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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Volta potential</span></span>
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<p>The <b>Volta potential</b> (also called <b>Volta effect,</b><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> <b>Volta potential difference</b>, <b>contact potential difference</b>, <b>outer potential difference</b>, Δψ, or "delta psi") in <a href="Electrochemistry" title="Electrochemistry">electrochemistry</a>, is the <a href="Electrostatic_potential" class="mw-redirect" title="Electrostatic potential">electrostatic potential</a> difference between two <a href="Metals" class="mw-redirect" title="Metals">metals</a> (or one metal and one <a href="Electrolyte" title="Electrolyte">electrolyte</a>) that are in contact and are in <a href="Thermodynamic_equilibrium" title="Thermodynamic equilibrium">thermodynamic equilibrium</a>. Specifically, it is the potential difference between a point close to the surface of the first metal and a point close to the surface of the second metal (or <a href="Electrolyte" title="Electrolyte">electrolyte</a>).<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
</p><p>The Volta potential is named after <a href="Alessandro_Volta" title="Alessandro Volta">Alessandro Volta</a>.
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<div class="mw-heading mw-heading2"><h2 id="Description">Description</h2></div>
<p>When two metals are electrically isolated from each other, an arbitrary potential difference may exist between them. However, when two different neutral metal surfaces are brought into electrical contact (even indirectly, say, through a long electro-conductive wire), electrons will flow from the metal with the higher <a href="Fermi_level" title="Fermi level">Fermi level</a> to the metal with the lower Fermi level until the Fermi levels in the two phases are equal.
Once this has occurred, the metals are in thermodynamic equilibrium with each other (the actual number of electrons that passes between the two 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 <a href="Work_function" title="Work function">work function</a>, and so dissimilar metals can show an electric potential difference even at equilibrium.
</p><p>The Volta potential is <i>not</i> an intrinsic property of the two bulk metals under consideration, but rather is determined by <a href="Work_function" title="Work function">work function</a> differences between the metals' surfaces. Just like the work function, the Volta potential depends sensitively on surface state, contamination, and so on.
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<div class="mw-heading mw-heading2"><h2 id="Measurement">Measurement</h2></div>
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</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="Kelvin_clip" class="mw-redirect" title="Kelvin clip">Kelvin clip</a>.</div>
<p>The Volta potential can be significant (of order 1 volt) but it cannot be measured directly by an ordinary <a href="Voltmeter" title="Voltmeter">voltmeter</a>.
A voltmeter does not measure vacuum electrostatic potentials, but instead the difference in <a href="Fermi_level" title="Fermi level">Fermi level</a> between the two materials, a difference that is exactly zero at equilibrium.
</p><p>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 <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Q}">
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</math></span><img src="./4fc55753007cd3c18576f7933f6f089196732029.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.766ex; height:2.176ex;" alt="{\displaystyle C}" loading="lazy"></span> between the two objects, by the relation <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Q=C\Delta \psi }">
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<annotation encoding="application/x-tex">{\displaystyle Q=C\Delta \psi }</annotation>
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<annotation encoding="application/x-tex">{\displaystyle \Delta \psi }</annotation>
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</math></span><img src="./56cd77c82e3bef469ed0c406ca046620a4d8d2fe.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.449ex; height:2.509ex;" alt="{\displaystyle \Delta \psi }" loading="lazy"></span> is 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.
</p><p>The Volta potential difference between a metal and an electrolyte can be measured in a similar fashion.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
The Volta potential of a metal surface can be mapped on very small scales by use of a <a href="Kelvin_probe_force_microscope" title="Kelvin probe force microscope">Kelvin probe force microscope</a>, based on <a href="Atomic_force_microscopy" title="Atomic force microscopy">atomic force microscopy</a>. Over larger areas on the order of millimeters to centimeters, a <a href="Scanning_Kelvin_probe" class="mw-redirect" title="Scanning Kelvin probe">scanning Kelvin probe</a> (SKP), which uses a wire probe of tens to hundreds of microns in size, can be used. In either case the capacitance change is not known—instead, 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.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Electrode_potential" title="Electrode potential">Electrode potential</a></li>
<li><a href="Absolute_electrode_potential" title="Absolute electrode potential">Absolute electrode potential</a></li>
<li><a href="Electric_potential" title="Electric potential">Electric potential</a></li>
<li><a href="Galvani_potential" title="Galvani potential">Galvani potential</a></li>
<li><a href="Potential_difference" class="mw-redirect" title="Potential difference">Potential difference</a> (voltage)</li>
<li><a href="Band_bending" title="Band bending">Band bending</a></li>
<li><a href="Volt" title="Volt">Volt</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.merriam-webster.com/dictionary/Volta%20effect">"Definition of VOLTA EFFECT"</a>. <i>www.merriam-webster.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">2025-01-28</span></span>.</cite></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://goldbook.iupac.org/C01293.html">IUPAC Gold Book, definition of contact (Volta) potential difference.</a></span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text">V.S. Bagotsky, "Fundamentals of Electrochemistry", Willey Interscience, 2006.</span>
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