25.7: Capacitor With A Dielectric
Parallel plate capacitors consist of two conducting plates separated by a certain distance. However, it is mechanically difficult to hold the large plates parallel to each other without actual contact. Hence, a dielectric layer is commonly placed between the plates, which provides an easy solution for holding the plates together with a small gap and increases the capacitance of the capacitor.
Dielectrics are non-conducting materials with no free or loosely bound electrons. When a dielectric is placed in an electric field, the dipoles align along the field's direction. The same phenomenon occurs when a dielectric layer fills the gap between the plates of a capacitor. The dipole alignment reduces the voltage across the plates, increasing the capacitance. However, the amount of charge is conserved.
The capacitance always increases when a dielectric layer is introduced between the plates, and the factor by which it increases is the dielectric constant. If C0 is the initial capacitance and C is the capacitance after introducing a dielectric, they are related in terms of the dielectric constant:
The electrical energy stored by a capacitor is also affected by the presence of a dielectric. When the energy stored in an empty capacitor is U0, the energy U stored in a capacitor with a dielectric is reduced by a factor of κ, as given in equation (2):
