How EMF and Internal Resistance Affect Electric Circuits
A cell which we have already discussed in our previous articles or an electrochemical cell is a device that can be said as it is capable of obtaining electrical energy from chemical reactions or vice versa. We have definitely seen a cell the small one is called as AAA or AA batteries we use in our remotes.
An battery or we can also say that an electric battery is a device which is made up of two or we can say more cells that make use of the chemical energy which is stored in the chemicals and converts it into energy or the electrical energy.
Note: A battery which we often have seen in our day to day lives is used to provide a continuous steady current source that is by the way of providing constant EMF or Electromotive force to an electrical circuit or we can say a machine.
Cells Electromotive Force Explained
Each of the cells comprises two half-cells which are connected in series by a conductive electrolyte containing cations and the anions. One of the half-cell which is made up of the electrolyte and the electrode which is negative the Anode.
The one which is negatively charged ions also called as Anions or the migrate to the Anode. The other half-cell which generally includes the electrolyte and the electrode which is positive and the Cathode to which cations positively charged ions migrate.
How do Redox Reactions Occur?
The Redox reaction is said to be a reduction and oxidation process which occur simultaneously and this powers the battery. The Cations are reduced and it gains electrons at the cathode during charging while anions are oxidized by the process of losing electrons at the anode during charging. During this whole process of discharge the process is reversed. The electrodes which we already do not touch each other but are electrically connected by the electrolyte.
EMF Physics Class 12
An electric power supply is also a cell or the Electric cell. The Cells generate the electricity and also derive the reaction which is the chemical reactions. One or more cells which are electrochemical are batteries.
Every cell that has two terminals which were namely
The Anode:
The Anode is said to be an terminal from where the flow of current is in from out that is that it provides an incoming channel for the current to enter the circuit or the device.
The Cathode:
When there is present current which is in the device or the circuit which is electrical and there’s a voltage drop also which is in source voltage or battery source which is internal resistance. It is caused due to material electrolytic in batteries or other the source voltage.
The Internal Resistance which is r = (E – V)/I
Where, E is the emf of the device that is the V which is the potential difference that is between the device the capital letter I is the current in the device. The resistance which is the Internal Resistance is the result of the resistance in the battery or we can say that the accumulation in the battery. the equation which is used to derive this is as follows equations:
V = (E – Ir).
What is a Cell in Physics?
We know that current that is electrical current is the flow of charged particles. It is the flow of electrons that take through a circuit.
A cell is said to be a device that maintains the potential difference which exists between the two electrodes that too due to chemical reaction. A collection which is of two or more cells that are connected in parallel or series is called a Battery. Thus we will obtain the voltage which is required or current.
We know that a battery which is an energy source that converts energy which is said as the chemical energy to electrical energy. It is otherwise called a cell which is electrochemical. The energy that is stored in the form of chemical form which is inside a battery. The Batteries give us a perfect convenient source of energy for energizing devices without wires and cables. When it is connected to a circuit it produces energy or electrical energy.
A battery consists of two terminals that is a negative and a Positive terminal. The terminal which is positive is known as Cathode and the terminal which is negative is known as Anode. They are also known as the electrodes of a Cell. These electrodes will be dipped later in the study in a solution which is known as the electrolyte. It is liquid which is ionic and electricity which is conducted by them. The voltage which is said to be the output of a battery depends upon the elements that are used as electrodes the size which is of the electrodes and the type of electrolyte that is used in it.
FAQs on Understanding Cells: Electromotive Force and Internal Resistance
1. What is an electric cell as studied in Physics?
An electric cell is a fundamental device that converts chemical energy into electrical energy. It consists of two electrodes (a positive and a negative terminal) immersed in a substance called an electrolyte. Through chemical reactions within the electrolyte, a potential difference is established between the electrodes, which can drive current through an external circuit.
2. What is meant by the electromotive force (EMF) of a cell?
The electromotive force, or EMF (ε), of a cell is defined as the maximum potential difference between its two terminals when no current is being drawn from it (i.e., in an open circuit). It represents the total work done by the cell in moving a unit positive charge throughout the entire circuit, including through the cell itself. Its unit is the Volt (V).
3. What is the internal resistance of a cell?
The internal resistance (r) of a cell is the opposition to the flow of electric current offered by the materials inside the cell itself, namely the electrodes and the electrolyte. When current flows, this resistance causes a potential drop within the cell, leading to energy loss, usually in the form of heat. Its unit is the Ohm (Ω).
4. How is the EMF of a cell different from its terminal voltage?
The key difference lies in the circuit condition. EMF (ε) is the total potential difference when the cell is in an open circuit (no current), while terminal voltage (V) is the potential difference across the terminals when the cell is in a closed circuit and supplying current. Due to the voltage drop across the internal resistance (Ir), the terminal voltage is typically less than the EMF (V = ε - Ir).
5. What is the mathematical relationship between EMF, terminal voltage, and internal resistance?
The relationship is given by the equation: V = ε - Ir. In this formula:
- V is the terminal voltage across the cell.
- ε is the electromotive force (EMF) of the cell.
- I is the current flowing from the cell.
- r is the internal resistance of the cell.
6. Why does every practical chemical cell have some internal resistance?
Every practical cell has internal resistance because the components it's made of are not perfect conductors. The electrolyte, which facilitates the movement of ions between the electrodes, offers resistance to this ionic flow. Similarly, the electrode materials themselves have some resistance. This inherent opposition from the cell's physical and chemical makeup is unavoidable and manifests as internal resistance.
7. What are the main factors that affect the internal resistance of a cell?
The internal resistance of a cell depends on several factors:
- Area of the electrodes: Larger electrode area immersed in the electrolyte decreases the resistance.
- Distance between electrodes: Increasing the separation between electrodes increases the resistance.
- Nature and concentration of the electrolyte: The type of electrolyte and its concentration significantly impact its conductivity and thus the internal resistance.
- Temperature: For most cells, an increase in temperature decreases the internal resistance as it increases the mobility of ions in the electrolyte.
8. How does a cell's internal resistance impact its performance in a real-world application like starting a car?
A car's starter motor requires a very large current to operate. According to the formula V = ε - Ir, if the internal resistance (r) is high, a large current (I) would cause a significant voltage drop (Ir). This would drastically reduce the terminal voltage (V) available to the motor, preventing it from working. Therefore, car batteries are designed to have an extremely low internal resistance to deliver high current while maintaining a sufficient terminal voltage.
9. Is it possible for the terminal voltage of a cell to be greater than its EMF?
Yes, it is possible under a specific condition: when the cell is being charged. During charging, an external power source pushes current into the cell against its natural direction. In this case, the external source must overcome both the cell's EMF and its internal resistance. The equation becomes V = ε + Ir, making the terminal voltage (V) across the cell greater than its EMF (ε).
10. Can you explain why a new, unused battery shows a higher voltage reading than an old, used one?
Over time and with use, the chemical composition of a battery's electrolyte changes, and degradation can occur on the electrodes (like sulfation in lead-acid batteries). These changes almost always lead to an increase in the internal resistance (r) of the cell. Even if the EMF (ε) remains relatively constant, the higher internal resistance of an old battery causes a much larger internal voltage drop (Ir) when a load is connected. This results in a lower terminal voltage (V) and reduced ability to supply power effectively compared to a new battery with low internal resistance.
