Why does a lamp not obey Ohms 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, given that the temperature and material properties remain constant. Mathematically, it is expressed as:
\[ V = I \cdot R \]
where:
- \( V \) is the voltage across the conductor,
- \( I \) is the current flowing through the conductor,
- \( R \) is the resistance of the conductor.
For many simple conductors and resistors, Ohm's Law holds true because their resistance remains relatively constant over a range of applied voltages and currents. However, not all devices, like lamps, behave according to Ohm’s Law. Here's why:
### Non-Ohmic Behavior of Lamps
1. **Temperature Dependence**: The resistance of a lamp, especially an incandescent lamp, changes significantly with temperature. When you first turn on a lamp, its filament is cold and has a lower resistance. As the filament heats up due to the electrical current, its resistance increases significantly. This temperature dependence means that the relationship between voltage and current isn't constant, which violates the assumptions of Ohm’s Law.
2. **Non-linear Resistance**: In many types of lamps, especially incandescent ones, the resistance is not constant. For example, as the filament heats up, its resistance increases. This is because the filament is made of a material (typically tungsten) that has a positive temperature coefficient of resistance. As the temperature rises, the atomic vibrations increase, making it harder for electrons to pass through, thus increasing resistance.
3. **Non-Ohmic Devices**: Lamps can be considered non-Ohmic devices because their resistance changes with operating conditions. Unlike a resistor, where the resistance is constant and thus obeys Ohm’s Law, a lamp’s resistance varies with temperature and other factors, causing a non-linear relationship between voltage and current.
4. **Complex Behavior**: In addition to simple incandescent lamps, other types of lamps like fluorescent and LED lamps have even more complex behavior. They may involve electronic components like ballasts or drivers, which can introduce non-linearities and further deviate from Ohm's Law. For instance, a fluorescent lamp needs a ballast to regulate the current, and LED lamps have drivers that adjust the current and voltage, making their behavior more complex.
### Summary
A lamp does not obey Ohm's Law primarily because its resistance changes with temperature and other operational factors. Unlike an ideal resistor, the filament in a lamp heats up and its resistance increases significantly with higher currents, leading to a non-linear relationship between voltage and current. Additionally, various types of lamps incorporate electronic components that introduce additional complexities, making their behavior deviate from Ohm's Law.
\[ V = I \cdot R \]
where:
- \( V \) is the voltage across the conductor,
- \( I \) is the current flowing through the conductor,
- \( R \) is the resistance of the conductor.
For many simple conductors and resistors, Ohm's Law holds true because their resistance remains relatively constant over a range of applied voltages and currents. However, not all devices, like lamps, behave according to Ohm’s Law. Here's why:
### Non-Ohmic Behavior of Lamps
1. **Temperature Dependence**: The resistance of a lamp, especially an incandescent lamp, changes significantly with temperature. When you first turn on a lamp, its filament is cold and has a lower resistance. As the filament heats up due to the electrical current, its resistance increases significantly. This temperature dependence means that the relationship between voltage and current isn't constant, which violates the assumptions of Ohm’s Law.
2. **Non-linear Resistance**: In many types of lamps, especially incandescent ones, the resistance is not constant. For example, as the filament heats up, its resistance increases. This is because the filament is made of a material (typically tungsten) that has a positive temperature coefficient of resistance. As the temperature rises, the atomic vibrations increase, making it harder for electrons to pass through, thus increasing resistance.
3. **Non-Ohmic Devices**: Lamps can be considered non-Ohmic devices because their resistance changes with operating conditions. Unlike a resistor, where the resistance is constant and thus obeys Ohm’s Law, a lamp’s resistance varies with temperature and other factors, causing a non-linear relationship between voltage and current.
4. **Complex Behavior**: In addition to simple incandescent lamps, other types of lamps like fluorescent and LED lamps have even more complex behavior. They may involve electronic components like ballasts or drivers, which can introduce non-linearities and further deviate from Ohm's Law. For instance, a fluorescent lamp needs a ballast to regulate the current, and LED lamps have drivers that adjust the current and voltage, making their behavior more complex.
### Summary
A lamp does not obey Ohm's Law primarily because its resistance changes with temperature and other operational factors. Unlike an ideal resistor, the filament in a lamp heats up and its resistance increases significantly with higher currents, leading to a non-linear relationship between voltage and current. Additionally, various types of lamps incorporate electronic components that introduce additional complexities, making their behavior deviate from Ohm's Law.
Ohm's Law states that the voltage across a resistor is directly proportional to the current flowing through it, and this relationship is given by the formula:
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. This relationship assumes that the resistance \( R \) is constant. However, many lamps and light bulbs do not obey Ohm's Law exactly. This deviation occurs because the resistance of a lamp, particularly incandescent bulbs, changes with temperature.
Here’s a more detailed explanation of why a lamp might not obey Ohm’s Law:
### 1. **Temperature Dependency of Resistance**
Incandescent light bulbs operate by heating a filament (usually tungsten) until it glows and emits light. When the filament is first turned on, it is at room temperature and has a relatively low resistance. As the filament heats up, its resistance increases significantly.
- **Initial Resistance:** When the lamp is first switched on, the filament is cold, and its resistance is relatively low. For instance, an incandescent bulb might have a cold resistance of just a few ohms.
- **Operational Resistance:** As the filament heats up, its temperature rises, causing the resistance to increase. At operating temperature, the filament’s resistance can be several times higher than its cold resistance.
This change in resistance with temperature means that the relationship between voltage and current is not linear over time, and thus the lamp does not follow Ohm's Law precisely during its entire operation cycle.
### 2. **Non-Ohmic Materials**
Ohm's Law applies to ohmic materials, which are materials where the resistance is constant regardless of the applied voltage. However, the materials used in lamps, especially incandescent bulbs, are non-ohmic. For non-ohmic materials, the resistance changes with the applied voltage or with other external factors like temperature.
### 3. **Electrical Characteristics of Different Lamps**
Different types of lamps behave differently:
- **Incandescent Bulbs:** As discussed, their resistance increases significantly with temperature.
- **Fluorescent Lamps:** These lamps use gas discharge to produce light. The relationship between voltage and current in these lamps is more complex due to the presence of gas discharge tubes and other electronic components.
- **LED Lamps:** LEDs (Light Emitting Diodes) are semiconductor devices. Their current-voltage relationship is nonlinear because of the semiconductor properties, which means they do not follow Ohm's Law.
### 4. **Power Supply Characteristics**
In some cases, the behavior of a lamp might also be influenced by the power supply it’s connected to. For instance, if a lamp is connected to a dimmer switch or a power supply with varying voltage, the voltage-current relationship may become more complex, further deviating from Ohm’s Law.
### Summary
In essence, the primary reason a lamp does not obey Ohm’s Law is due to the temperature dependency of its resistance, especially in incandescent bulbs. The filament's resistance changes as it heats up, leading to a nonlinear relationship between voltage and current. For other types of lamps, the non-linear characteristics of the materials and components used also contribute to deviations from Ohm’s Law.
\[ V = IR \]
where \( V \) is the voltage, \( I \) is the current, and \( R \) is the resistance. This relationship assumes that the resistance \( R \) is constant. However, many lamps and light bulbs do not obey Ohm's Law exactly. This deviation occurs because the resistance of a lamp, particularly incandescent bulbs, changes with temperature.
Here’s a more detailed explanation of why a lamp might not obey Ohm’s Law:
### 1. **Temperature Dependency of Resistance**
Incandescent light bulbs operate by heating a filament (usually tungsten) until it glows and emits light. When the filament is first turned on, it is at room temperature and has a relatively low resistance. As the filament heats up, its resistance increases significantly.
- **Initial Resistance:** When the lamp is first switched on, the filament is cold, and its resistance is relatively low. For instance, an incandescent bulb might have a cold resistance of just a few ohms.
- **Operational Resistance:** As the filament heats up, its temperature rises, causing the resistance to increase. At operating temperature, the filament’s resistance can be several times higher than its cold resistance.
This change in resistance with temperature means that the relationship between voltage and current is not linear over time, and thus the lamp does not follow Ohm's Law precisely during its entire operation cycle.
### 2. **Non-Ohmic Materials**
Ohm's Law applies to ohmic materials, which are materials where the resistance is constant regardless of the applied voltage. However, the materials used in lamps, especially incandescent bulbs, are non-ohmic. For non-ohmic materials, the resistance changes with the applied voltage or with other external factors like temperature.
### 3. **Electrical Characteristics of Different Lamps**
Different types of lamps behave differently:
- **Incandescent Bulbs:** As discussed, their resistance increases significantly with temperature.
- **Fluorescent Lamps:** These lamps use gas discharge to produce light. The relationship between voltage and current in these lamps is more complex due to the presence of gas discharge tubes and other electronic components.
- **LED Lamps:** LEDs (Light Emitting Diodes) are semiconductor devices. Their current-voltage relationship is nonlinear because of the semiconductor properties, which means they do not follow Ohm's Law.
### 4. **Power Supply Characteristics**
In some cases, the behavior of a lamp might also be influenced by the power supply it’s connected to. For instance, if a lamp is connected to a dimmer switch or a power supply with varying voltage, the voltage-current relationship may become more complex, further deviating from Ohm’s Law.
### Summary
In essence, the primary reason a lamp does not obey Ohm’s Law is due to the temperature dependency of its resistance, especially in incandescent bulbs. The filament's resistance changes as it heats up, leading to a nonlinear relationship between voltage and current. For other types of lamps, the non-linear characteristics of the materials and components used also contribute to deviations from Ohm’s Law.