EMF (Electromotive Force)
Definition and meaning of EMF (Electromotive Force) in chemistry.
EMF (electromotive force) is the maximum electrical potential difference a voltaic cell can produce, measured when no current flows through the external circuit. It reflects the total driving force pushing electrons from the anode to the cathode in an electrochemical cell.
In more detail
EMF is measured under zero-current (open-circuit) conditions using a high-resistance voltmeter, so no energy is lost to internal resistance. Once current flows, the measured terminal voltage drops below the EMF because of the internal resistance of the cell and electrode polarization. The standard cell EMF, E°cell, is calculated from standard reduction potentials as E°cathode − E°anode, and the Nernst equation relates the actual EMF to E°cell and the concentrations (activities) of reactants and species under nonstandard conditions.
Key facts
| Field | Physical Chemistry |
|---|---|
| SI Unit | volt (V) = J/C |
| Key Equation | E = E° − (RT/nF) ln Q (Nernst equation) |
| Measurement Condition | Zero current (open circuit) |
A Daniell cell (Zn/Zn2+ || Cu2+/Cu) has a standard EMF of +1.10 V, calculated from E°(Cu2+/Cu) = +0.34 V minus E°(Zn2+/Zn) = −0.76 V, reflecting the spontaneous transfer of electrons from zinc to copper.
Frequently asked questions
Is EMF the same as voltage?
Not exactly. EMF is the theoretical maximum potential difference at zero current, while the measured voltage (terminal potential) under load is lower due to internal resistance and polarization losses.
How is EMF related to Gibbs free energy?
EMF is related to the spontaneity of the cell reaction through ΔG° = −nFE°cell, where n is moles of electrons transferred and F is Faraday's constant; a positive EMF corresponds to a negative ΔG° and a spontaneous reaction.