13 Jul 2026

Dielectric Constant of Solid and Liquid Samples by Capacitance Method

practical pg-iii dielectric capacitance permittivity

Aim

To determine the dielectric constant of solid and liquid samples by measuring the capacitance of a parallel-plate capacitor.

Apparatus

Parallel-plate capacitor, LCR meter, solid dielectric sheets, liquid cell, micrometer, connecting leads, and insulating supports.

Experimental arrangement

The empty cell is measured first; the solid sheet or liquid sample is then placed between the capacitor electrodes and the capacitance is measured again.

Theory

Two conducting plates separated by distance $d$ store equal and opposite charges when a potential difference is applied. In air or vacuum, the capacitance is

\[C_0=\frac{\epsilon_0A}{d}.\]

When an insulating material is placed between the plates, its molecules polarise. The induced dipoles reduce the effective electric field for a given charge, so a larger charge is required to maintain the same voltage. The capacitance becomes

\[C=\frac{\epsilon_r\epsilon_0A}{d}=\epsilon_r C_0.\]

Therefore the relative permittivity or dielectric constant is

\[\epsilon_r=\frac{C}{C_0}.\]

For a liquid, the empty-cell capacitance is first measured and the cell is then filled without air bubbles. The same capacitance ratio is used after allowing the liquid to settle.

Observations

Empty-cell capacitance: $C_0=48$ pF.

Sample	Filled capacitance $C$ (pF)	Dielectric constant $\epsilon_r$
Glass	336	7.00
Mica	244	5.08
Transformer oil	98	2.04
Distilled water	216	4.50

Graph

Comparison of the measured dielectric constants of the supplied samples.

Calculation

For glass,

\[\epsilon_r=\frac{C}{C_0}=\frac{336}{48}=7.00.\]

Result

The dielectric constants are

\[\boxed{\epsilon_r(\text{glass})=7.00},\qquad \boxed{\epsilon_r(\text{mica})=5.08},\qquad \boxed{\epsilon_r(\text{oil})=2.04}.\]

Viva Questions

1. What is dielectric polarisation? It is the displacement or alignment of bound charges inside an insulating material.
2. Why must air bubbles be removed from the liquid cell? They introduce a second dielectric and alter the effective capacitance.
3. Why is an LCR meter used? It measures capacitance at a controlled AC frequency.