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Linear systems combine homogeneity and additivity. Homogeneity means multiplying an input by a constant multiplies the output by the same constant. Additivity means the response to multiple inputs equals the sum of individual responses. Together, these properties enable linear systems like resistor, capacitor, and inductor circuits to obey the superposition principle.

A causal system's current response depends only on present and past inputs, not future values. A car's movement exemplifies causality—it cannot predict future driving actions. Noncausal systems like ideal filters depend on future inputs and cannot be physically realized because they violate the principle of causality.

Dynamic systems exhibit memory, where output depends on past, present, and future inputs. Electrical circuits with capacitors or inductors demonstrate this memory effect. Static systems are memoryless, producing output based solely on present input. A simple resistor circuit exemplifies a static system where output voltage directly reflects current input voltage.

Rectifiers and diodes fail to obey the linearity equation combining homogeneity and additivity. These devices do not maintain proportional input-output relationships or satisfy the superposition principle. Their nonlinear behavior makes them unsuitable for applications requiring predictable linear system responses.

Additivity means that when multiple inputs are applied to a linear system, the total output equals the sum of individual outputs for each input separately. This property, combined with homogeneity, allows linear systems to satisfy the superposition principle, enabling engineers to analyze complex inputs by decomposing them into simpler components.

Homogeneity dictates that scaling an input by a constant scales the output by the same constant. If input x(t) is multiplied by constant c, output y(t) is also multiplied by c. This scaling property is fundamental to linear systems and distinguishes them from nonlinear systems where this proportional relationship does not hold.

Memory determines whether a system's output depends on historical inputs. Dynamic systems possess memory, with outputs influenced by past inputs, such as circuits with capacitors or inductors. Static systems lack memory, producing instantaneous outputs based only on present input values, exemplified by simple resistor circuits.