Electromotive Force versus Electrical Potential Difference: Approaching (but Not Yet at) Equilibrium

• Autores: Leandro da Silva Rodrigues, Jones de Andrade, Luiz H. S. Gasparotto
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 95, Nº 10, 2018, págs. 1811-1815
• Idioma: inglés
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• Resumen

  • One of the most elusive concepts in electrochemistry is that of electromotive force (EMF). Often students, and even instructors, mistakenly take it as “electric potential difference” (EPD) or “voltage difference”. To clarify this issue properly, here we demonstrate an activity that was conducted with readily available and inexpensive resources. We employ the widely known Daniell Cell slightly modified to meet our need of measuring EMF and EPD. The key of the demonstration is to connect to the cell a potentiometer, which is an element that provides a controllable variable resistance. When the slider of the potentiometer is set to the maximum resistance, only negligible charge circulates, which brings the system to a partial equilibrium at which the potential annotated is exactly equivalent to the electromotive force. This is equivalent to opening the circuit of the cell. Upon turning the slider toward lower resistances, charge circulates with the electromotive force no longer measurable because the electrochemical cell is far from a reversible condition. Other aspects such as available work versus energy dissipation, maximum attainable work, and power comparisons are also discussed.