Electromotive Force, commonly abbreviated as “EMF”, is one of the most fundamental concepts in the study of electricity and magnetism. Despite its name, EMF is not a force but quite a measure of the energy provided by a source such as a cell, battery or generator to move electric charges in a circuit. It represents the work done per unit charge in moving the charge once around the complete circuit.
Definition of Electromotive Force (EMF)
The “Electromotive Force” of a source is defined as the work done by the source in moving a unit positive charge across the entire circuit (including both external and internal parts of the circuit).
Mathematically, it can be expressed as: € = W / q
Where:
* € = EMF of the cell (in volts)
* W = Work done by the source (in joules)
* q = Charge moved (in coulombs)
The SI unit of EMF is volt (V), which is equivalent to joule per coulomb.
Nature of EMF
Although the term includes the word “force,” EMF does not represent a mechanical force. Instead, it is a potential difference generated by non-electrostatic forces inside the source.
For example, in a battery, chemical reactions separate charges and maintain a potential difference between the terminals. In a generator, it is the result of electromagnetic induction. Thus, EMF can be considered the “driving energy” that keeps the current flowing in a circuit.
Sources of EMF
Electrochemical cells – e.g., primary cells like dry cells and secondary cells like lead-acid batteries.
Electromagnetic induction – produced in generators, dynamos, or transformers.
Thermoelectric effect – generated when two dissimilar metals are joined and heated at one junction.
Photovoltaic effect – solar cells convert light energy into electrical EMF.
Each source uses different physical processes to maintain the separation of charges and provide EMF.
EMF and Potential Difference
It is important to distinguish between EMF and potential difference (p.d.):
* EMF refers to the energy supplied by the source to move unit charge across the entire circuit (open or closed). It is the cause of current.
* Potential difference refers to the energy used per unit charge when charges move between two points in the circuit (generally across a resistor, bulb or any other element).
In short, EMF is like the total energy available per charge, while potential difference is the actual energy consumed per charge in specific components.
Internal Resistance of a Cell
Every real source of EMF possesses some internal resistance (r) due to the opposition offered by the electrolyte or internal components. When a current flows through the cell, this internal resistance causes a voltage drop inside the source itself.
If V is the terminal potential difference across the cell, E is the EMF, and I is the current drawn, then the relation is: V = E – Ir
This shows that the terminal voltage is always less than the EMF when the current is flowing, due to energy lost in overcoming internal resistance. Only in open circuit (no current), terminal voltage equals EMF.
Measurement of EMF
EMF can be measured using instruments such as:
Potentiometer – gives the most accurate measurement as it is a null method, not drawing any current from the cell.
Voltmeter – can measure EMF if connected across a cell in open circuit, but not as precise due to internal resistance of the meter.
Factors Affecting EMF
Nature of source – type of electrolyte or material.
Temperature – especially in thermoelectric and electrochemical cells.
Concentration of electrolyte – in cells like Daniell cell, concentration affects EMF.
Design and construction of source – such as area of electrodes or efficiency of generator.
Applications of EMF
* Used in defining and comparing the efficiency of cells and batteries.
* Helps in understanding the working principle of electrical circuits, generators, and transformers.
* Important in experiments like potentiometer and meter bridge.
* Solar panels and thermocouples depend on the concept of EMF for energy conversion.
