Devices called capacitors are used to store energy and electric charge. The dielectric material between a capacitor’s plates is one of the key elements that determines how much charge it can hold.
What is a Dielectric?
An insulating substance that can sustain an electrostatic field but does not carry electricity is called a dielectric. To put it simple, it is a non-conductive substance that, when exposed to an electric field, can become polarised.
Effect of Dielectric on Capacitance
Common examples of dielectric materials are tabulated below:
Air
Paper
Glass
Mica
Plastic
Ceramics
These materials are placed between the plates of a capacitor to increase its capacitance.
Capacitance Without Dielectric
First we review the formula for the capacitance of a parallel plate capacitor without a dielectric in order to understand the impact of a dielectric.
C0 = ε0A / d
Where:
C0 is the capacitance without dielectric
ε0 is the permittivity of free space
A is the area of the plates
d is the distance between the plates
Capacitance With Dielectric
A capacitor’s capacitance rises when a dielectric material is positioned between its plates. The updated capacitance turns into:
C = K ⋅ C0 = Kε0A / d
Here, K is called the dielectric constant or relative permittivity of the material. It is a measure of how well the material can increase the capacitance.
If K = 1, the material is like a vacuum (or air).
If K > 1, the material increases the capacitance.
Example: If a capacitor has a capacitance of 10 µF in air, and we insert a dielectric with K = 4, the new capacitance becomes:
C = 4 × 10 = 40 μF
Why Does Capacitance Increase?
A dielectric becomes polarised in the electric field when it is positioned between the plates. This indicates that the dielectric’s positive and negative charges line up in opposing directions. The effective electric field between the plates is lowered as a result.
Because the electric field is weaker, the capacitor can hold more charge for the same voltage, which means higher capacitance.
Effect on Electric Field and Potential
What happens when a dielectric is inserted while keeping the battery connected:
The charge on the plates increases.
The voltage across the plates stays constant (due to the battery).
The electric field decreases inside the dielectric.
The capacitance increases.
If the battery is disconnected before inserting the dielectric:
The charge remains constant (no path to flow).
The voltage across the plates decreases.
The electric field decreases.
The capacitance still increases.
Energy Stored in a Capacitor
The energy stored in a capacitor is given by:
U = 1 / 2CV2
So, inserting a dielectric changes the stored energy depending on whether the voltage or charge is kept constant.
If voltage V is constant (battery connected), energy increases because C increases.
If charge Q is constant (battery disconnected), energy decreases because U = Q2 / 2C and C increases.
When a conducting slab thickness t is less than d is introduced in the space between the plates of parallel plate capacitor, then the electric field inside conductor becomes zero and hence potential difference between the plates reduces.
V = E0 * (d-t) = Q / A ε0 (d – t) or Q / V = ε0 A/ d – t
C = ε0 A /A – t or C = C0 / (1 – t / d)
Capacitance of a partially filled capacitor (with dielectric of thickness t)
C = K ε0A / K(d-1)+1
Applications
Almost every capacitor in electronics uses dielectrics. The stability and dielectric constant of various materials are taken into consideration. For example:
Ceramic capacitors (high K)
Plastic film capacitors (stable and reliable)
Mica capacitors (used in radio circuits)
Summary
A dielectric is an insulating material that increases the capacitance of a capacitor.
It does so by reducing the effective electric field, allowing more charge to be stored.
The new capacitance becomes C = K ⋅ C0, where K is the dielectric constant.
The behavior depends on whether the battery is connected or disconnected during the insertion.
Dielectrics are widely used in electronics to enhance capacitor performance.
Any non-conductive substance that can become polarized in an electric field is called a dielectric. It influences the behaviour of electric fields between capacitor plates but prevents current from flowing through. Paper, plastic, glass and air are classic examples.
A capacitor’s capacitance is increased by a dielectric. It decreases the effective electric field when placed between the plates, increasing the capacitor’s capacity to hold charge at the same voltage. With K as the dielectric constant, the new capacitance is C = K ⋅ C0.
The dielectric constant (K), also called relative permittivity, is a number that tells us how effective a dielectric is at increasing capacitance.
K = 1 for a vacuum or air
K > 1 for other dielectrics
The higher the K, the greater the increase in capacitance.
Yes, adding a dielectric always makes a capacitor more capacitive. This is because more charge may be stored for the same potential difference when the dielectric weakens the electric field.
If the battery remains connected:
The voltage across the plates stays constant.
The charge increases.
The capacitance increases.
The energy stored in the capacitor also increases.
If the battery is disconnected before inserting the dielectric:
The charge remains constant.
The voltage decreases.
The capacitance increases.
The energy stored in the capacitor decreases.
Dielectrics are vital in electronics because they:
Increase the capacitance without needing larger capacitors
Improve energy storage
Provide insulation between plates
Allow compact, efficient circuit designs
that’s why materials like ceramic, plastic, and mica are widely used in capacitors.