The fundamental passive linear circuit elements are the resistor (R), capacitor (C) and inductor (L). These circuit elements can be combined to form an electrical circuit in four distinct ways: the RC circuit, the RL circuit, the LC circuit and the RLC circuit with the abbreviations indicating which components are used. These circuits exhibit important types of behaviour that are fundamental to analogue electronics. In particular, they are able to act as passive filters. This article considers the RL circuit in both series and parallel as shown in the diagrams.In practice, however, capacitors (and RC circuits) are usually preferred to inductors since they can be more easily manufactured and are generally physically smaller, particularly for higher values of components.
In an RL circuit, with a battery of emf V, if the switch is closed
at time t = 0, then the current in the circuit will be given by I ( t ) = ( V / R )*[ 1 exp ( -t / T )], where
T is the time constant of the RL circuit and is given by L / R. Note that the current rises gradually from 0 to a
maximum value of V / R.
After the current has reached its maximum value, if we then
disconnect the battery at time t = 0, so that we end up with an RL circuit without a battery, then the current
decreases in time according to I ( t ) = ( V / R )*exp( - t / T ).
Just like a capacitor, when charged, stores electric energy in the
space between the plates, an inductor, when it carries a current, stores magnetic energy. The magnetic energy
stored in the inductor is ( 1 / 2 ) L I^2.
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