Alternating field

The above electrokinetic model has implicitly only considered endurance under a steady electric field. Obviously many applications of polymeric insulation concern alternating fields, and it is well known that AC conditions are much more conducive to ageing and treeing than are DC ones. Hence it is important to extend our considerations to alternating fields. 

In an alternating field E = fsosinaj/ there will be a cyclic mechanical tensile stress 

Taking the usual equation for the rate of energy dissipation per unit volume, W, under a cyclic stress to sinojf, 

We note firstly that the energy dissipation depends on Eq and so will be particularly strong where the field is locally enhanced, e.g. at electrodes and where space charges are present. Secondly, the appropriate value of the loss compliance will be that for the craze region in the amorphous phase, and this is likely to be greater than the average macroscopic value, which includes the crystalline phase. 

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Linked scan. A scan, in an instrument with two or more analyzers, in which two or more of the analyzer fields are scanned simultaneously to preserve a predetermined relationship between parameters that characterize these fields. Often these parameters are the field strengths, but they can be the frequencies in the case of analyzers that use alternating fields. 

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

Wechsel-feld, n. (Elec.) alternating field, -fie-bcr, n. intermittent fever, -gesprach, n. [Pg.507]

A classical setup for microwave conductivity measurements is based on the utilization of the waveguides. A simple installation consists of a microwave generator (typically a gun diode) which, when the Ka-band is used, can be operated in the frequency region of 28-40 Gc/s this is protected by an isolator against back-reflections from the rest of the microwave circuit. The microwave power is conducted by an attenuator across a circulator into the microwave conductor branch at the end of which the electrochemical cell is mounted. The microwave power reflected from the electrochemical sample is conducted via the circulator into the microwave detector. It typically consists of a diode that acts as an antenna, receiving the electrical alternating field, rectifying it, and con-... 

The dissipation factor of a polymer (which we also refer to as tan 5) is the ratio of energy lost to the energy stored when it is placed in an alternating field. The dissipation factor is analogous to a mechanical tan 8 describing rheological behavior. The dissipation factor at a specific frequency is defined according to Eq. 8.14. 

As nylons absorb water, their electrical properties deteriorate. Their performance as insulators declines and they become more susceptible to energy dissipation in alternating fields. 

Shapiro and Watson (S6) and Frickel (F2) considered a device which in principle is similar to the quadruple mass spectrometer to be discussed later. In this device individual particles are suspended and balanced in an alternating field in such a way that they cannot escape. The frequency of... 

A quadrupole mass analyzer is made of four hyperbolic or circular rods placed in parallel with identical diagonal distances from each other. The rods are electrically connected in diagonal. In addition to an alternating radiofrequency (RE) potential (V), a positive direct current (DC) potential (U) is applied on one pair of rods while a negative potential is applied to the other pair (Fig. 1.17). The ion trajectory is affected in x and y directions by the total electric field composed by a quadrupolar alternating field and a constant field. Because there is only a two-dimensional quadrupole field the ions, accelerated after ionization, maintain their velocity along the z axis. 

To be successful in the pharmaceutical market it is vital that data on how your product is progressing and what the competition is doing is constantly updated and evaluated. It is also important to keep track of what might be occurring in alternative fields, which may have an impact on the market. A new surgical technique or new medical device may be introduced that completely alters the way a disease is managed. 

The pendant groups may oscillate in an electric field at temperatures below the T but the polymer chain does not oscillate below the Tr It is important to note that the Tg as measured electrically may not coincide with the Tg obtained by other techniques. The energy consumed in aligning the polymer dipoles in an alternating field is called the power loss A, i.e., the loss of energy per second. The power loss is zero for a perfect dielectric. 

Due to the dependence on the absolute value of the field strength (Eq. 6) a solution of a weak electrolyte has also rectifying properties for high alternating fields and Is therefore not the frequency of the high alternating field ... 

Fig. 49 a and b. Diagrammatic representation of a neutral particle in an inhomogeneous electric field, a) the particles are able to migrate, because the field intensity is not equal on both sides resulting in a net force so-called dielectrophoresis, b) The direction of dielectrophoresis is independent of the polarity of the field. In an alternating field all particles will move in the direction of higher field strengths 611... 

An alternating magnetic field Bl with frequency irradiating an ensemble of nuclear spins precessing in the static field B0 may overcome the energy difference AE if it meets two conditions The vector of the alternating field B1 must rotate in the plane of precession with the Larmor frequency v0 of the nuclei to be observed (Fig. 1.4(a)). 

In order to observe NMR, a sample containing nuclear spins (e.g. H, 13C) is placed in a static magnetic field B0. An alternating field B with radio frequency v, is applied perpendicularly to BQ. Usually, v, is increased or decreased slowly and continuously... 

Relaxation can be described in terms of the magnetization vector components. At resonance, the equilibrium magnetization M0 parallel to B0 decreases to Mz, due to the transitions between the nuclear magnetic energy levels caused by the alternating field B,. Following resonance, the equilibrium of the nuclear spins with their lattice and with each other is restored by relaxation. 

At resonance, the magnetization vector M precesses about the vector B, i of the alternating field according to eq. (1.34). As a result, a component of transverse magnetization My.j rotates in the x y plane at the Larmor frequency v0. If a receiver coil is placed in the x y plane, the rotating magnetic vector Myj induces an electromotive force measurable as an inductance current. This process is called nuclear induction [5, 8]. The orientation of the coil axis will affect the phase relative to B1 i but not the magnitude of the induction current. 

A radio-frequency (rf) alternating field initiates any NMR experiment. At resonance, the field vector B, rotates with Larmor frequency (v, = v0) perpendicularly to the static field vector B0, as shown in Fig. 2.1. In this situation, the nuclear magnetic moments will precess about both fields B0 and B,. Provided B1 extends along the x axis at a certain instant, the double cone of precession will rotate about the x axis in the yz plane (Fig. 2.1 (a) -> (b)). The flip angle 0 relative to the z axis at a given field strength B, depends on the pulse width t of the rf field, in the range of some ps, so that... 

Spin decoupling or nuclear magnetic double resonance (NMDR) is achieved by irradiating an ensemble of nuclei not only with a radio frequency fl, at resonance with the nuclei to be observed, but additionally with a second alternating field B2 at resonance with the nuclei to be decoupled e.g. H). Decoupling experiments can be carried out to convert homonuclear ( H— JH, 19F —19F) or heteronuclear multiplets (19F—1H,... 

At low frequencies of the alternating field, the dielectric loss is normally zero and e is indistinguishable from the dielectric constant edc measured with a static field. Debye has shown that... 

As an alternative, field crews may communicate with the chemist in the office by telephone and by fax for immediate resolution of questions and problems that may arise during sampling. 

An excellent reference describing appropriate ways of measuring the piezoelectric coefficients of bulk materials is the IEEE Standard for Piezoelectricity [1], In brief, the method entails choosing a sample with a geometry such that the desired resonance mode can be excited, and there is little overlap between modes. Then, the sample is electrically excited with an alternating field, and the impedance (or admittance, etc.) is measured as a function of frequency. Extrema in the electrical responses are observed near the resonance and antiresonance frequencies. As an example, consider the length extensional mode of a vibrator. Here the elastic compliance under constant field can be measured from... 

The goal of the following discussion is to study the low-frequency dynamic birefringence induced by an alternating field in a suspension of acicular particles with dipole moments, that is, the system of type 1 according to the classification given above. The results presented are obtained avoiding any... 

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