Beatrice Lang
The electrical resistance of a material generally increases with temperature. This is because, as the temperature rises, the atoms in the material vibrate more, causing more collisions with the electrons flowing through the material. This increases the resistance to the electron flow. However, the relationship between temperature and resistance is complex and depends on the type of material. For example, in some materials, such as semiconductors, an increase in temperature can lead to a decrease in resistance as more electrons are freed to conduct electricity. Additionally, different materials have different temperature coefficients, which determine how their resistance changes with temperature. Understanding the temperature dependence of resistance is crucial in designing electronic circuits, as engineers must select materials with suitable conductivity and resistance properties within the expected temperature range of operation.
| Characteristics | Values |
|---|---|
| Effect of temperature on resistance | As temperature increases, atoms vibrate more, causing more collisions with electrons, leading to increased resistance. |
| Conductor behaviour | In conductors, resistance generally increases with temperature due to higher atomic vibrations and more collisions with electrons. |
| Semiconductor behaviour | In semiconductors, increased temperature frees more electrons for conduction, increasing current and reducing resistance. |
| Insulator behaviour | In insulators, high temperatures can free electrons, reducing resistance and allowing current flow. |
| Material-specific behaviour | Different materials have different temperature coefficients, with conductors having low positive coefficients and insulators having low negative coefficients. |
| Temperature coefficient | A positive temperature coefficient indicates resistance increases with temperature, while a negative coefficient indicates resistance decreases with temperature. |
| Resistivity and temperature | Resistivity generally increases with temperature in conductors and decreases with temperature in insulators. |
| Conductor examples | Elemental metals like copper show an increase in resistance with higher temperatures, approximately +0.4% per Kelvin near room temperature. |
| Semiconductor examples | Nichrome, manganin, and constantan are metallic alloys with high resistivities and low temperature coefficients. |
| Insulator examples | Silicon is an example of a material with a negative temperature coefficient, where resistance decreases as temperature increases. |
| Resistance independence | Combining a resistor with a positive temperature coefficient and one with a negative coefficient can create a resistance almost independent of temperature. |
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What You'll Learn
The temperature coefficient of resistance
The value of alpha (α) varies depending on the material. Pure metals typically have a positive temperature coefficient, indicating that their resistance increases with a rise in temperature. This is because, as temperature increases, electrons in metals gain more kinetic energy, leading to more frequent collisions. These collisions between electrons and metal atoms hinder the flow of electrons, resulting in increased resistance.
On the other hand, some materials, such as semiconductors and insulators, exhibit a negative temperature coefficient of resistance. In these materials, the number of charge carriers (free electrons) per unit volume increases as the temperature rises. This increase in charge carriers compensates for the decrease in resistivity and resistance caused by the shorter intervals between collisions. As a result, the overall resistance of these materials decreases as temperature increases.
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The effect of temperature on the atomic structure of conductors and insulators
Conductors typically have a positive temperature coefficient, meaning their resistance increases with an increase in temperature. This is due to the increased thermal agitation of atoms, which causes them to vibrate more violently. In a conductor, which already has a large number of free electrons flowing through it, this vibration causes more collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron, hindering their flow and causing resistance.
However, insulators generally exhibit the opposite behaviour, with a negative temperature coefficient. At high temperatures, the resistance of an insulator can decrease, sometimes dramatically. This is because heating an insulating material causes its atoms to vibrate, and if heated sufficiently, the atoms vibrate violently enough to shake some of their captive electrons free. This creates additional free electrons, increasing the conductance and reducing resistance. Practical insulators like glass and plastic only exhibit this behaviour at very high temperatures and remain good insulators over the range of temperatures they are likely to encounter in use.
The behaviour of semiconductors is similar to that of insulators, with their resistance decreasing as temperature increases. This is because the vibration energy due to the increase in temperature breaks covalent bonds and creates additional free electrons. The increase in the number of electrons that can transmit charge outweighs the effect of increased vibrations, resulting in a net decrease in resistance.
The distinct behaviours of conductors, insulators, and semiconductors with respect to temperature can be exploited in the design of electronic systems. For example, thermistors and resistance temperature detectors (RTDs) in digital thermometers use the change in resistance with temperature to provide accurate temperature readings.
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How temperature affects the movement of electrons
Temperature has a significant influence on the movement of electrons, particularly in the context of electrical resistance. As temperature rises, atoms and molecules within a conductor vibrate more vigorously, impeding the flow of electrons. This increased resistance to electron movement is a result of heightened collisions between the vibrating atoms and the electrons.
In a conductor, numerous free electrons facilitate the flow of electric current. When the temperature rises, these free electrons collide more frequently with the captive electrons, resulting in energy loss for the free electrons. Consequently, resistance increases with temperature. However, this relationship is not universal, and materials like silicon exhibit a decrease in resistance with increasing temperature due to the liberation of additional charge carriers.
The impact of temperature on electron movement is also influenced by the type of material. In semiconductors, for instance, most electrons are engaged in valence bonding between atoms, with only a few free electrons available for conduction. As temperature rises, more electrons are released from their valence duties, increasing the conductivity of the semiconductor. This behaviour is described by the temperature coefficient of resistance, which quantifies the fractional increase in resistivity per unit rise in temperature.
The temperature also affects the kinetic energy of electrons. Higher temperatures correspond to higher kinetic energy in electrons, leading to an increase in their velocity. This relationship is observed in both free and bound electrons. However, it is important to note that while thermal energy contributes to higher electron velocity, it is the slower drift velocity that predominantly facilitates electrical conduction.
In summary, temperature influences the movement of electrons by affecting the resistance they encounter and their kinetic energy. The vibrating atoms in a conductor due to higher temperatures create obstacles for the flowing electrons, increasing resistance. At the same time, higher temperatures translate to higher kinetic energy in electrons, resulting in elevated electron velocities.
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The impact of temperature on resistivity
Now, let's delve into how temperature influences resistivity. In general, as the temperature rises, the atoms within a material vibrate more vigorously, leading to increased collisions between electrons. These collisions impede the flow of electrons, resulting in higher resistance. Consequently, materials tend to exhibit increased resistivity with higher temperatures. This relationship is particularly notable in conductors, where the number of free electrons is already substantial, and the additional collisions caused by thermal agitation contribute significantly to resistance.
However, it is essential to recognize that not all materials follow this pattern. Some materials, notably semiconductors, display a negative temperature coefficient of resistance. This means that as temperature rises, their resistivity decreases. In semiconductors, an increase in temperature can lead to a higher number of free electrons, facilitating greater current flow and lower resistance. This unique behaviour of semiconductors has significant implications for their application in electronic devices.
Additionally, it is worth noting that the choice of materials for resistors in electronic circuits is carefully made. Conductors with very low positive temperature coefficients are selected to ensure that resistivity and resistance exhibit only slight changes over a given temperature range. Conversely, insulators are chosen with very low negative temperature coefficients, ensuring that their resistance remains high even at elevated temperatures.
In summary, the impact of temperature on resistivity is multifaceted and material-dependent. While most materials show increased resistivity with higher temperatures due to increased atomic vibrations and electron collisions, semiconductors stand out with their negative temperature coefficient of resistance. The careful selection of materials for electronic components helps mitigate drastic changes in resistivity over typical temperature ranges.
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How temperature affects different materials
Temperature has a significant impact on the electrical resistance of various materials, and this relationship is crucial in understanding the performance of electronic circuits. The effect of temperature on resistance is influenced by the unique characteristics of different materials, categorised broadly as conductors, insulators, and semiconductors.
Conductors
Conductors, such as metals, generally experience an increase in resistance with a rise in temperature. This is due to the presence of free electrons within the material. As temperature increases, the atoms in the conductor vibrate more vigorously, causing greater collisions between the free electrons and the atoms. These collisions hinder the flow of electrons, leading to increased resistance. The specific resistance values for conductors are typically provided at a standard temperature of 20° Celsius, and any deviation from this temperature will result in a change in resistance.
Insulators
Insulating materials, such as glass and plastic, exhibit a decrease in resistance as temperatures rise. Insulators have very few free electrons, and at lower temperatures, the electrons are tightly bound within their atoms. However, when heated, the atoms in the insulator vibrate, and if the temperature is high enough, the vibrations can be strong enough to shake some electrons free. This increase in free electrons contributes to a decrease in resistance.
Semiconductors
Semiconductors, including materials like carbon, silicon, and germanium, demonstrate a decrease in resistance as temperatures rise. In semiconductors, most electrons are involved in valence bonding between atoms, but there are a small number of free electrons available for conduction. As the temperature increases, additional electrons are shaken free from their valence duties, increasing the overall conductivity of the material. This leads to a reduction in resistance.
It is important to note that the temperature coefficient of resistance, denoted as "alpha" (α), quantifies the change in resistance per degree of temperature change. Materials with a positive temperature coefficient exhibit increasing resistance with temperature, while those with a negative coefficient show a decrease in resistance as temperature rises.
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Frequently asked questions
Electrical resistance is the hindrance to the flow of electrons through a conductor.
As temperature increases, atoms and molecules vibrate more, causing more collisions with electrons. This increases resistance to electron flow. Therefore, resistance generally increases with temperature.
Different materials respond differently to temperature changes. Materials with a positive temperature coefficient, like metals, experience an increase in resistance with temperature. Materials with a negative temperature coefficient, like insulators, experience a decrease in resistance with temperature.
Semiconductors have a high number of valence electrons at low temperatures, and these electrons do not contribute to current flow. As temperature increases, more valence electrons gain energy and move to the conduction band, creating more charge carriers and increasing conductivity. Therefore, the resistance of semiconductors decreases with increasing temperature.
Insulators have very few free electrons at low temperatures, and most electrons are tightly bound to their atoms. As temperature increases, the atoms vibrate more violently, shaking free some electrons and creating additional charge carriers. This results in a decrease in resistance for insulators as temperature increases.
