In which one of the following cases Ohm’s law is not valid?

Ohm’s law states that the potential difference across a conductor is directly proportional to the current flowing through it, provided physical conditions like temperature remain constant [c1][c5]. Devices that follow this linear relationship are called ohmic. A junction diode is a semiconductor device that exhibits non-linear I-V characteristics [t1]. In a diode, the current does not vary linearly with voltage; instead, it remains negligible until a threshold voltage is reached and then increases exponentially [t5]. While wire-wound resistors and potentiometers are designed to be ohmic [t4][t8], and electric bulbs (filament lamps) are ohmic at low voltages before temperature changes increase resistance [t2][t3], the junction diode is fundamentally non-ohmic due to its semiconductor PN-junction properties [t1]. Therefore, Ohm's law is not valid for a junction diode as its resistance is not constant across different voltages.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > What you have learnt > p. 192
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > Figure 11.3 > p. 176
3. [3] https://www.sciencedirect.com/topics/engineering/diode-characteristic-curve