Factors affecting capacitance:
The factors which affect the capacitance of a parallel-plate capacitor are:
- Overlapping area of the plates (A). The capacitance increases as the area of overlap increases since a larger plate area provides more room to accommodate the increase charge.
- Distance between the plates (d). The capacitance increases as the distance between the plates decreases, since the electric field then becomes more concentrated.
- Material between the plates. This introduces a constant called the absolute permittivity (ε). The constant ε is actually the product of two constants, the permittivity of space ( εo) which has a value of 885 x 10-12 Fm-1 and the relative permittivity (εr), which is basically a multiplication factor (no units) that indicates how many more times the material is able to concentrate the electric flux compared with space. For example, if waxed paper is inserted between the plates instead of air, the ability to concentrate a flux (the permittivity) is multiplied by approximately 3, therefore the relative permittivity ( εr) of waxed paper is approximately 3.
We may summarise this in equation form as:
C = εA/d
The units of 'C' are Farads if the units of the other quantities are:
- Area (a) - square metres (m2).
- Distance between plates (d) - metres (m).
- Absolute permittivity (ε) - farads per metre (Fm-1).
In the case of multi-plate capacitors, capacitance is calculated using the formula: C = [(n-1)εA]/d
Where n is the number of plates and A is the area of a single plate.