INDUCTANCEDC CircuitsThis is an example of Faraday’s Law, which states that a voltage is induced in a conductor whenthat conductor is moved through a magnetic field, or when the magnetic field moves past theconductor. When the EMF is induced in Wire B, a current will flow whose magnetic fieldopposes the change in the magnetic field that produced it.For this reason, an induced EMF is sometimes called counter EMF or CEMF. This is anexample of Lenz’s Law, which states that the induced EMF opposes the EMF that caused it.The three requirements forFigure 2 Induced EMF in Coilsinducing an EMF are:1. a conductor,2. a magnetic field,and3. relative motionbetween the two.The faster the conductor moves, orthe faster the magnetic fieldcollapses or expands, the greaterthe induced EMF. The inductioncan also be increased by coilingthe wire in either Circuit A or Circuit B, or both, as shown in Figure 2.Self-induced EMF is anotherFigure 3 Self-Induced EMFphenomenon of induction. Thecircuit shown in Figure 3 containsa coil of wire called an inductor(L). As current flows through thecircuit, a large magnetic field isset up around the coil. Since thecurrent is not changing, there is noEMF produced. If we open theswitch, the field around theinductor collapses. This collapsingmagnetic field produces a voltagein the coil. This is calledself-induced EMF.The polarity of self-induced EMFis given to us by Lenz’s Law.The polarity is in the direction that opposes the change in the magnetic field that induced theEMF. The result is that the current caused by the induced EMF tends to maintain the samecurrent that existed in the circuit before the switch was opened. It is commonly said that aninductor tends to oppose a change in current flow.ES-03 Page 2 Rev. 0