Electromotive Force
The strength of the pile is expressed in terms of its electromotive force, or emf, given in volts. Volta characterized the emf of a pair of metals in terms of the difference in their voltages, which he could measure. His theory of contact tension considered that the emf, which drives the electric current through a circuit containing a voltaic cell, occurs at the contact between the two metals.
The emf between the ends of the pile is the number of cells multiplied by the difference of the standard electrode potential for each of the two half reactions in each cell. For the 6-cell pile illustrated above, a table of standard electrode potentials gives
- Cu2+ + 2 e− Cu with E0 = +0.34 V
- Zn2+ + 2 e− Zn with E0 = −0.76 V
giving a standard potential for the reaction of +0.34 V − (−0.76 V) = 1.10 V for each cell. Joining six cells in series results in a total potential difference of 6.60 V. As zinc has a more negative standard electrode potential than copper, it forms the anode or negative terminal, while copper forms the cathode or positive terminal.
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