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Dielectrics

Permittivity
  • Altering the properties of a capacitor changes how much charge it can store at a given voltage (its capacitance).
  • One of the things you can change is the dielectric material separating the two conducting plates. This changes the capacitance because different materials have different relative permittivities.
  • Permittivity is a measure of how difficult it is to generate an electric field in a medium. The higher the permittivity of a material, the more charge is needed to generate an electric field of a given size.
  • Relative permittivity is the ratio of the permittivity of a material to the permittivity of free space:

Equation for permittivity
\[\varepsilon_{r}=\frac{\varepsilon_{1}}{\varepsilon_{0}}\]
Equation for the capacitance of a capacitor
\[C=\frac{A\varepsilon_{0}\varepsilon_{r}}{d}\]
Practise Questions: Application
  1. \(\frac{1.99\times10^{-11}}{8.85\times10^{-12}}\)
  2. \(\frac{2.7\times10^{-5}\times 86\times 8.85\times10^{-12}}{0.0005}=4.10994\times 10^{-11}\)
  3. \(16.6nF\times\frac{2.55}{3}=14.11nF\)
Practise Questions: Fact Recall
  1. A dielectric material is an insulating material that is placed between two plates of a capacitor.
  2. Permittivity is how difficult it is to generate an electric field in a certain medium.
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