Jawaharlal Nehru Government Engineering College,

Sundernagar

Presentation on

Topic:- Dielectric properties of FibreSubmitted to:- Er. Priya Jaswal

Submitted by:- Suraj Raghuvanshi

T.E.-4th sem.

What is Dielectrics?

• Materials such as glass, ceramics, polymers and paper are non-conducting

materials i.e. they prevent flow of current through them.

• When the main function of these material to provide electrical insulation, they

are called insulators.

• When these materials are placed in an electric field, then charge storage is

main function, the materials are called dielectric.

Dielectric Properties• The permittivity, ε, of a material may be defined either in terms of the capacitance, C, of a

condenser with the material between parallel plates of area A and separation d,

or

In terms of the force F between two charges Q1 and Q2 at a distance r in the material.

• The ratio of permittivity of material to permittivity of vacuum is called the relative

permittivity or dielectric constant.

i.e. εr = ε/ε0

where ε0 is the permittivity of a vacuum, which is a fundamental physical quantity with the

value 8·854 × 10–12 F/m.

Dielectric Properties

• Physically, dielectric effects are due to polarisation in the medium (Fig. 21.1).

• This gives rise to a reverse field, which reduces the force between two charges and reduces

the potential difference between the charged plates of a condenser, which thus increases its

capacitance (given by charge/potential difference).

Dielectric Properties

• The polarization may be due either to the alignment of permanent dipoles, such as the water

molecule (Fig. 21.2), or to the separation of charge, which forms induced dipoles (Fig. 21.3).

• Because of its influence on capacitance, the relative permittivity is important in alternating

current electricity.

Measurement

• To measure the dielectric properties, the material must be placed between the plates of a condenser.

• In case of fibres they can be placed parallel or perpendicular to the plate.

• In case of yarn conical electrodes are provided and a layer of yarn, about 2 mm thick, is wound on

the inner cone, and the outer cone is then pressed on it.

• With this arrangement, densities of packing of about 80% by volume were obtained with

continuous-filament yarns, and of about 50% with staple-fibre yarns.

Factor Affecting Dielectric Properties

• Effect of Frequency• Effect of Moisture • Effect of Temperature• Presence of Impurities

The Effect of Frequency

The Effect of Frequency• At low frequency, the dipoles line up in the field, reverse direction when the field

reverses, and so contribute to a high permittivity.

• Dipoles take certain time to reverse direction, i.e. relaxation time.

• As time taken for reversal is small, the energy loss will be low.

• At higher frequency, the reversals of field takes place at intervals comparable to the

relaxation time dipole will stop to follow the change, due to which force will increases and

permittivity will decreases.

• So at high frequency movement of dipole is negligible therefore energy loss will be low.

• If frequency corresponds to relaxation time, dipole are moving throughout the cycle the

energy loss will be high.

The Effect of Moisture

• At high frequency permittivity will be almost constant as the moisture constantly

increases.

• At low frequency, as moisture constantly increases → permittivity also increases.

The Effect of Temperature

• A rise in temperature will results in reduction in restraints.

• Due to which dipole get disorganized and less regular alignment of dipole will decreases

the permittivity of liquid and gases.

• For solid material, rise in temperature will reduce restraints, polarization will takes place,

due to which capacitance (C) will increases and permittivity will also increases.

Presence of ImpuritiesFibre Permittivity

(Unextracted)Permittivity (Extracted)

Nylon fibre 2.23 1.73

Acrylic(Orlon)

2.34 2.43

NOTE:-• Permittivity of fibre varies with direction in which electric field is applied.• For cotton axial permittivity will be twice then transverse.

THANK YOU