Capacitors 03 - Dielectrics and Permittivity

Define permittivity.

Permitivity 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 in the material.

What is relative permittivity?

The ratio of permittivity of a material to the permittivity of free space.

It is also called the dielectric constant.

Give the equation for relative permittivity.

ε_r = ε₁ / ε₀

_{Relative Permittivity of material 1 = permittivity of material 1 / permittivity of free space}

What is a dielectric?

An electric insulator made of polar molecules (molecules that have positive and negative ends to them).

What happens in a dielectric capacitor when a charge is applied to one plate?

When a charge is applied to a plate, the negative ends of the polar molecules is attracted to the positively charged plate, and vice versa. This causes the randomly oriented dielectric molecules to rotate and align themselves anti-parallel to the electric field between the plates.

The molecules now have an opposing electric field. The larger the permittivity, the larger this opposing field is.

This reduces the overall field between the plates, reducing the p.d. needed to transfer a given charge to the capacitor, increasing the capacitance.

Give the equation for calculating capacitance from the area of one plate, the relative permittivity of a material and the distance between the plates.

Capacitance = (Area of one plate x relative permittivity x permittivity of free space) / distance between the plates

C = (A ε₀ ε_r) / d