Electrical circuits are systems with a closed circle, so the electricity can return throughout the loop. The system may have many pieces, but if it is not closed, it is not a circuit.Electric systems that have voltage, other pieces such as diodes or resistors can be analyzed by mathematical means to find transient, DC and AC response.A system that has active electronic pieces generally cannot be analyzed by the same mathematical techniques, but will need something more sophisticated in order to be created or understood.To create an electrical system, electrical engineers should calculate the voltages and other elements for various locations around the loop. Some mathematical techniques work well when the system has the same input and output specifications. Other systems will need more sophistication or perhaps can only be estimated. Software is available to do this kind of analysis.