Dielectric and Polarisation: Definition, Application, Problems & Class 12 Physics Notes

An insulating material that does not allow the flow of electric current but supports an electrostatic field is called a dielectric. It is placed in a capacitor between the plates. An insulating material increases the ability to store charge by reducing the effective electric field in a capacitor.

Examples: Plastics, air, ceramic, mica, and glass.

Types of Dielectrics

1. Polar Dielectrics:

• Molecules have a permanent dipole moment
• Example: HCL and Water
• Randomly oriented with no external electric field.
• Partially aligned along the field in an external field.

2. Non-Polar Dielectric:

• Molecules have no permanent dipole moment
• Example: Oxygen, Nitrogen, and Carbon dioxide.
• In an external field, charge displacement with molecules produces a temporary dipole. 
  

1. Without Dielectric:

• Parallel plate capacitor with air/vacuum C0 = ε0A / d 

Here, 

• C0 is capacitance without a dielectric 
• ε0 is the permittivity of free space 
• A is the area of the plate 
• d is the space between plates 

2. With Dielectric

• When a dielectric slab of dielectric constant K fills the space:

C=KC0 

C = Kε0A / d 

• Effect: Capacitance increases by the dielectric constant K. 

In the electric field, the dielectric gets polarised. The opposing field, due to polarisation, reduces the effective electric field between plates. Potential difference decreases for the same charge. 

Two Common Cases 

Case 1: Constant Voltage 

• Capacitance increases the stored charge increases: Q′ = CV 
• More charge flows from the voltage source to the plates. 
• Energy stored increases: $U=\frac{1}{2}C{V}^2$

 Case 2: Constant Charge (Q) fixed: 

• Capacitance increases, voltage decreases: V′= Q / C 
•  Energy stored decreases: U = $U=\frac{Q^2}{2C}$

The polarisation is the process of shifting positive and negative charges inside molecules in the opposite direction when a dielectric is placed in an electric field.

• Without Electric Field: Due to molecules being randomly oriented, there is no net dipole moment.
• With Electric Field: Charges rearrange, creating tiny dipoles. 

Formula of Polarisation: 

P = Net dipole moment / Volume

Important Links: NCERT Solution for Class 11 & 12

Mechanism of Polarisation

1. Electronic polarization
2. Ionic polarization
3. Orientation polarization
4. Space charge polarisation

The maximum electric field that a dielectric material can oppose without undergoing electrical breakdown is the strength of the dielectric.

Formula: Dielectric Strength = Breakdown voltage/thickness of material. 

Factors Affecting Dielectric Strength 

1. Purity of material
2. Temperature
3. Moisture content
4. Frequency of applied Voltage

Related Topics:

Example 1: A parallel plate capacitor has capacitance 𝐶0 = 50pF with air. A dielectric ( 𝜀𝑟 = 4 ) is inserted to fill the gap. Calculate the new capacitance and the factor by which the electric field changes.

Solution: 

$C={\epsilon }_r{C}_0=4\cdot 50\times {10}^{-12}=200\mathrm{pF}$

Electric field reduction: $E^{\prime }=\frac{E_0}{{\epsilon }_r}=\frac{E_0}{4}$