Schematic of a parallel plate capacitor with a dielectric spacer.  Two plates with area <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle A}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mi>A</mi></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle A}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/7daff47fa58cdfd29dc333def748ff5fa4c923e3" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.338ex; width:1.743ex; height:2.176ex;" alt="A"></span> are separated by a distance <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle d}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mi>d</mi></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle d}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e85ff03cbe0c7341af6b982e47e9f90d235c66ab" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.338ex; width:1.216ex; height:2.176ex;" alt="{\displaystyle d}"></span>.  When a charge <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \pm {}Q}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mo>±<!-- ± --></mo><mrow class="MJX-TeXAtom-ORD"></mrow><mi>Q</mi></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle \pm {}Q}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/64d4b94abea52155c75c458d161682aeee7821c5" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.671ex; width:3.646ex; height:2.509ex;" alt="{\displaystyle \pm {}Q}"></span> is moved between the plates, an electric field <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle E}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mi>E</mi></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle E}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/4232c9de2ee3eec0a9c0a19b15ab92daa6223f9b" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.338ex; width:1.776ex; height:2.176ex;" alt="E"></span> exists in the region between the plates.  The dielectric material becomes polarised due to the charge displacement, reducing the total internal field, and increasing the capacitance.
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<a href="//commons.wikimedia.org/wiki/File:Capacitor_schematic_with_dielectric.svg" title="File:Capacitor schematic with dielectric.svg">File:Capacitor schematic with dielectric.svg</a> by <a href="//commons.wikimedia.org/wiki/User:Papa_November" title="User:Papa November">Papa November</a>