Voltaic Pile

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Edited by Pichet Adstamongkonkul, AP225, Fall 2011

Introduction

A voltaic pile is a set of individual Galvanic cells, consisting of two different metal plates, placed in series. The voltaic pile was invented by Alessandro Volta in the late 18th century and known as the first battery. He could demonstrate that when two metals with salt solution-soaked cloth or cardboard were arranged in series, they can produce electric current.[1]

VolaticPile-Kenyon .png

The setup of Voltaic pile and how it works

VoltaicPile.png

A voltaic pile comprises of n wet contacts, in which the two metals, copper and zinc, are separated by soaked disc of cardboard or cloth with acidic/alkaline solution (or copper sulphate solution in the later improvement by Daniell to prevent the formation of hydrogen gas and subsequent polarization of the original silver plates), and n dry contacts, in which the two metals are touching each other directly[2]. Since the two different metals have different electrochemical potentials of electrons, when the two metals come into contact, the electrons rapidly flow from one metal to another, since they are both conductive, until the electrochemical potentials of electrons balance out. As a result, there is no potential difference or voltage between the two metals in dry contacts, but there is transfer in electrons between them.

However, since the two metals have different electrochemical potentials or, in other words, there are different numbers of electrons in each metal, this generates an electric field across the two metal plates. Since the two metals that form a Galvanic cell always have the same arrangement (Zinc, acting as a negative end, sitting on top of Copper, acting as a positive end of the cell), the electric fields are all aligned in the same direction.

This paragraph is a little shaky. Electrochemical potentials are not related to different numbers of electrons.
Why is there an electric field across two metal plates?

Across the wet contacts, when the metal plates are separated, there is no change in the voltage across the plates, which indicates no electric field between them is formed. The water molecules in the soaked cardboard become ionized and form electric double layers on the surfaces of the metals and neutralize the effect between the charges on the surface. However, the voltage across the wet contact still exists because of the imbalance number of electrons.

Why is there no voltage across the wet contact? Seems to me that voltage is easily measured.

Regarding the electrochemical reactions, the acid in the cardboard or the cloth will try to dissolve some of the zinc discs. This causes each zinc atom to give away two of its electrons, and becomes ionized. The electrons can flow through the soaked cloth to the copper plate. Since the zinc ions are electrophiles compared to the neutral copper, they would “steal” electrons from the copper atoms of the next plate in dry contact. The zinc ions then turn back to “normal.”[3] If the pile is arranged so that there are only wet contacts, the voltage across the whole pile will be equal to that of a single cell, since there are alternating increases and decreases in the voltage when one goes from one cell to the next. In contrast, a pile with only dry contacts would have the voltage across the pile equal to zero, as there is no potential difference between each metal plates. In the case when the wet contacts and dry contacts are alternating, the voltage can be built up through the pile. When the whole pile is connected to an external “load”, this generates a current as the electrons flow through the closed circuit. This becomes the fundamental concept in building modern batteries.[4]

References

[1] Wikipedia contributors. "Voltaic pile." Wikipedia, The Free Encyclopedia. 26 Nov 2011.

[2] Lecture on Charged interfaces, AP225, Fall 2011

[3] Wikipedia contributors. "Voltaic pile." Simple English Wikipedia. 3 Dec 2011.

[4] Franco, Decker. "Volta and the "Pile"." Electrochemistry Encyclopedia. N.p., Jan 2005. <http://electrochem.cwru.edu/encycl/art-v01-volta.htm>.

Keyword in references:

Controlling the Kinetics of 'Contact Electrification' with Patterned Surfaces