Ohm's law is not applicable in all the following cases except

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Insulators

Explanation: Ohm's Law ($V = IR$) is valid for linear, bilateral, and ohmic conductors where resistance remains constant with changes in voltage or current. Insulators follow this linear relationship for extremely small currents before breakdown occurs, whereas non-ohmic devices exhibit non-linear V-I characteristics.

Working Principle: Ohm's Law holds when temperature and physical dimensions remain constant. Electrolysis, arc lamps, and vacuum valves have non-linear characteristics because their internal resistance changes with the magnitude or direction of current flow (e.g., ionization in arc lamps or space charge effects in vacuum tubes). Insulators behave as high-value linear resistors until the dielectric strength limit is exceeded.

Key Formulas: $V = IR$ — Ohm's law definition for linear elements; $R = \rho \frac{L}{A}$ — Resistance formula relating to physical material properties

Key Points: Ohm's law applies strictly to linear conductors at constant temperature.; Non-ohmic devices (diodes, vacuum tubes) have a changing dynamic resistance.; Arc lamps involve gas discharge, leading to negative differential resistance.; Electrolysis involves electrochemical reactions which create back-EMF, violating simple linear proportionality.

Advantages: Simplifies circuit analysis for linear networks; Predictive power for voltage/current relationships in ohmic materials

Disadvantages: Inapplicable to semiconductors and non-linear components; Fails to account for temperature coefficient variations

Applications: Resistor design; Circuit simulation models; Electrical power distribution (linear loads)

Additional Info: The question asks for the case where Ohm's law IS applicable.. Arc lamps (B) are non-ohmic due to ionized gas discharge characteristics.. Vacuum tubes (D) are non-ohmic due to space-charge-limited current flow.. Electrolytes (A) change conductance based on ionic mobility and concentration.

Answer: C is correct — Insulators behave as linear resistors and follow Ohm's law at low voltages, unlike arc lamps, vacuum tubes, and electrolytes which are inherently non-linear.

Exam Tip: Always identify the component type first; if it's a gas-discharge tube or semiconductor, Ohm's law is almost certainly inapplicable.

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