Absorption alternating currents

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

Phase-modulation immunoassay measurements are made with sinusoidally modulated light. Since the emission is a forced response to the excitation, the emitted light has the same periodicity as the excitation. Due to the time lag between absorption and emission, the emission is delayed in comparison with the excitation. The time delay between the zero crossing of one period of the excitation and of the emission is measured as the phase angle (Figure 14.11). The emission is also demodulated, due to a decrease in the alternating current (AC) component of the AC to direct current (DC) ratio. 

Cole, K. S., and R. H. Cole, 1941. Dispersion and absorption in dielectrics I. Alternating current characteristics,/. Chem. Phys., 9, 341-351. 

A vitally important aspect of ion trap operation is the ability to impart translational energy selectively to ions via resonance absorption of alternating current (ac) voltages (10-450 kHz) applied to the endcap. Unlike for linear quadrupole (or other multipole) collision cells, the absorption of energy is m/z specific as each m/z in the trap precesses at a specific set of frequencies, the most important of which for MS/MS is ooz, the fundamental frequency of motion in the z dimension, which is defined by... 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

K. S. Cole and R. H. Cole, "Dispersion and Absorption in Dielectrics 1 Alternating Current Characteristics," Journal of Chemical Physics, 9 (1941) 341-351. 

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