Hi, Since the primary side of a transformer is a closed circuit (like the cubical adapter for the iphone charger), it would make sense that even if my iphone wasn't plugged into the secondary, the primary would still draw power. I know this is the case for at least some adapters, because i see arcing sometimes when i plug my laptop adapter into the wall without my laptop being plugged into the secondary. But some adapters only start getting hot when i plug in load at the secondary. Why? also When the secondary load is connected to make the secondary a closed circuit, does its induced emf change the impedance for the primary side?