Why Resistance Changes — The Physics Explanation

⚙️ Why Resistance Changes — The Physics Explanation

In a Filament Lamp: Resistance Increases with Temperature

The tungsten filament in a bulb is a metal. In metals, conduction electrons move relatively freely through a lattice of positive ions. When current flows, energy is transferred — the ions vibrate more vigorously. At higher temperatures, these vibrations are larger, making it harder for electrons to pass through without colliding. More collisions = greater resistance to electron flow.

This explains the curved I-V graph: as you increase voltage, the filament heats up, resistance rises, and the current increases less rapidly than voltage. The gradient of the I-V curve gets smaller (less steep) at high currents.

In a Diode: Resistance Depends on Direction

A diode is made from semiconductor materials (typically silicon). It has a threshold voltage of about 0.6 V in the forward direction. Below this, the internal electric field within the diode opposes current flow. Above it, the barrier is overcome and current flows freely.

In the reverse direction, the electric field within the diode strongly opposes current flow — the resistance is effectively infinite (millions of ohms) for typical reverse voltages.

This one-way conductance makes diodes extremely useful for rectification — converting alternating current (AC) to direct current (DC) by allowing current to flow in only one direction.