary circuit:
Vs = Is x Rs
8 Vp = 25 A x 16 ohms
Vp = 3200 V

In an Ideal 1:8 Step-Down Transformer, the Primary Power Is