DC CircuitsINDUCTANCEThe example that follows shows how a circuit with an inductor in parallel with a resistor reactsto changes in the circuit. Inductors have some small resistance, and this is shown schematicallyas a 1W resistor (Figure 9).1. While the switch is closed, aFigure 9 Inductor and Resistor in Parallelcurrent of 20 v/1W = 20 ampsflows through the inductor. Thiscauses a very large magnetic fieldaround the inductor.2. When we open the switch, there isno longer a current through theinductor. As the magnetic fieldbegins to collapse, a voltage isinduced in the inductor. Thechange in applied voltage isinstantaneous; the counter EMF isof exactly the right magnitude toprevent the current from changinginitially. In order to maintain thecurrent at 20 amps flowingthrough the inductor, theself-induced voltage in theinductor must be enough to push20 amps through the 101W ofresistance. The CEMF =(101)(20) = 2020 volts.3. With the switch open, the circuitlooks like a series RL circuitwithout a battery. The CEMFinduced falls off, as does thecurrent, with a time constant TL of:TLLR.TL4H101W0.039 secRev. 0 Page 7 ES-03
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