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Transcript
Inductor Physical Coil A circuit component designed for inductance is an inductor. • Symbol suggests coiled wire Ideal inductors act like wires unless the current changes. • Emf based on L Real inductors have some resistance. • Resistivity of length of wire RL Circuit An inductor resists changes in L I(t) current. • Creates emf V R A switch can be used to VL (t ) L I t I V L IR 0 t complete a circuit with a resistor. • One-loop Kirchhoff’s law • Treat as additional emf source Inductive Time Constant The voltage across an inductor falls with time. • Change in current decreases • Never reaches zero • Overall current increases The period of time for a decrease by a factor of e is the time constant t. I R I t L I I t t t L/ R Exponential Decay The emf of an inductor can be represented by an exponential curve. • Depends on battery voltage VL (t ) V0 e t /t Inductor Current Inductor and resistor voltages sum to battery voltage. • Kirchhoff’s voltage law Circuit current can be determined from Ohm’s law VR (t ) V0 (1 e t /t ) VR V0 t / t I (t ) (1 e ) R R Disconnection Opening a switch with current L I(t) V flowing is a change in current. R The inductor will create an emf to resist the change. VL (t ) L I t I V L IR 0 t For a large current the emf can create a spark. Safe Discharge A pair of opposite switches can provide a safe path for current in an inductor. • A and B always in opposite position SA I(t) V SB When switch A is closed the battery powers the circuit. When switch B is closed the inductor current flows through the switch and resistor. I L IR 0 t L R