Why all conductors do not obey Ohm's law?
2 Answers
Ohm's law is a fundamental principle in electrical engineering and physics that states the relationship between voltage, current, and resistance in an electrical circuit. Specifically, Ohm's law is expressed as:
\[ V = IR \]
where \( V \) is the voltage across the conductor, \( I \) is the current flowing through it, and \( R \) is the resistance of the conductor. This law implies that the resistance \( R \) remains constant as the voltage and current change. However, not all conductors obey Ohm's law. Here's why:
### 1. **Non-Linear Resistance**
Some materials have non-linear resistance, meaning their resistance changes with voltage or current. This behavior is typical of:
- **Semiconductors**: In materials like silicon and germanium, resistance depends on factors like temperature and the amount of current. For instance, in a diode, the resistance is not constant but changes as the voltage exceeds a certain threshold.
- **Thermistors**: These are temperature-sensitive resistors where resistance varies significantly with temperature changes.
### 2. **Temperature Dependence**
For many materials, resistance is not constant but varies with temperature. As current flows through a conductor, it heats up. This heating increases the lattice vibrations within the material, which can increase resistance. For instance:
- **Metallic Conductors**: In most metals, resistance increases with temperature (positive temperature coefficient).
- **Superconductors**: At very low temperatures, some materials exhibit zero resistance. In this superconducting state, Ohm’s law does not apply because the resistance is effectively zero.
### 3. **High Current or High Voltage Effects**
At very high current levels or voltages, certain materials may exhibit behaviors that deviate from Ohm's law due to:
- **Non-Ohmic Materials**: Materials like electrolytes or gases might not follow Ohm’s law under extreme conditions.
- **Breakdown Effects**: High voltages can cause breakdown phenomena such as arcing or ionization in some materials, which leads to non-linear current-voltage characteristics.
### 4. **Physical and Chemical Changes**
Some materials change their properties under different conditions:
- **P-N Junctions**: In semiconductor devices like transistors or diodes, the resistance is not constant but varies based on the junction's configuration and the applied voltage.
- **Metal-Oxide Varistors (MOVs)**: These devices are used for over-voltage protection and exhibit a highly non-linear current-voltage relationship.
### 5. **Complex Materials and Components**
Certain complex materials or components, like:
- **Graphene**: Although theoretically exhibiting near-Ohmic behavior, practical applications can show deviations due to edge effects or defects.
- **Non-Linear Resistors**: Specialized resistors designed for particular applications, like varistors or light-dependent resistors (LDRs), display non-linear characteristics.
### Summary
In essence, while Ohm's law provides a fundamental understanding of how voltage, current, and resistance interact in simple, idealized cases, real-world materials and devices often exhibit more complex behaviors. These deviations can be attributed to factors such as temperature dependence, high current/voltage effects, and inherent material properties. Understanding these deviations is crucial for designing and working with electronic circuits and systems effectively.
\[ V = IR \]
where \( V \) is the voltage across the conductor, \( I \) is the current flowing through it, and \( R \) is the resistance of the conductor. This law implies that the resistance \( R \) remains constant as the voltage and current change. However, not all conductors obey Ohm's law. Here's why:
### 1. **Non-Linear Resistance**
Some materials have non-linear resistance, meaning their resistance changes with voltage or current. This behavior is typical of:
- **Semiconductors**: In materials like silicon and germanium, resistance depends on factors like temperature and the amount of current. For instance, in a diode, the resistance is not constant but changes as the voltage exceeds a certain threshold.
- **Thermistors**: These are temperature-sensitive resistors where resistance varies significantly with temperature changes.
### 2. **Temperature Dependence**
For many materials, resistance is not constant but varies with temperature. As current flows through a conductor, it heats up. This heating increases the lattice vibrations within the material, which can increase resistance. For instance:
- **Metallic Conductors**: In most metals, resistance increases with temperature (positive temperature coefficient).
- **Superconductors**: At very low temperatures, some materials exhibit zero resistance. In this superconducting state, Ohm’s law does not apply because the resistance is effectively zero.
### 3. **High Current or High Voltage Effects**
At very high current levels or voltages, certain materials may exhibit behaviors that deviate from Ohm's law due to:
- **Non-Ohmic Materials**: Materials like electrolytes or gases might not follow Ohm’s law under extreme conditions.
- **Breakdown Effects**: High voltages can cause breakdown phenomena such as arcing or ionization in some materials, which leads to non-linear current-voltage characteristics.
### 4. **Physical and Chemical Changes**
Some materials change their properties under different conditions:
- **P-N Junctions**: In semiconductor devices like transistors or diodes, the resistance is not constant but varies based on the junction's configuration and the applied voltage.
- **Metal-Oxide Varistors (MOVs)**: These devices are used for over-voltage protection and exhibit a highly non-linear current-voltage relationship.
### 5. **Complex Materials and Components**
Certain complex materials or components, like:
- **Graphene**: Although theoretically exhibiting near-Ohmic behavior, practical applications can show deviations due to edge effects or defects.
- **Non-Linear Resistors**: Specialized resistors designed for particular applications, like varistors or light-dependent resistors (LDRs), display non-linear characteristics.
### Summary
In essence, while Ohm's law provides a fundamental understanding of how voltage, current, and resistance interact in simple, idealized cases, real-world materials and devices often exhibit more complex behaviors. These deviations can be attributed to factors such as temperature dependence, high current/voltage effects, and inherent material properties. Understanding these deviations is crucial for designing and working with electronic circuits and systems effectively.
Ohm's Law is a fundamental principle in electrical engineering and physics, stating that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points, provided the temperature remains constant. The law is mathematically expressed as:
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. Ohm's Law is commonly associated with ohmic materials, which are materials that have a constant resistance and follow this relationship. However, not all conductors obey Ohm's Law. Here's why:
### 1. **Non-Ohmic Materials**
Many materials do not have a constant resistance; their resistance changes with the applied voltage or current. These are called non-ohmic materials. For such materials, the relationship between voltage and current is not linear. Common examples include:
- **Semiconductors**: Materials like silicon and germanium, which are used in diodes and transistors, have a nonlinear current-voltage relationship. Their resistance changes significantly with the applied voltage due to the movement of charge carriers.
- **Superconductors**: At very low temperatures, materials like lead and niobium exhibit superconductivity, where their resistance drops to zero. In this state, Ohm's Law does not apply because the resistance is zero, making the voltage-current relationship trivial.
### 2. **Temperature Effects**
In many conductors, resistance varies with temperature. For example, most metals have a positive temperature coefficient, meaning their resistance increases with temperature. When the temperature changes significantly, the resistance of the material changes, causing deviations from Ohm's Law.
- **Thermistors**: These are resistors with a resistance that varies significantly with temperature. They are often used in temperature sensing and compensation.
### 3. **High-Voltage and High-Current Effects**
At high voltages or currents, some materials exhibit behaviors that deviate from Ohm's Law:
- **Breakdown Voltage**: In semiconductors and insulators, applying a high voltage can cause breakdown, where the material starts conducting uncontrollably. This results in a non-linear current-voltage relationship.
- **Electromigration**: In conductors carrying high currents, the flow of electrons can cause physical changes in the conductor (such as migration of metal atoms), altering the resistance over time.
### 4. **Frequency Dependence**
For alternating current (AC) circuits, reactance (the opposition to AC) can also affect the overall resistance and cause deviations from Ohm's Law.
- **Inductors and Capacitors**: In AC circuits, inductors and capacitors introduce reactance, which varies with the frequency of the AC signal. This reactance alters the effective resistance, leading to deviations from Ohm's Law.
### 5. **Quantum Effects**
At very small scales, quantum effects become significant. For instance:
- **Quantum Tunneling**: In very small or thin conductors, quantum effects like tunneling can cause unexpected conductance behavior that does not follow Ohm's Law.
### Summary
In essence, Ohm's Law is a useful approximation for many materials and conditions but does not apply universally. Non-ohmic materials, temperature effects, high-voltage and high-current phenomena, frequency-dependent behaviors, and quantum effects can all lead to deviations from the simple linear relationship described by Ohm's Law. Understanding these exceptions helps in designing and analyzing electronic components and systems that behave differently from idealized ohmic conductors.
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. Ohm's Law is commonly associated with ohmic materials, which are materials that have a constant resistance and follow this relationship. However, not all conductors obey Ohm's Law. Here's why:
### 1. **Non-Ohmic Materials**
Many materials do not have a constant resistance; their resistance changes with the applied voltage or current. These are called non-ohmic materials. For such materials, the relationship between voltage and current is not linear. Common examples include:
- **Semiconductors**: Materials like silicon and germanium, which are used in diodes and transistors, have a nonlinear current-voltage relationship. Their resistance changes significantly with the applied voltage due to the movement of charge carriers.
- **Superconductors**: At very low temperatures, materials like lead and niobium exhibit superconductivity, where their resistance drops to zero. In this state, Ohm's Law does not apply because the resistance is zero, making the voltage-current relationship trivial.
### 2. **Temperature Effects**
In many conductors, resistance varies with temperature. For example, most metals have a positive temperature coefficient, meaning their resistance increases with temperature. When the temperature changes significantly, the resistance of the material changes, causing deviations from Ohm's Law.
- **Thermistors**: These are resistors with a resistance that varies significantly with temperature. They are often used in temperature sensing and compensation.
### 3. **High-Voltage and High-Current Effects**
At high voltages or currents, some materials exhibit behaviors that deviate from Ohm's Law:
- **Breakdown Voltage**: In semiconductors and insulators, applying a high voltage can cause breakdown, where the material starts conducting uncontrollably. This results in a non-linear current-voltage relationship.
- **Electromigration**: In conductors carrying high currents, the flow of electrons can cause physical changes in the conductor (such as migration of metal atoms), altering the resistance over time.
### 4. **Frequency Dependence**
For alternating current (AC) circuits, reactance (the opposition to AC) can also affect the overall resistance and cause deviations from Ohm's Law.
- **Inductors and Capacitors**: In AC circuits, inductors and capacitors introduce reactance, which varies with the frequency of the AC signal. This reactance alters the effective resistance, leading to deviations from Ohm's Law.
### 5. **Quantum Effects**
At very small scales, quantum effects become significant. For instance:
- **Quantum Tunneling**: In very small or thin conductors, quantum effects like tunneling can cause unexpected conductance behavior that does not follow Ohm's Law.
### Summary
In essence, Ohm's Law is a useful approximation for many materials and conditions but does not apply universally. Non-ohmic materials, temperature effects, high-voltage and high-current phenomena, frequency-dependent behaviors, and quantum effects can all lead to deviations from the simple linear relationship described by Ohm's Law. Understanding these exceptions helps in designing and analyzing electronic components and systems that behave differently from idealized ohmic conductors.