What happens to resistance with increase in temperature?
2 Answers
The resistance of a material generally changes with temperature, and the effect depends on the type of material. Here’s a detailed explanation of what happens to the resistance when the temperature increases:
### 1. **Conductors (Metals)**
For most conductors, such as metals (e.g., copper, aluminum), the resistance **increases** with an increase in temperature. This behavior can be explained by considering the movement of electrons within the metal:
- **Electron Movement**: Metals have free electrons that move through the lattice structure of the metal atoms. These free electrons carry electric current when a voltage is applied.
- **Thermal Vibrations**: As the temperature increases, the metal atoms vibrate more intensely due to increased thermal energy.
- **Increased Collisions**: The more intense vibrations cause more frequent collisions between the free electrons and the vibrating metal atoms. These collisions impede the flow of electrons, thereby increasing the electrical resistance.
The relationship between temperature and resistance in conductors is given by the formula:
\[
R_T = R_0(1 + \alpha \Delta T)
\]
Where:
- \(R_T\) is the resistance at temperature \(T\),
- \(R_0\) is the resistance at a reference temperature (usually 0°C or room temperature),
- \(\alpha\) is the temperature coefficient of resistance (positive for metals),
- \(\Delta T\) is the change in temperature.
The positive value of \(\alpha\) indicates that resistance increases with temperature.
### 2. **Semiconductors (e.g., Silicon, Germanium)**
For semiconductors, such as silicon and germanium, the resistance **decreases** with an increase in temperature. This is due to the following reasons:
- **Carrier Generation**: In semiconductors, the number of charge carriers (electrons and holes) is very temperature-dependent. At low temperatures, there are relatively few free electrons available to conduct current.
- **Thermal Excitation**: As the temperature rises, more electrons gain enough energy to break free from their atomic bonds and become conduction electrons. This increase in free electrons and holes (charge carriers) improves the material’s conductivity and decreases its resistance.
- **Negative Temperature Coefficient**: Unlike metals, semiconductors have a **negative temperature coefficient** of resistance, meaning that the resistance decreases as the temperature increases.
### 3. **Insulators (e.g., Glass, Rubber)**
Insulators typically have very high resistance at room temperature, and the effect of temperature on their resistance can vary:
- **Very High Initial Resistance**: Insulators do not conduct electricity well because they have very few free charge carriers.
- **Decrease in Resistance**: At very high temperatures, even insulators can start to show a decrease in resistance. This is because, with enough thermal energy, some electrons can break free and become charge carriers. However, this effect usually only becomes significant at very high temperatures, which can lead to breakdowns.
### 4. **Alloys (e.g., Manganin, Constantan)**
Alloys are often used in applications where it is desirable to have a stable resistance over a wide range of temperatures:
- **Low Temperature Coefficient**: Alloys like manganin and constantan have a very low temperature coefficient of resistance. This means that their resistance changes very little with temperature. They are often used in precision resistors and measurement instruments where stable resistance is needed.
### Summary
- **Metals**: Resistance **increases** with temperature due to increased atomic vibrations and electron collisions.
- **Semiconductors**: Resistance **decreases** with temperature as more charge carriers are thermally excited.
- **Insulators**: Resistance may slightly decrease at very high temperatures due to thermal excitation but remains generally high.
- **Alloys**: Resistance remains relatively **stable** with temperature changes.
Understanding these principles is crucial for designing electrical circuits and selecting materials suitable for different environmental conditions and applications.
### 1. **Conductors (Metals)**
For most conductors, such as metals (e.g., copper, aluminum), the resistance **increases** with an increase in temperature. This behavior can be explained by considering the movement of electrons within the metal:
- **Electron Movement**: Metals have free electrons that move through the lattice structure of the metal atoms. These free electrons carry electric current when a voltage is applied.
- **Thermal Vibrations**: As the temperature increases, the metal atoms vibrate more intensely due to increased thermal energy.
- **Increased Collisions**: The more intense vibrations cause more frequent collisions between the free electrons and the vibrating metal atoms. These collisions impede the flow of electrons, thereby increasing the electrical resistance.
The relationship between temperature and resistance in conductors is given by the formula:
\[
R_T = R_0(1 + \alpha \Delta T)
\]
Where:
- \(R_T\) is the resistance at temperature \(T\),
- \(R_0\) is the resistance at a reference temperature (usually 0°C or room temperature),
- \(\alpha\) is the temperature coefficient of resistance (positive for metals),
- \(\Delta T\) is the change in temperature.
The positive value of \(\alpha\) indicates that resistance increases with temperature.
### 2. **Semiconductors (e.g., Silicon, Germanium)**
For semiconductors, such as silicon and germanium, the resistance **decreases** with an increase in temperature. This is due to the following reasons:
- **Carrier Generation**: In semiconductors, the number of charge carriers (electrons and holes) is very temperature-dependent. At low temperatures, there are relatively few free electrons available to conduct current.
- **Thermal Excitation**: As the temperature rises, more electrons gain enough energy to break free from their atomic bonds and become conduction electrons. This increase in free electrons and holes (charge carriers) improves the material’s conductivity and decreases its resistance.
- **Negative Temperature Coefficient**: Unlike metals, semiconductors have a **negative temperature coefficient** of resistance, meaning that the resistance decreases as the temperature increases.
### 3. **Insulators (e.g., Glass, Rubber)**
Insulators typically have very high resistance at room temperature, and the effect of temperature on their resistance can vary:
- **Very High Initial Resistance**: Insulators do not conduct electricity well because they have very few free charge carriers.
- **Decrease in Resistance**: At very high temperatures, even insulators can start to show a decrease in resistance. This is because, with enough thermal energy, some electrons can break free and become charge carriers. However, this effect usually only becomes significant at very high temperatures, which can lead to breakdowns.
### 4. **Alloys (e.g., Manganin, Constantan)**
Alloys are often used in applications where it is desirable to have a stable resistance over a wide range of temperatures:
- **Low Temperature Coefficient**: Alloys like manganin and constantan have a very low temperature coefficient of resistance. This means that their resistance changes very little with temperature. They are often used in precision resistors and measurement instruments where stable resistance is needed.
### Summary
- **Metals**: Resistance **increases** with temperature due to increased atomic vibrations and electron collisions.
- **Semiconductors**: Resistance **decreases** with temperature as more charge carriers are thermally excited.
- **Insulators**: Resistance may slightly decrease at very high temperatures due to thermal excitation but remains generally high.
- **Alloys**: Resistance remains relatively **stable** with temperature changes.
Understanding these principles is crucial for designing electrical circuits and selecting materials suitable for different environmental conditions and applications.