25.7:
Capacitor With A Dielectric
Parallel plate capacitors consist of two conducting plates separated by a small distance. However, it is mechanically difficult to hold the plates closer without any contact. This can be resolved by placing a non-conducting medium sandwiched between the capacitor plates.
Most commonly, capacitors are made of long strips of metal foils separated by a dielectric medium. Dielectrics are non-conducting materials with no loosely bound or free electrons.
Consider a fully charged parallel plate capacitor with a dielectric slab filling the gap between the plates. The dipoles in the dielectric align along the direction of the electric field.
The alignment of the negative charges toward the positive plate and the positive charges toward the negative plate reduces the voltage across the plates, although the total charge remains the same. The decrease in voltage increases the capacitance.
The factor by which the capacitance increases is the material's dielectric constant. The presence of a dielectric always increases the capacitance. Hence, the dielectric constant is always greater than unity.
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):
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from Pp. 365–367 https://openstax.org/books/university-physics-volume-2/pages/8-4-capacitor-with-a-dielectric