Dielectric dissipation tangent

The extent to which a material is heated when subjected to microwave radiation depends on two parameters the dielectric constant e and the dielectric loss factor e". The dielectric constant describes the ease with which a material is polarized by an electric field, while fhe loss factor measures fhe efficiency wifh which the electromagnetic radiation is converted into heat. The ratio of these properties gives the dielectric loss tangent or dissipation factor ... 

Dielectric Loss Tangent - See Dielectric Dissipation Factor. 

The ratio e"/e is the amount of energy dissipated per cycle divided by the amount of energy stored per cycle and known as the dielectric loss tangent or dissipation factor (tan (5). 

The dielectric loss tangent or dissipation factor is a material property characterizing the loss behavior of the dielectrics itself. It is defined as follows ... 

Dissipation factor (loss tangent) lEC 250. As explained in the chapter, this is the tangent of the dielectric loss angle and is now more commonly used than the power factor, which is the sine of the loss angle. When the angle is small the two are almost identical (e.g. for a loss angle of 10° the difference is about 1.5%). 

The dissipation factor is a ratio of the real power (in-phase power) to the reactive power (power 90° out of phase). It is also defined as (1) IT is the ratio of conductance of a capacitor in which the material is the dielectric to its susceptance, (2) IT is the ratio of its parallel reactance to its parallel resistance it is the tangent of the loss angle and the cotangent... 

The power factor of a material may be described loosely as the fraction of the electrical energy stored by the condenser in each cycle which is lost as heat. This arises because the phase difference between voltage and current deviates from 90° (which it would be for a perfect dielectric, e.g. vacuum) by the loss angle, 8. The dissipation factor is the tangent of the loss angle, tan 8. 

When an a.c. voltage is applied to a perfect capacitor, no energy is dissipated. However, a real capacitor dissipates energy because of lead and electrode resistances, d.c. leakage resistance and, most importantly, dielectric losses. These account for the capacitor s dissipation factor or loss tangent tan 3. It is sometimes convenient to regard the lossy capacitor as an ideal capacitor shunted by a resistance Rp or in series with a resistance rs, as shown in Fig. 5.5. 

The final quantity to be defined has been the source of much confusion. The loss tangent of the sample is the same as the dissipation factor defined above however, the loss tangent of the medium is a dielectric property. To distinguish between them, we shall refer to tan 8X as the sample loss tangent, having the value... 

Power factor. As used in dielectric measurements, the cosine of the angle whose tangent is the ratio of loss factor to dielectric constant, and hence a measure of both stored and dissipated energy within the dielectric medium. 

We would expect intuitively that tan 0 emd the Deborah number De are related, since both refer to the ratio between the rates of an imposed process and that (or those) of the system. The exact shape of this relationship depends on the number and nature(s) of the releixation process(es). So let us anticipate [3.6.4 la] for the loss tangent of a monolayer in oscillatory motion, which describes a special case of [3.6,12], namely -tan0 = t]°(o/K°. Here, (o is the imposed frequency, equal to the reciprocal time of observation, t(obs) =< . The quotient K° /t]° also has the dimensions of a time in fact it is the surface rheological equivalent of the Maxwell-Wagner relaxation time in electricity, (Recall from sec. 1.6c that for the electrostatic case relaxation is exponential ith T = e/K where e e is the dielectric permittivity and K the conductivity of the relaxing system. In other words, T is the quotient between the storage and the dissipative part.) For the surface rheological case T therefore becomes The exponential decay that is required for such a... 

The dielectric permittivity of a medium (relative to the permittivity of free space, 8q = 8.85 X 10 F/m) is given by e and measures the polarization of the medium per unit applied electric field. The dielectric loss factor arises from energy loss during time-dependent polarization and bulk conduction. The loss factor is written as a". The loss tangent or dissipation of the medium, tan<5 is defined by e"/e. The orientation of molecular dipoles has a characteristic time r. Typically is short early in the cure but grows large at the end of the cure. 

The electrical properties of polymers are important in many applications [1]. The most widespread electrical application of polymers is the insulation of cables. In recent years, high-performance polymers have become important in the electronics industry as encapsulants for electronic components, as interlayer dielectrics, and as printed wiring board materials. The dielectric constant (or permittivity) and the dissipation factor (or power factor or electrical loss tangent) tan 8, which are dimensionless quantities, are the key electrical properties. 

The dielectric properties of soil determine the amount of RF power that can be dissipated in the soil. These properties are the relative dielectric constant (e ) and the loss-tangent. The loss-tangent, tan 6, is defined as o/oieQe where a is the apparent conductivity, w is the frequency of the applied electric field, radians/sec, and is the permittivity of free space which equals 8.85 X 10 Farads/meter. All the dielectric properties are a function of soil temperature, the frequency of the applied field, and the composition of the soil. 

The amount of RF power dissipated in the soil is directly related to the frequency of the applied electric field, to the square of the amplitude, to the relative dielectric constant, and to the loss-tangent (8). 

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