Electromotive Force
Definition and meaning of Electromotive Force in chemistry.
Electromotive force (EMF) is the maximum potential difference between the two electrodes of a galvanic cell, measured when no current flows through the circuit. It represents the total energy per unit charge available to drive electrons from the anode to the cathode through an external circuit.
In more detail
EMF arises from the difference in reduction potentials of the cell's two half-reactions and reflects the cell's full thermodynamic driving force before any energy is lost to internal resistance. It is measured with a potentiometer or high-impedance voltmeter that draws essentially zero current, since any current flow causes an IR drop that lowers the observed terminal voltage below the true EMF. Under standard conditions, EMF equals E°cell = E°cathode − E°anode, and it connects directly to thermodynamics through ΔG° = −nFE°cell, linking electrochemistry to spontaneity and equilibrium constants.
Key facts
| Symbol | E or ε |
|---|---|
| SI Unit | volt (V) |
| Key Relation | ΔG° = −nFE°cell |
| Field | Physical Chemistry |
For the Daniell cell, Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s), the standard EMF is E°cell = E°(Cu2+/Cu) − E°(Zn2+/Zn) = 0.34 V − (−0.76 V) = 1.10 V.
Frequently asked questions
Is EMF the same as terminal voltage?
No. EMF is the theoretical maximum, measured at zero current; the terminal voltage under load is lower, V = E − Ir, because of the cell's internal resistance.
How is EMF measured experimentally?
With a potentiometer or a high-impedance voltmeter that draws negligible current, avoiding the IR drop that would otherwise reduce the measured value.