Which statement about a filament bulb's I–V behavior is true?

Which statement about a filament bulb's I–V behavior is true?

• A. The current is not proportional to voltage; the graph is curved; the bulb does not obey Ohm's Law
• B. The current is proportional; the graph is straight
• C. The resistance remains perfectly constant
• D. Voltage changes do not affect current

Answer: (A)

When a filament bulb operates, the filament heats up as current flows. As the temperature increases, the metal’s resistance rises. Ohm’s Law in its simple form (V = IR) assumes a constant resistance, so if the resistance changes with temperature, the current isn’t simply proportional to the voltage. As voltage grows, the bulb gets hotter, resistance increases, and the I–V relationship becomes curved rather than a straight line. That’s why the statement that the current is not proportional to voltage, the graph is curved, and the bulb does not obey Ohm’s Law is the true description. The other ideas—that the current is proportional and the graph is straight, or that resistance stays constant, or that voltage doesn’t affect current—don’t match how a real filament behaves.

When a filament bulb operates, the filament heats up as current flows. As the temperature increases, the metal’s resistance rises. Ohm’s Law in its simple form (V = IR) assumes a constant resistance, so if the resistance changes with temperature, the current isn’t simply proportional to the voltage. As voltage grows, the bulb gets hotter, resistance increases, and the I–V relationship becomes curved rather than a straight line. That’s why the statement that the current is not proportional to voltage, the graph is curved, and the bulb does not obey Ohm’s Law is the true description. The other ideas—that the current is proportional and the graph is straight, or that resistance stays constant, or that voltage doesn’t affect current—don’t match how a real filament behaves.