1. What a Cell Really Does
A cell simply provides energy to move charges. Inside the cell, chemical reactions push positive and negative charges apart, creating a potential difference. When we connect the cell in a circuit, this potential difference drives current.
The cell tries to maintain this separation of charges as long as the chemicals allow.
1.1. Chemical Action and Charge Separation
Inside the cell, chemical energy is converted into electrical energy. Positive charges accumulate at one terminal and negative charges at the other. This separation creates an electric field that pushes charges in the external circuit.
2. Understanding EMF
EMF (electromotive force) is the total work done by the cell to move a unit positive charge from one terminal to the other inside the cell.
Even though the name contains the word 'force', EMF is actually a potential difference.
2.1. Formal Definition
EMF of a cell is the energy supplied by the cell per unit charge when no current is being drawn from it.
\( E = \dfrac{W}{q} \)
This is the maximum possible voltage the cell can provide.
2.1.1. Key Point
EMF is measured when the circuit is open (no current flowing). That’s why the ideal reading is taken without any load.
3. Internal Resistance of a Cell
Every real cell has some resistance inside it. This resistance comes from the chemicals and the internal structure of the cell. It is called internal resistance.
When current flows, some energy is used to overcome this internal resistance, so the voltage available at the terminals is less than the EMF.
3.1. Definition of Internal Resistance
Internal resistance is the opposition offered by the cell’s internal components to the flow of current.
It increases with age, decreases when chemicals are fresh, and depends on temperature.
4. EMF vs Terminal Voltage
The voltage measured across the terminals of a cell when current is flowing is called terminal voltage. It is usually less than EMF because some voltage is lost inside the cell due to internal resistance.
4.1. Relation Between EMF and Terminal Voltage
If current I flows through a cell of EMF E and internal resistance r, then the terminal voltage V is:
\( V = E - Ir \)
This loss \( Ir \) is often called 'internal drop'.
4.1.1. Interpretation
More current → larger internal drop → terminal voltage decreases.
When no current flows (open circuit), terminal voltage = EMF.
5. Current Supplied by a Cell
The current in an external circuit depends on both the external resistance and the internal resistance of the cell.
5.1. Formula for Current
For a cell connected to an external load resistance \( R \):
\( I = \dfrac{E}{R + r} \)
This clearly shows that internal resistance reduces the available current.
5.1.1. Meaning of the Formula
If internal resistance is small, most of the EMF appears across the external circuit. If internal resistance is large, the cell behaves weakly and supplies much less current.
6. Example to Visualise Internal Drop
Suppose a cell has EMF \( 1.5\,\text{V} \) and internal resistance \( 0.5\,\Omega \). When a current of \( 1\,\text{A} \) flows, the voltage available at the terminals is:
\( V = 1.5 - (1\times 0.5) = 1.0\,\text{V} \)
This reduction happens entirely inside the cell.