In electrical circuit considerations, fixed and time-varying elements are types of components and their behavior with time:
- Fixed Elements: These components have constants values which do not change with time. A couple of examples of these are resistors with a fixed resistance, capacitors with a fixed capacitance, and inductors with a fixed inductance. Their behavior remains unaltered unless the environment, such as temperature, alters its properties significantly.
- Time Varying Elements: These elements or constraints change with time. This could be due to the changing environmental circumstances or an external cue. In an AC circuit, e.g., the voltage across a capacitor, or the current flowing past an inductor, may vary with time. The resistance of certain materials is also a time-varying parameter that dependents on temperature.
In circuit theory, these ideas can be used to analyze transient responses in RLC circuits whereby voltages and currents change across time following a switch activation or deactivation.