# Resistance of a Filament Lamp

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A series of free GCSE/IGCSE Physics Notes and Lessons.

Resistance of a Filament Lamp

In this lesson, we will
• Describe the current/potential difference graphs for an ohmic conductor and for a filament lamp.
• Explain the shape of this graph.

Current through a resistor is directly proportional to the potential difference. The resistance is constant. Such a resistor is called an ohmic conductor.

The current through a filament lamp is not directly proportional to the potential difference. This is because the filament gets hot, which causes the resistance to increase. At high temperatures, the atoms in the filament vibrate more. The electrons in the current now collide more with the atoms. More energy is needed to push the current through the filament. As the potential difference increases, the current no longer increases as much.

Resistance in filament light bulbs
Ohm's Law states that
"The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor"

A filament light bulb is a non-ohmic conductor.

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