What are two devices which do not obey Ohm's law?
2 Answers
Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and material properties remain constant. Mathematically, it's expressed as \( V = IR \), where \( V \) is voltage, \( I \) is current, and \( R \) is resistance.
However, not all electrical devices obey Ohm's Law. Devices that do not follow this law are typically those where resistance changes with voltage or current, or those that have nonlinear voltage-current characteristics. Here are two common examples:
### 1. **Diodes**
**Description**: A diode is a semiconductor device that allows current to flow in one direction only. It has a characteristic curve where the relationship between the voltage across it and the current flowing through it is nonlinear.
**Why It Doesn't Obey Ohm's Law**: For diodes, the current-voltage relationship is exponential rather than linear. In the forward-biased condition (where the diode is conducting), the current increases exponentially with the increase in voltage after surpassing a certain threshold voltage (typically about 0.7 volts for silicon diodes). In the reverse-biased condition (where the diode blocks current flow), the current remains very small and nearly constant regardless of the voltage until breakdown occurs.
**Characteristic Curve**: The current-voltage (I-V) characteristic of a diode is described 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 across the diode, \( V_T \) is the thermal voltage, and \( n \) is the ideality factor.
### 2. **Transistors**
**Description**: Transistors are semiconductor devices used to amplify or switch electronic signals. There are various types, such as bipolar junction transistors (BJTs) and field-effect transistors (FETs).
**Why They Don't Obey Ohm's Law**: Transistors have nonlinear characteristics. In a BJT, for example, the relationship between the base current (I_B) and the collector current (I_C) is not linear; it's exponential due to the transistor’s active region. The collector-emitter voltage (V_CE) and collector current (I_C) also do not follow Ohm’s Law, as the transistor’s behavior depends on the control signal applied to the base (in BJTs) or gate (in FETs). The transfer characteristics of a transistor are governed by more complex relationships involving the transistor’s parameters and operating conditions.
**Characteristic Curves**:
- For BJTs, the collector current is related to the base current and the collector-emitter voltage through the transistor's current gain (\( \beta \)) and is described by the Ebers-Moll model.
- For FETs, the drain current (\( I_D \)) is related to the gate-source voltage (\( V_{GS} \)) and the drain-source voltage (\( V_{DS} \)), and it’s often described by quadratic or exponential equations depending on the operating region (linear or saturation).
In both cases, the voltage-current relationship is not straightforward and linear, thus not adhering to Ohm's Law.
However, not all electrical devices obey Ohm's Law. Devices that do not follow this law are typically those where resistance changes with voltage or current, or those that have nonlinear voltage-current characteristics. Here are two common examples:
### 1. **Diodes**
**Description**: A diode is a semiconductor device that allows current to flow in one direction only. It has a characteristic curve where the relationship between the voltage across it and the current flowing through it is nonlinear.
**Why It Doesn't Obey Ohm's Law**: For diodes, the current-voltage relationship is exponential rather than linear. In the forward-biased condition (where the diode is conducting), the current increases exponentially with the increase in voltage after surpassing a certain threshold voltage (typically about 0.7 volts for silicon diodes). In the reverse-biased condition (where the diode blocks current flow), the current remains very small and nearly constant regardless of the voltage until breakdown occurs.
**Characteristic Curve**: The current-voltage (I-V) characteristic of a diode is described 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 across the diode, \( V_T \) is the thermal voltage, and \( n \) is the ideality factor.
### 2. **Transistors**
**Description**: Transistors are semiconductor devices used to amplify or switch electronic signals. There are various types, such as bipolar junction transistors (BJTs) and field-effect transistors (FETs).
**Why They Don't Obey Ohm's Law**: Transistors have nonlinear characteristics. In a BJT, for example, the relationship between the base current (I_B) and the collector current (I_C) is not linear; it's exponential due to the transistor’s active region. The collector-emitter voltage (V_CE) and collector current (I_C) also do not follow Ohm’s Law, as the transistor’s behavior depends on the control signal applied to the base (in BJTs) or gate (in FETs). The transfer characteristics of a transistor are governed by more complex relationships involving the transistor’s parameters and operating conditions.
**Characteristic Curves**:
- For BJTs, the collector current is related to the base current and the collector-emitter voltage through the transistor's current gain (\( \beta \)) and is described by the Ebers-Moll model.
- For FETs, the drain current (\( I_D \)) is related to the gate-source voltage (\( V_{GS} \)) and the drain-source voltage (\( V_{DS} \)), and it’s often described by quadratic or exponential equations depending on the operating region (linear or saturation).
In both cases, the voltage-current relationship is not straightforward and linear, thus not adhering to Ohm's Law.
Two common devices that do not obey Ohm's Law are:
1. **Diodes**: Diodes are semiconductor devices that allow current to flow in one direction only. The voltage-current relationship for a diode is non-linear. In the forward-biased region, the current increases exponentially with the applied voltage rather than linearly. This non-linear behavior means that a diode does not exhibit a constant resistance, hence it does not obey Ohm's Law.
2. **Transistors**: Transistors, specifically bipolar junction transistors (BJTs) and field-effect transistors (FETs), are also non-ohmic devices. The current through a transistor is not directly proportional to the voltage across it but is controlled by other parameters such as the base current (for BJTs) or gate voltage (for FETs). The relationship between voltage and current in transistors is complex and varies depending on their operating region, so they do not follow Ohm's Law.
Both of these devices have characteristics that make their electrical behavior more complex than the simple linear relationship described by Ohm's Law.
1. **Diodes**: Diodes are semiconductor devices that allow current to flow in one direction only. The voltage-current relationship for a diode is non-linear. In the forward-biased region, the current increases exponentially with the applied voltage rather than linearly. This non-linear behavior means that a diode does not exhibit a constant resistance, hence it does not obey Ohm's Law.
2. **Transistors**: Transistors, specifically bipolar junction transistors (BJTs) and field-effect transistors (FETs), are also non-ohmic devices. The current through a transistor is not directly proportional to the voltage across it but is controlled by other parameters such as the base current (for BJTs) or gate voltage (for FETs). The relationship between voltage and current in transistors is complex and varies depending on their operating region, so they do not follow Ohm's Law.
Both of these devices have characteristics that make their electrical behavior more complex than the simple linear relationship described by Ohm's Law.