Parallel plate capacitor (A-level only) Flashcards – AQA A-level Physics (7408)

What is the capacitance of a parallel-plate capacitor in terms of plate area $A$, separation $d$, free-space permittivity $\varepsilon_0$ and relative permittivity $\varepsilon_r$?

$C = \frac{A\varepsilon_0\varepsilon_r}{d}$

What is relative permittivity, also called the dielectric constant?

The ratio of a material’s permittivity to the permittivity of free space. It has no units.

For the same plate geometry, how is relative permittivity related to capacitance with a dielectric and with a vacuum?

It is the ratio of capacitance with the dielectric to capacitance with a vacuum.

What assumptions about the dielectric and edge effects apply to the parallel-plate capacitance formula?

The dielectric is uniform, and edge effects are negligible.

What is a permanent electric dipole in a polar molecule?

A separation of positive and negative charges that gives the molecule oppositely charged ends.

Why does a polar molecule rotate towards alignment in an electric field?

Forces on its oppositely charged ends exert a turning effect.

What overall alignment does an electric field produce in a dielectric containing polar molecules?

A partial alignment of the molecular dipoles.

How does the electric field produced by polarisation relate to the applied field inside a dielectric?

It opposes the applied field.

Why does a polar dielectric increase a capacitor’s capacitance?

It reduces the field and potential difference for a given charge, allowing more charge to be stored per unit potential difference.