When a filament bulb is dim, the filament is cold and has

When a filament bulb is dim, the filament is cold and has

• A. low resistance
• B. high resistance
• C. variable resistance
• D. no resistance

Answer: (A)

Resistance in a metal filament depends on temperature. Metals have a positive temperature coefficient, so as they heat up, their resistance increases. When a filament is cold, there are fewer atomic vibrations, so electrons collide less often and can pass through more easily, giving a lower resistance. A dim bulb means the filament isn’t hot, so it remains in this cooler, lower-resistance state. The other possibilities don’t fit because the filament would only have high resistance if it were hot, or zero resistance would imply a perfect conductor, which a tungsten filament is not.

Resistance in a metal filament depends on temperature. Metals have a positive temperature coefficient, so as they heat up, their resistance increases. When a filament is cold, there are fewer atomic vibrations, so electrons collide less often and can pass through more easily, giving a lower resistance. A dim bulb means the filament isn’t hot, so it remains in this cooler, lower-resistance state. The other possibilities don’t fit because the filament would only have high resistance if it were hot, or zero resistance would imply a perfect conductor, which a tungsten filament is not.