There are materials which have permanent dipoles. These are referred to as polar. The application of an electric field tends to cause these dipoles to rotate and align with the field, figure 3. The magnitude of P is dependant on the applied electric field as well as the type of material which is reflected in the dielectric constant. As with all capacitors, there needs to be a charging current due to polarization. The higher the dielectric constant, the greater the charging current.
In AC circuits, this current needs to be supplied every half cycle. There is an emphasis on testing cables in the field to determine their acceptability. Methods using transmission line theory will also measure the cable capacitance. A variation in this value over time would indicate a change in the dielectric constant which implies changes in the insulating materials.
Such changes could be the result of aging and the environment i. The dielectric constant is a also a function of frequency.
As frequency increases, the value decreases due to polarization mechanisms no longer being able to follow the rapidly changing field. A shift in the frequency where these decreases occur can also be an indication of cable aging. These affects are being investigated through dielectric spectroscopy. In summary, the dielectric constant indicates the degree of polarization and thus, the amount of charging current needed. Shifts in its value can serve as a metric for measuring cable health.
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The dielectric of an ideal capacitor is free space that has a relative dielectric constant of unity and is free of polarization and leakage. If the applied voltage and frequency are kept constant and a solid dielectric or a liquid dielectric, or a combination of both, is made to displace the free space the current will increase because of polarization. For a given applied voltage and frequency, the relative dielectric constant of an imperfect dielectric is the ratio of the current phasor to that which would result by replacing the imperfect dielectric with free space.
Because of the dielectric energy loss the relative dielectric constant is a complex quantity rather than a real number. The term expressed by Eq. Another term that is used is the complex permittivity.
This is synonymous with absolute dielectric constant and is expressed as. Typical values of relative dielectric constant and dissipation factor are shown in Table
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