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The EMF (Electromotive Force) of a cell is defined as the maximum potential difference between its two terminals when no current is being drawn from the cell (i.e., in an open circuit).
In more technical terms, it is the work done by the cell's non-electrical (chemical) forces in moving a unit of positive charge from the negative terminal to the positive terminal inside the cell.
Key Points:
Misleading Name: The term "force" is a historical misnomer. EMF is not a force; it is a potential difference, or energy per unit charge.
Unit: The SI unit for EMF is the Volt (V), which is equivalent to a Joule per Coulomb (J/C).
Ideal Voltage: EMF represents the "ideal" or "total" voltage a cell can supply. In practice, the voltage you measure when the cell is in use (called the terminal voltage) is slightly lower due to internal resistance.
Symbol: EMF is usually denoted by the symbol ℰ or E.
Analogy: The Water Pump
Imagine a water pump lifting water to a certain height.
The EMF (ℰ) of the cell is like the maximum pressure the pump can generate, which determines the maximum height it can lift the water.
The electric charge is like the water.
The wires of the circuit are like the pipes the water flows through.
The internal resistance (r) of the cell is like the friction or inefficiency inside the pump itself. When the pump is running, some of its energy is lost internally.
Because of this internal resistance, the actual voltage available to the external circuit (the terminal voltage, V) is given by the equation:
V = ℰ - Ir
where I is the current and r is the internal resistance. When the circuit is open, I = 0, and therefore V = ℰ.
The EMF of a cell is determined by its fundamental chemistry. It depends on:
The Nature of the Electrodes: The EMF is a direct result of the difference in the chemical potentials of the two materials used for the anode (negative electrode) and the cathode (positive electrode). Different pairs of materials (e.g., zinc-carbon vs. lithium-ion) produce different potential differences.
* Example: A standard Daniell cell using zinc and copper electrodes has an EMF of about 1.1V. A common alkaline battery using zinc and manganese dioxide has an EMF of about 1.5V.
The Nature and Concentration of the Electrolyte: The electrolyte is the medium that contains ions and facilitates the chemical reaction. Its chemical composition and the concentration of ions within it directly influence the electrode reactions and, therefore, the cell's EMF.
* Example: In a lead-acid car battery, the EMF changes slightly depending on the concentration (specific gravity) of the sulfuric acid electrolyte.
Temperature: Since the EMF is a product of chemical reactions, and the rates and equilibrium of chemical reactions are temperature-dependent, the EMF of a cell also varies with temperature. For most cells, this effect is relatively small under normal operating conditions but can become significant at extreme temperatures.
It is equally important to know what EMF does not depend on, as these are common misconceptions:
The Size or Shape of the Electrodes: A large 'D' cell battery and a small 'AAA' battery can both have an EMF of 1.5V if they are made of the same materials. The size affects the capacity (how long the battery lasts) and the internal resistance (larger batteries generally have lower internal resistance), but not the EMF itself.
The Amount of Electrolyte: Similar to the size of the electrodes, having more electrolyte increases the cell's capacity, allowing it to supply current for a longer time, but it does not change the voltage (EMF).
The Distance Between the Electrodes: The separation between the electrodes primarily affects the cell's internal resistance, not its EMF. A larger distance increases the path ions must travel, increasing internal resistance.
