Temperature has a significant effect on the **resistivity** of insulators. Resistivity is a material's ability to resist the flow of electric current. Here's how temperature influences this property in insulators:
### 1. **Basic Relationship Between Temperature and Resistivity:**
In insulators, resistivity **decreases** as temperature **increases**. This is the opposite of what happens in most conductors, where resistivity increases with temperature. The decrease in resistivity in insulators occurs because thermal energy excites electrons, allowing them to overcome the energy gap between the valence band and conduction band.
### 2. **Energy Band Model of Insulators:**
- **Valence Band**: In insulators, the electrons are tightly bound to their atoms and reside in the valence band.
- **Conduction Band**: The conduction band, where free electrons can move and contribute to electric current, is separated from the valence band by a large **energy gap**.
- **At low temperatures**: Very few electrons have enough energy to jump from the valence band to the conduction band, so the material behaves as an insulator with very high resistivity.
- **At high temperatures**: As temperature increases, more electrons gain enough energy to cross the energy gap, leading to a decrease in resistivity. However, the number of free electrons remains relatively small compared to conductors, so the material does not become a good conductor.
### 3. **Thermal Excitation of Electrons:**
- The key reason for the decrease in resistivity is the **thermal excitation** of electrons. In an insulator, when temperature increases, more electrons gain the thermal energy needed to break free from their bound state (valence band) and move into the conduction band.
- Once in the conduction band, these electrons can participate in electrical conduction, thus reducing the resistivity of the material.
### 4. **Practical Impact:**
- At room temperature, insulators have very high resistivity, making them effective for preventing current flow.
- At very high temperatures, the resistivity decreases, but the material remains an insulator as long as the number of thermally excited electrons remains low compared to the total number of atoms.
### Summary:
For insulators, resistivity decreases with increasing temperature because more electrons gain enough energy to participate in conduction. However, even with this decrease, insulators typically retain their insulating properties unless they are subjected to extremely high temperatures.