### Resistance of a metallic conductor and temperature

The filament of a **filament lamp **is metal (tungsten), but the resistance does not remain constant. The graph of current against resistance is curved. As the current and PD rise the energy rate of energy change (power) in the filament increases and the temperature rises. The resistance therefore increases and so the gradient of the graph (which is the inverse of resistance) falls. This causes the free electrons to become more excited and they no longer flow as easily, therefore raising the resistance.

If the conductors are of similar sizes the resistance of the semi conductor will always be much higher.

### A thermistor

A thermistor is made from a semi-conductor. These materials conduct electricity better when they are hot (but never as well as a metal). It is only the free electrons that will conduct a current. In semiconductors there are a few, but not many. (Metals have lots). When cold there are hardly any free electrons to conduct a current. When hot some of the electrons are sufficiently excited to move into higher energy levels, able to move from atom to atom. These free electrons can then conduct a current. The hotter the material is the greater the number of free electrons, allowing a current to flow more easily, reducing the resistance.

**Other pages on resistance you might find useful:**

**Electrical resistance and Ohm's Law notes and video Resistance and temperature graphs Resistance of a filament bulb The characteristics of a diode Electrical resistivity Internal resistance and EMF**