INDUCTANCEDC CircuitsThe voltage drop across an inductor is directly proportional to the product of the inductance andthe time rate of change of current through the inductor, as shown in Equation (3-6).V_{L}= (3-6)L^{DI}DtwhereV_{L}= voltage drop across the inductor (volts)L = inductance (henries)= time rate of change of current (amp/sec)DIDtAfter five time constants, circuit parameters normally reach their final value. Circuits thatcontain both inductors and resistors are called RL circuits. The following example will illustratehow an RL circuit reacts to changes in the circuit (Figure 8).1. Initially, the switch is inFigure 8 Voltage Applied to an InductorPosition 1, and no current flowsthrough the inductor.2. When we move the switch toPosition 2, the battery attempts toforce a current of 10v/100W =0.1A through the inductor. But ascurrent begins to flow, theinductor generates a magneticfield. As the field increases, acounter EMF is induced thatopposes the battery voltage. As asteady state is reached, the counterEMF goes to zero exponentially.3. When the switch is returned toPosition 1, the magnetic fieldcollapses, inducing an EMF thattends to maintain current flow inthe same direction through theinductor. Its polarity will beopposite to that induced when theswitch was placed in Position 2.ES-03 Page 6 Rev. 0

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