Characteristic Curves Of Components

NCERT Class 12, Chapter 3 points out that Ohm's law holds only when physical conditions, especially temperature, remain constant, and it identifies devices whose characteristics are non-linear. A filament lamp is a classic non-ohmic conductor: as the current increases, the filament heats up, and because the resistivity of a metal rises with temperature, its resistance increases. Consequently the current grows less than proportionally with voltage, so the current-voltage graph bends toward the voltage axis rather than staying straight. The option claiming perfect Ohm's law obedience is wrong because the observed curvature directly contradicts a constant-resistance straight line. The option claiming resistance falls because the metal cools is wrong because a current-carrying filament heats up, not cools, and metallic resistance rises with temperature. The option blaming instrument errors is wrong because the curvature is a genuine physical effect reproducible across careful measurements. Quantitatively, the resistance of a metal rises roughly linearly with temperature through its positive temperature coefficient, so at higher currents the filament may glow white-hot and its resistance can be several times its cold value. This is also why the inrush current when a lamp is first switched on is much larger than its steady operating current. A consistency check confirms that rising temperature increasing metallic resistance fully explains the bending characteristic of the filament lamp, distinguishing it clearly from an ideal ohmic resistor whose graph stays straight.