Volta

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Introduction

(18 February 1745 – 5 March 1827) Encyclopedia Britannica

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Voltaic pile

A Voltaic pile (from Wikipedia) This illustration shows the basic components in a voltaic pile; Metal and "soaked" disks, along with common terminals to the entire pile, and annotations. The numbers in the picture refers to:

1. One element 2. Copper disc 3. Negative terminal for the entire pile 4. Positive terminal for the entire pile 5. Soaked disc of cardboard or leather, with acidic or alkaline solution. 6. Zinc disc

Reference
The Voltaic pile has n wet contacts and n dry contacts. The voltage between the top and the bottom is n times the voltage of a single wet contact, VCu/wet/Zn .

There are no voltages across the metal contacts; there are electric fields, all aligned in the same direction. The potential comes from the dry contacts, the voltage from the wet contacts.

R.M. Lichtenstien in Nineteenth-Century Attitudes: Men of Science, Ross, S. 1991.


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"Early failures"

EarlyFailuresVoltaicPileA.png
Alternating Cu/Zn plates, with dry contacts. No voltage at all.
EarlyFailuresVoltaicPileB.png
Alternating Cu/Zn plates, with wet contacts. The voltage of just one cell.


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Volta potential

Volta potential (also called Volta potential difference, or contact potential difference, or outer potential difference, Δψ) in electrochemistry, is the electric potential difference between two points in the vacuum: (1) close to the surface of metal M1 (2) close to the surface of metal M2; where M1 and M2 are two uncharged metals brought into contact.

When two metals are electrically isolated from each other, an arbitrary potential difference may exist between them. However, when two different metals are brought into contact, electrons will flow from the metal with a lower work function to the metal with the higher work function until the electrochemical potential of the electrons in the bulk of both phases are equal. The actual numbers of electrons that passes between the two phases is small, and the occupancy of the Fermi levels is practically unaffected.


"Electrochemical cell"

The wet contacts in a Voltaic cell actually form an electrochemical battery. Originally, the Zinc and Copper plates had cardboard soaked in salt water between them (that acted as the electrolyte).

"An electrochemical cell is a device used for generating an electromotive force (voltage) and current from chemical reactions. The current is caused by the reactions releasing and accepting electrons at the different ends of a conductor. A common example of an electrochemical cell is a standard 1.5-volt battery. Batteries are composed of usually multiple Galvanic cells. An electrochemical cell consists of two half-cells. The two half-cells may use the same electrolyte, or they may use different electrolytes. Each half-cell consists of an electrode, and an electrolyte. One half-cell undergoes oxidation, and the other one undergoes reduction. " [[1]]

ElectrochemCell.png

"Finally, a salt bridge [shown] is often employed to provide electrical contact between two half-cells with very different electrolytes—to prevent the solutions from mixing"[[2]]

"As electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate." [[3]]


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New-Age “voltaic cells”

With a major energy crisis upon us, people around the world are looking for alternative and clean sources of energy. One of the most promising options is, of course, solar energy. The question of how to turn sunrays into a usable (and storable) form of energy is an extremely important scientific question. Luckily, many smart and socially savvy scientists have worked to develop what is known as a solar, or photovoltaic, cell. These cells have many applications today: they power electricity in houses, calculators, radios, flashlights, and more. Giant solar cell grids have been created that disseminate electricity to a vast number of people.

Solar cells are large area electronic devices that convert solar energy into electricity via the photovoltaic effect. This effects is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or, in the case of solar energy, visible light.

The photovoltaic effect was first recognized in 1839 by A. E. Becquerel (a French physicist). Charles Fritts built the first solar cell in 1883 by coating selenium (a semiconductor) with an extremely thin layer of gold to form junctions. The device was about 1% efficient (which by today’s standards is not very good!) In 1954 Bell Labs discovered that silicon doped with some impurities was very sensitive to light, which lead the development of the first solar-powered satellite. These cells have efficiencies of around 6%. Since then, solar cells have become more efficient and sophisticated. Today advanced cells (many of which utilize a gallium-arsenic heterostructure) have efficiencies reaching 40%, which is pretty exciting. Note that high efficiency solar cells generate electricity at higher efficiencies than conventional solar cells. This is obviously advantageous as we want to be able to convert as much solar energy as possible into usable electricity.

A simple explanation as to how solar cells work is the following: First photons (ie. sunlight) hit solar panels and are absorbed by semiconducting materials (a classic example of such is silicon). Then electrons escape their binding atoms, which allows them to flow through the material to produce electricity. Whether or not a flow of electrons occurs depends on the energy of the photons. If the photons are too low in energy they pass right through the semiconductor. These electrons only flow in one direction. The complementary positive charges (which are called holes) flow in the opposite direction. This electron flow is then converted into a usable amount of direct current electricity.

This is only a very brief introduction to solar cells (with some history included to parallel the historic evolution of voltaic cells in class). Perhaps some other people can fill in more information about how solar cells work, advances such as thin-film processing or metamorphic multijunction solar cells, and the future of research for this technology.


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