Why does the terminal voltage of cell decrease when current is drawn from it?
2 Answers
The terminal voltage of a cell decreases when current is drawn from it due to several factors related to the internal resistance of the cell and the electrochemical processes occurring within it. Here are the main reasons:
1. **Internal Resistance**: Every cell has some internal resistance, which causes a voltage drop when current flows through it. This resistance is due to the materials and components within the cell, including electrolyte, electrodes, and other physical barriers. The voltage drop (\( V_{\text{drop}} \)) across the internal resistance can be calculated using Ohm's Law:
\[
V_{\text{drop}} = I \times r_{\text{internal}}
\]
where \( I \) is the current drawn and \( r_{\text{internal}} \) is the internal resistance.
2. **Electrochemical Reactions**: When a current is drawn from a cell, electrochemical reactions occur at the electrodes to supply energy. These reactions can lead to a decrease in concentration of reactants, affecting the cell's ability to generate voltage. As reactants are consumed and products accumulate, the cell’s efficiency may decrease, resulting in a lower terminal voltage.
3. **Voltage Regulation**: Cells are designed to maintain a certain voltage under no load conditions (open circuit). When a load is applied, the terminal voltage decreases because the cell cannot maintain its open-circuit voltage under load due to the aforementioned internal resistance and reaction dynamics.
4. **Temperature Effects**: Current flow can generate heat within the cell, which can affect the electrolyte's conductivity and the overall electrochemical processes. This change in temperature can lead to a further decrease in voltage.
5. **State of Charge**: As a cell discharges (especially in lead-acid or lithium-ion cells), its state of charge decreases, leading to a reduction in the cell's ability to maintain its nominal voltage under load.
In summary, when current is drawn from a cell, the internal resistance causes a voltage drop, and the ongoing electrochemical processes can further reduce the terminal voltage due to changes in reactant concentrations and other factors.
1. **Internal Resistance**: Every cell has some internal resistance, which causes a voltage drop when current flows through it. This resistance is due to the materials and components within the cell, including electrolyte, electrodes, and other physical barriers. The voltage drop (\( V_{\text{drop}} \)) across the internal resistance can be calculated using Ohm's Law:
\[
V_{\text{drop}} = I \times r_{\text{internal}}
\]
where \( I \) is the current drawn and \( r_{\text{internal}} \) is the internal resistance.
2. **Electrochemical Reactions**: When a current is drawn from a cell, electrochemical reactions occur at the electrodes to supply energy. These reactions can lead to a decrease in concentration of reactants, affecting the cell's ability to generate voltage. As reactants are consumed and products accumulate, the cell’s efficiency may decrease, resulting in a lower terminal voltage.
3. **Voltage Regulation**: Cells are designed to maintain a certain voltage under no load conditions (open circuit). When a load is applied, the terminal voltage decreases because the cell cannot maintain its open-circuit voltage under load due to the aforementioned internal resistance and reaction dynamics.
4. **Temperature Effects**: Current flow can generate heat within the cell, which can affect the electrolyte's conductivity and the overall electrochemical processes. This change in temperature can lead to a further decrease in voltage.
5. **State of Charge**: As a cell discharges (especially in lead-acid or lithium-ion cells), its state of charge decreases, leading to a reduction in the cell's ability to maintain its nominal voltage under load.
In summary, when current is drawn from a cell, the internal resistance causes a voltage drop, and the ongoing electrochemical processes can further reduce the terminal voltage due to changes in reactant concentrations and other factors.
The terminal voltage of a cell decreases when current is drawn from it due to the internal resistance of the cell. Here’s a detailed explanation of the factors involved:
### 1. **Internal Resistance**
Cells and batteries have internal resistance, which is the resistance to the flow of current within the cell itself. This resistance arises from the materials and the construction of the cell, including the electrolyte, electrodes, and connectors. When a current flows through the cell, the internal resistance causes a voltage drop inside the cell.
The relationship can be expressed by Ohm’s Law:
\[ V_{\text{internal}} = I \cdot R_{\text{internal}} \]
where:
- \( V_{\text{internal}} \) is the voltage drop across the internal resistance,
- \( I \) is the current flowing through the cell,
- \( R_{\text{internal}} \) is the internal resistance of the cell.
### 2. **Terminal Voltage**
The terminal voltage (\( V_{\text{terminal}} \)) is the voltage measured across the terminals of the cell when it is delivering current. It is given by:
\[ V_{\text{terminal}} = E - I \cdot R_{\text{internal}} \]
where:
- \( E \) is the electromotive force (EMF) of the cell, which is the voltage when no current is drawn,
- \( I \) is the current flowing from the cell,
- \( R_{\text{internal}} \) is the internal resistance of the cell.
### 3. **Effect of Current Draw**
When you draw current from the cell, the voltage drop across the internal resistance (\( I \cdot R_{\text{internal}} \)) reduces the terminal voltage. The higher the current drawn, the greater the voltage drop across the internal resistance, leading to a more significant decrease in the terminal voltage.
### 4. **Chemical and Physical Factors**
- **Chemical Reactions:** The internal resistance can increase if the cell's chemical reactions are not optimal or if the cell is aging, leading to increased resistance over time.
- **Electrolyte Concentration:** As current is drawn, the concentration of the electrolyte can change, which can affect the internal resistance.
- **Temperature:** Internal resistance can also be affected by temperature. Higher temperatures can decrease resistance, but in practical terms, high current draw often generates heat, which might lead to performance degradation.
### Summary
The decrease in terminal voltage when current is drawn from a cell is mainly due to the internal resistance of the cell. This internal resistance causes a voltage drop within the cell as current flows, leading to a reduction in the terminal voltage.
### 1. **Internal Resistance**
Cells and batteries have internal resistance, which is the resistance to the flow of current within the cell itself. This resistance arises from the materials and the construction of the cell, including the electrolyte, electrodes, and connectors. When a current flows through the cell, the internal resistance causes a voltage drop inside the cell.
The relationship can be expressed by Ohm’s Law:
\[ V_{\text{internal}} = I \cdot R_{\text{internal}} \]
where:
- \( V_{\text{internal}} \) is the voltage drop across the internal resistance,
- \( I \) is the current flowing through the cell,
- \( R_{\text{internal}} \) is the internal resistance of the cell.
### 2. **Terminal Voltage**
The terminal voltage (\( V_{\text{terminal}} \)) is the voltage measured across the terminals of the cell when it is delivering current. It is given by:
\[ V_{\text{terminal}} = E - I \cdot R_{\text{internal}} \]
where:
- \( E \) is the electromotive force (EMF) of the cell, which is the voltage when no current is drawn,
- \( I \) is the current flowing from the cell,
- \( R_{\text{internal}} \) is the internal resistance of the cell.
### 3. **Effect of Current Draw**
When you draw current from the cell, the voltage drop across the internal resistance (\( I \cdot R_{\text{internal}} \)) reduces the terminal voltage. The higher the current drawn, the greater the voltage drop across the internal resistance, leading to a more significant decrease in the terminal voltage.
### 4. **Chemical and Physical Factors**
- **Chemical Reactions:** The internal resistance can increase if the cell's chemical reactions are not optimal or if the cell is aging, leading to increased resistance over time.
- **Electrolyte Concentration:** As current is drawn, the concentration of the electrolyte can change, which can affect the internal resistance.
- **Temperature:** Internal resistance can also be affected by temperature. Higher temperatures can decrease resistance, but in practical terms, high current draw often generates heat, which might lead to performance degradation.
### Summary
The decrease in terminal voltage when current is drawn from a cell is mainly due to the internal resistance of the cell. This internal resistance causes a voltage drop within the cell as current flows, leading to a reduction in the terminal voltage.