13 Jul 2026

Dielectric Constant of a Material as a Function of Frequency

practical ug-vi dielectric frequency capacitance

Experimental arrangement

The specimen is placed between capacitor electrodes and the LCR instrument measures the frequency-dependent capacitance.

Aim

To determine the dielectric constant of a material at different frequencies.

Apparatus

Parallel-plate capacitor, dielectric specimen, LCR meter, oscillator, and connecting leads.

Theory

For a parallel-plate capacitor, insertion of a dielectric changes the capacitance from $C_0$ to $C$. The relative dielectric constant is

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

At high frequency, orientational polarisation cannot follow the applied field and the dielectric constant generally decreases.

Observations

Frequency (kHz)	$C_0$ (pF)	$C$ (pF)	$\epsilon_r$
1	102	408	4.00
10	101	394	3.90
100	100	360	3.60
1000	99	322	3.25

Graph

Frequency dependence of the measured relative dielectric constant.

Result

The dielectric constant decreases from approximately $4.0$ at low frequency to $3.25$ at $1\,\text{MHz}$ in the sample range.

Precautions

1. Keep the dielectric specimen flat between the plates.
2. Remove stray capacitance by proper instrument zeroing.
3. Use screened leads at high frequency.

Viva Questions

1. What is dielectric polarisation? It is the displacement or orientation of bound charges in an electric field.
2. Why does dielectric constant fall with frequency? Slow polarisation mechanisms cannot follow rapid field reversals.
3. What is relative permittivity? It is the ratio of the material permittivity to vacuum permittivity.

Maxima Code

Download the Maxima calculation file.