Filament bulb
A light source consisting of a thin wire (filament) that glows when heated by electric current. The filament's resistance increases with temperature, producing a nonlinear I-V characteristic. Filament bulbs are nonohmic—their resistance is not constant.
Real World
When you dim a vintage incandescent bulb in a theatre, the filament cools and its resistance drops — this is why dimmers must account for the non-constant resistance of tungsten filaments.
Exam Focus
For 'explain the shape' questions, link the curve explicitly to increasing temperature causing increasing resistance — two separate marking points.
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