en two currents. Next, we will examine the phenomenon of induced currents, where a changing magnetic field can produce a current. Changing Field --* CurrentFaraday's ExperimentsIn 1831, Michael Faraday and Joseph Henry conducted similar experiments that demonstrated the following phenomena:In the above illustration, movement of a magnet through a wire loop induces a current in that wire. Reversing the direction in which the magnet travels reverses the current direction.In this experimental setup, opening or closing switch S induces a momentary current, i. The direction of i when closing the switch is opposite the direction when opening the switch.Faraday's Law of Induction We can conclude from the previous experiments that a change in the magnetic field through a current loop produces an current in that wire. More scientifically, we say that a change in magnetic flux (field through a given area) induces a current in the loop to oppose the change in flux. Quantitatively, we find that the negative rate of change in flux is equal to the electromotive force (EMF) in the wire:For a coil of N turns, the induced EMF is the sum of the voltages from each turn:The direction of these induced currents, according to a principle known as Lenz' Law, always opposes the change in magnetic flux that produced it.Eddy Currents When a large peice of conducting material moves through a magnetic field in such a way that the magnetic flux through the material changes, currents are induced in the material:These currents, known as eddy currents , may produce desired or undesired effects, depending on the situation. Of particular interest to magnetic levitation is the magnetic force produced, which opposes motion through the magnetic field:This induced magnetic force is somewhat analgous to frictional forces: it opposes motion in or out of the magnetic field. In our example of magnetic levitation, this effect becomes significant, as we will see later. Summary We ...
