Why is there an iron core in a transformer? Is it necessary? In a transformer, how does the changing magnetic field on the primary induce a current on the secondary? Can we think of the changing magnetic field causing a changing force on the secondary and causing it to move back and forth, cutting field lines? Or is it more like constantly switching the polarity of a magnet? Where does Lenz's law come into it? Say we have a coil and an ammeter in a complete circuit (ammeter will read zero because there is no power supply). When we move a magnet through the coil, why is a current registered? There is no current in the circuit so the coil does not have its own field and the magnet is not interacting with another field. Is it because the metal of the coil physically cuts the magnetic field lines? http://img269.imageshack.us/img269/7002/induction.jpg [Broken] That's a circuit with a coil and an ammeter. The magnet is moved to the right and through the coil. Lenz's law tells us that the coil will be north on the left and south on the right and these will switch around when the magnet gets past the middle of the coil. So in the first half of the motion, when the coil is north on the left and south on the right, what is the direction of the current? I don't just want the answer; I want to know how to compute it. I was thinking of Fleming's left hand rule but then what is the direction of the force? One would think that it's the force opposing the magnet, but then wouldn't it be parallel to the magnetic field? Thanks in advance.