Device properties: linearity, time variance, and passivity Assess the correctness of these statements: An iron-cored choke is a nonlinear, passive device. A carbon resistor kept in sunlight is time-invariant and passive. A dry cell is a time-varying, active device. An air capacitor is time-invariant and passive.

Introduction / Context:
Classifying components by linearity, time variance, and passivity is basic to circuit modeling. Correct classification influences small-signal analysis, biasing, and stability in power and signal circuits.

Given Data / Assumptions:

• Definitions: “passive” delivers no net energy; “active” can supply energy (e.g., sources).
• “Time-invariant” means parameters do not explicitly depend on time (for a given operating condition).
• Nonlinearity arises if device parameters depend on signal level (e.g., core magnetization).

Concept / Approach:

(1) Iron-cored choke: B–H is nonlinear; device stores but does not generate energy → nonlinear, passive. (2) Carbon resistor: ohmic, parameter R largely independent of signal; sunlight does not create an intrinsic time dependence in the i–v law (though ambient changes alter R slowly), hence time-invariant and passive in standard classification. (3) Dry cell: a source of electromotive force that changes with state of discharge → time-varying and active. (4) Air capacitor: linear C (within limits), parameters independent of time for fixed geometry → time-invariant and passive.

Step-by-Step Solution:

Verification / Alternative check:

Textbook classifications align: resistors, capacitors, inductors are passive; magnetic cores introduce nonlinearity; batteries are active sources whose parameters drift with usage.

Why Other Options Are Wrong:

Options B, C, D omit at least one correct statement; only Option A includes all four truths.

Common Pitfalls:

Interpreting environmental changes as intrinsic “time variance” of the model; confusing nonlinearity (iron core) with activity.

Final Answer:

1, 2, 3 and 4 are correct