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Volta potential 1

Volta potential

Volta potential (also called Volta potential difference, or contact potential difference, or outer potential

difference, , delta psi) in electrochemistry, is the electrostatic potential difference between two points ("1" and

"2") in the vacuum:

point "1" close to the surface of metal M1;

point "2" close to the surface of metal M2

(or electrolyte);

where M1

and M2

are two metals that are in contact and in thermodynamic equilibrium.[1]

The Volta potential is named after Alessandro Volta.

Volta potential between two metals

When the two metals depicted here are in thermodynamic equilibrium with eachother as shown (equal Fermi levels), the vacuum electrostatic potential is not flat

due to a difference in work function.

When two metals are electrically isolated

from each other, an arbitrary potential

difference may exist between them.

However, when two different neutrally

charged metals are brought into electrical

contact (even indirectly, say, through a long

wire), electrons will flow from the metal

with a higher Fermi level to the metal with

the lower Fermi level until the Fermi levels

in the both phases are equal. Once this has

occurred, the metals are in thermodynamic

equilibrium with each other (the actual

number of electrons that passes between thetwo phases is usually small). Just because

the Fermi levels are equal, however, does

not mean that the electric potentials are

equal. The electric potential outside each material is controlled by its work function, and so dissimilar metals can

show an electric potential difference even at equilibrium.

The Volta potential is notan intrinsic property of the two bulk metals under consideration, but rather is determined

by work function differences between the metals' surfaces. Just like the work function, the Volta potential depends

sensitively on surface state, contamination, and so on.

http://en.wikipedia.org/w/index.php?title=Work_functionhttp://en.wikipedia.org/w/index.php?title=Work_functionhttp://en.wikipedia.org/w/index.php?title=Fermi_levelhttp://en.wikipedia.org/w/index.php?title=File%3AWork_function_mismatch_gold_aluminum.svghttp://en.wikipedia.org/w/index.php?title=Work_functionhttp://en.wikipedia.org/w/index.php?title=Electrostatic_potentialhttp://en.wikipedia.org/w/index.php?title=Fermi_levelhttp://en.wikipedia.org/w/index.php?title=Alessandro_Voltahttp://en.wikipedia.org/w/index.php?title=Electrolytehttp://en.wikipedia.org/w/index.php?title=Electrostatic_potentialhttp://en.wikipedia.org/w/index.php?title=Electrochemistry

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Volta potential 2

Measurement of Volta potential (Kelvin probe)

Kelvin probe energy diagram at flat vacuum configuration, used for measuring

Volta potential between sample and probe.

The Volta potential can be significant (of

order 1 volt) but it cannot be measured

directly by an ordinary voltmeter. A

voltmeter does not measure vacuum

electrostatic potentials, but instead the

difference in Fermi level between the two

materials, a difference that is exactly zero at

equilibrium.

The Volta potential however corresponds to

a real electric field in the spaces between

and around the two metal objects, a field

generated by the accumulation of charges at

their surfaces. The total charge over

each objects' surface depends on the capacitance between the two objects, by the relation whereis the Volta potential. It follows therefore that the value of the potential can be measured by varying the

capacitance between the materials by a known amount (e.g., by moving the objects further from each other), and

measuring the displaced charge that flows through the wire that connects them.

The Volta potential difference between a metal and an electrolyte can be measured in a similar fashion.[2]

The Volta

potential of a metal surface can be mapped on very small scales by use of a Kelvin probe force microscope. In this

case the capacitance change is not knowninstead, a compensating DC voltage is added to cancel the Volta

potential so that no current is induced by the change in capacitance. This compensating voltage is the negative of the

Volta potential.

References

[1] IUPAC Gold Book, definition of contact (Volta) potential difference. (http://goldbook. iupac.org/C01293. html)

[2][2] V.S. Bagotsky, "Fundamentals of Electrochemistry", Willey Interscience, 2006.

http://goldbook.iupac.org/C01293.htmlhttp://en.wikipedia.org/w/index.php?title=Kelvin_probe_force_microscopehttp://en.wikipedia.org/w/index.php?title=Capacitancehttp://en.wikipedia.org/w/index.php?title=Fermi_levelhttp://en.wikipedia.org/w/index.php?title=Voltmeterhttp://en.wikipedia.org/w/index.php?title=File%3AKelvin_probe_setup_at_flat_vacuum.svg

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