There are primary and secondary coils in a transformers. So, what hat are the difference between when the secondary coil is in 1)open circuit, 2)closed circuit with a load, and 3) shorted circuit? When can the ideal transformer equation be used? It seems that not only is it related to whether the transformer is ideal or not, but also related to which case (1, 2, or 3 above) the secondary coil is in. I'm much confused. Please help clarify my concept!! Thanks.