reactance

Reactance is a term used in physics and psychology to describe distinct but related ideas. In electrical engineering, it denotes the opposition to changes in current in an alternating-current circuit caused by inductors and capacitors. In psychology, psychological reactance refers to a motive to regain threatened or removed freedoms when individuals perceive coercion or restrictions.

In AC circuits, inductors and capacitors present reactance that depends on frequency. Inductive reactance X_L is ωL, and capacitive reactance X_C is 1/(ωC), where ω = 2πf. The net reactance is X = X_L − X_C. The complex impedance is Z = R + jX, with R as resistance and j the imaginary unit. When X is zero, the circuit is at resonance; when X is positive, the circuit is inductive; when negative, capacitive.

Reactance determines how current and voltage relate in phase and affects power flow. For a purely reactive element, real power is zero, while reactive power Q equals I^2X (or V^2/X, with sign indicating inductive or capacitive behavior). Reactance also shapes filters and impedance matching in communication and power systems.

Psychological reactance theory, introduced by Jack Brehm, explains why people resist attempts to limit their choices. Perceived threats to autonomy can trigger counteractions, increasing attraction to the restricted option and sometimes reducing the effectiveness of persuasive messages. The degree of reactance depends on the importance of the freedom and the perceived threat.