Which elements do not obey Ohm's law?
2 Answers
Ohm’s law is a fundamental principle in electrical engineering and physics that describes the relationship between voltage (V), current (I), and resistance (R) in a circuit. According to Ohm's law, the current flowing through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance of the conductor. Mathematically, it is expressed as:
\[
V = I \times R
\]
However, not all materials behave according to Ohm's law. Elements or materials that do not obey Ohm's law are called **non-ohmic materials**. These materials exhibit a nonlinear relationship between voltage and current, meaning that their resistance changes with the applied voltage or current. Here are some examples of elements and materials that do not obey Ohm's law:
### 1. **Semiconductors**
- **Description**: Semiconductors, such as silicon and germanium, have electrical conductivity that varies with temperature and the presence of impurities (doping). Their resistance is not constant and can change significantly with applied voltage.
- **Behavior**: In forward bias (when voltage is applied in a direction that allows current to flow), their resistance decreases, allowing more current to flow with increasing voltage. In reverse bias, they may exhibit a very high resistance until breakdown occurs.
### 2. **Diodes**
- **Description**: Diodes are semiconductor devices that allow current to flow in one direction only. They have a nonlinear current-voltage (I-V) characteristic.
- **Behavior**: In the forward direction, a diode exhibits very low resistance after a certain threshold voltage (the forward voltage), leading to an exponential increase in current with voltage. In reverse bias, they block current until a certain reverse voltage is reached (breakdown).
### 3. **Transistors**
- **Description**: Transistors are semiconductor devices used to amplify or switch electronic signals. Their operation relies on the control of current flow.
- **Behavior**: The relationship between the input current/voltage and the output current/voltage is nonlinear, making them non-ohmic.
### 4. **Thermistors**
- **Description**: Thermistors are temperature-sensitive resistors made from ceramic materials that exhibit a significant change in resistance with temperature.
- **Behavior**: There are two types of thermistors: NTC (Negative Temperature Coefficient) thermistors, which decrease in resistance as temperature increases, and PTC (Positive Temperature Coefficient) thermistors, which increase in resistance with temperature. This non-linear behavior makes them non-ohmic.
### 5. **Light Emitting Diodes (LEDs)**
- **Description**: LEDs are diodes that emit light when current flows through them.
- **Behavior**: Similar to standard diodes, LEDs have a nonlinear I-V characteristic, where a certain threshold voltage must be exceeded for them to conduct significantly.
### 6. **Superconductors**
- **Description**: Superconductors are materials that can conduct electricity without resistance below a certain critical temperature.
- **Behavior**: Above the critical temperature, they behave as normal conductors and follow Ohm's law, but below this temperature, they exhibit zero resistance, which contradicts the traditional concept of Ohm’s law.
### 7. **Plasmas**
- **Description**: Plasmas are ionized gases with free-moving charged particles.
- **Behavior**: The relationship between current and voltage in plasmas can be very complex and depends on factors like ionization levels, making them non-ohmic.
### 8. **Carbon Nanotubes**
- **Description**: Carbon nanotubes exhibit unique electrical properties, including a variable resistance based on the applied electric field.
- **Behavior**: Depending on their structure and environment, they can show either metallic or semiconducting behavior, leading to non-linear I-V characteristics.
### 9. **Nonlinear Resistors**
- **Description**: Some materials, specifically designed as nonlinear resistors, change their resistance based on the voltage applied across them.
- **Behavior**: These resistors may be used in surge protectors and other applications where their resistance decreases with increased voltage.
### Conclusion
In summary, while many materials obey Ohm’s law under standard conditions, a variety of elements and materials, particularly semiconductors and certain types of resistors, exhibit non-ohmic behavior. Understanding the differences in how these materials respond to electrical forces is crucial for designing electronic components and systems that operate effectively in a range of conditions.
\[
V = I \times R
\]
However, not all materials behave according to Ohm's law. Elements or materials that do not obey Ohm's law are called **non-ohmic materials**. These materials exhibit a nonlinear relationship between voltage and current, meaning that their resistance changes with the applied voltage or current. Here are some examples of elements and materials that do not obey Ohm's law:
### 1. **Semiconductors**
- **Description**: Semiconductors, such as silicon and germanium, have electrical conductivity that varies with temperature and the presence of impurities (doping). Their resistance is not constant and can change significantly with applied voltage.
- **Behavior**: In forward bias (when voltage is applied in a direction that allows current to flow), their resistance decreases, allowing more current to flow with increasing voltage. In reverse bias, they may exhibit a very high resistance until breakdown occurs.
### 2. **Diodes**
- **Description**: Diodes are semiconductor devices that allow current to flow in one direction only. They have a nonlinear current-voltage (I-V) characteristic.
- **Behavior**: In the forward direction, a diode exhibits very low resistance after a certain threshold voltage (the forward voltage), leading to an exponential increase in current with voltage. In reverse bias, they block current until a certain reverse voltage is reached (breakdown).
### 3. **Transistors**
- **Description**: Transistors are semiconductor devices used to amplify or switch electronic signals. Their operation relies on the control of current flow.
- **Behavior**: The relationship between the input current/voltage and the output current/voltage is nonlinear, making them non-ohmic.
### 4. **Thermistors**
- **Description**: Thermistors are temperature-sensitive resistors made from ceramic materials that exhibit a significant change in resistance with temperature.
- **Behavior**: There are two types of thermistors: NTC (Negative Temperature Coefficient) thermistors, which decrease in resistance as temperature increases, and PTC (Positive Temperature Coefficient) thermistors, which increase in resistance with temperature. This non-linear behavior makes them non-ohmic.
### 5. **Light Emitting Diodes (LEDs)**
- **Description**: LEDs are diodes that emit light when current flows through them.
- **Behavior**: Similar to standard diodes, LEDs have a nonlinear I-V characteristic, where a certain threshold voltage must be exceeded for them to conduct significantly.
### 6. **Superconductors**
- **Description**: Superconductors are materials that can conduct electricity without resistance below a certain critical temperature.
- **Behavior**: Above the critical temperature, they behave as normal conductors and follow Ohm's law, but below this temperature, they exhibit zero resistance, which contradicts the traditional concept of Ohm’s law.
### 7. **Plasmas**
- **Description**: Plasmas are ionized gases with free-moving charged particles.
- **Behavior**: The relationship between current and voltage in plasmas can be very complex and depends on factors like ionization levels, making them non-ohmic.
### 8. **Carbon Nanotubes**
- **Description**: Carbon nanotubes exhibit unique electrical properties, including a variable resistance based on the applied electric field.
- **Behavior**: Depending on their structure and environment, they can show either metallic or semiconducting behavior, leading to non-linear I-V characteristics.
### 9. **Nonlinear Resistors**
- **Description**: Some materials, specifically designed as nonlinear resistors, change their resistance based on the voltage applied across them.
- **Behavior**: These resistors may be used in surge protectors and other applications where their resistance decreases with increased voltage.
### Conclusion
In summary, while many materials obey Ohm’s law under standard conditions, a variety of elements and materials, particularly semiconductors and certain types of resistors, exhibit non-ohmic behavior. Understanding the differences in how these materials respond to electrical forces is crucial for designing electronic components and systems that operate effectively in a range of conditions.
Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, given that the temperature and other physical conditions remain constant. The relationship is given by the formula:
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. This law holds true for ohmic materials, where the resistance \( R \) is constant regardless of the voltage and current.
However, not all materials and elements obey Ohm's Law. Here are some key examples of elements and materials that do not follow Ohm's Law:
### 1. **Semiconductors:**
- **Diodes:** Diodes are designed to allow current to flow in one direction only. The current-voltage relationship in a diode is nonlinear and exponential, rather than linear. For example, the voltage-current relationship for a diode is given by the Shockley diode equation:
\[ I = I_S \left( e^{\frac{V}{nV_T}} - 1 \right) \]
where \( I_S \) is the saturation current, \( V \) is the voltage, \( n \) is the ideality factor, and \( V_T \) is the thermal voltage.
- **Transistors:** Like diodes, transistors have a nonlinear current-voltage relationship. For example, in a Bipolar Junction Transistor (BJT), the current through the collector-emitter junction is related to the base-emitter voltage in a nonlinear manner.
### 2. **Nonlinear Resistors:**
- **Varistors:** These are voltage-dependent resistors whose resistance decreases with increasing voltage. The voltage-current relationship is nonlinear, typically modeled with a power-law function:
\[ V = kI^n \]
where \( k \) and \( n \) are constants.
- **Thermistors:** These are temperature-dependent resistors. Their resistance varies with temperature, and thus their relationship with current and voltage is not linear. Negative Temperature Coefficient (NTC) thermistors decrease in resistance as temperature increases, while Positive Temperature Coefficient (PTC) thermistors increase in resistance with temperature.
### 3. **Superconductors:**
- Superconductors exhibit zero electrical resistance below a certain critical temperature. When in the superconducting state, the voltage across a superconducting material is zero regardless of the current, which means Ohm's Law in its conventional form does not apply.
### 4. **Non-ohmic Materials:**
- **Electric Arcs:** In an electric arc, the resistance is not constant and can vary significantly with changes in the current and voltage. The relationship between current and voltage is complex and non-linear due to the ionized gas present in the arc.
- **Plasma:** Plasmas, consisting of ionized gases, exhibit a complex and nonlinear relationship between current and voltage.
### 5. **Carbon Nanotubes and Graphene:**
- **Carbon Nanotubes:** These have unique electrical properties that can vary depending on their structure (e.g., metallic or semiconducting behavior). The current-voltage relationship can be nonlinear.
- **Graphene:** In graphene, the electrical conductivity can depend on various factors including doping, electric fields, and strain, leading to a nonlinear current-voltage characteristic.
In summary, materials and elements that do not obey Ohm's Law generally exhibit nonlinear relationships between current and voltage. This nonlinearity can arise from intrinsic properties like semiconductor behavior, temperature dependence, or unique material structures.
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. This law holds true for ohmic materials, where the resistance \( R \) is constant regardless of the voltage and current.
However, not all materials and elements obey Ohm's Law. Here are some key examples of elements and materials that do not follow Ohm's Law:
### 1. **Semiconductors:**
- **Diodes:** Diodes are designed to allow current to flow in one direction only. The current-voltage relationship in a diode is nonlinear and exponential, rather than linear. For example, the voltage-current relationship for a diode is given by the Shockley diode equation:
\[ I = I_S \left( e^{\frac{V}{nV_T}} - 1 \right) \]
where \( I_S \) is the saturation current, \( V \) is the voltage, \( n \) is the ideality factor, and \( V_T \) is the thermal voltage.
- **Transistors:** Like diodes, transistors have a nonlinear current-voltage relationship. For example, in a Bipolar Junction Transistor (BJT), the current through the collector-emitter junction is related to the base-emitter voltage in a nonlinear manner.
### 2. **Nonlinear Resistors:**
- **Varistors:** These are voltage-dependent resistors whose resistance decreases with increasing voltage. The voltage-current relationship is nonlinear, typically modeled with a power-law function:
\[ V = kI^n \]
where \( k \) and \( n \) are constants.
- **Thermistors:** These are temperature-dependent resistors. Their resistance varies with temperature, and thus their relationship with current and voltage is not linear. Negative Temperature Coefficient (NTC) thermistors decrease in resistance as temperature increases, while Positive Temperature Coefficient (PTC) thermistors increase in resistance with temperature.
### 3. **Superconductors:**
- Superconductors exhibit zero electrical resistance below a certain critical temperature. When in the superconducting state, the voltage across a superconducting material is zero regardless of the current, which means Ohm's Law in its conventional form does not apply.
### 4. **Non-ohmic Materials:**
- **Electric Arcs:** In an electric arc, the resistance is not constant and can vary significantly with changes in the current and voltage. The relationship between current and voltage is complex and non-linear due to the ionized gas present in the arc.
- **Plasma:** Plasmas, consisting of ionized gases, exhibit a complex and nonlinear relationship between current and voltage.
### 5. **Carbon Nanotubes and Graphene:**
- **Carbon Nanotubes:** These have unique electrical properties that can vary depending on their structure (e.g., metallic or semiconducting behavior). The current-voltage relationship can be nonlinear.
- **Graphene:** In graphene, the electrical conductivity can depend on various factors including doping, electric fields, and strain, leading to a nonlinear current-voltage characteristic.
In summary, materials and elements that do not obey Ohm's Law generally exhibit nonlinear relationships between current and voltage. This nonlinearity can arise from intrinsic properties like semiconductor behavior, temperature dependence, or unique material structures.