What happens to the resistance of a filament as its temperature increases?

• A. Resistance increases
• B. Resistance decreases
• C. Resistance stays the same
• D. It becomes infinite

Answer: (A)

When a metal filament heats up, its resistance goes up. This happens because increasing temperature makes the metal’s lattice vibrate more, which scatters the free electrons more as they move through the material. That extra scattering raises the resistivity, and since resistance R is proportional to resistivity (R ∝ ρL/A), the filament’s resistance increases with temperature. This positive temperature coefficient is why tungsten filaments in bulbs heat up and become more resistant as they glow. It doesn’t stay the same, it doesn’t decrease with temperature, and it wouldn’t become infinite under normal heating (it would only reach an extremely high value if the filament melted or failed).