Ohmic currents

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

The Cyclic Voltammetry Experiment. Faradaic and Double-Layer Charging Currents. Ohmic Drop... 

Figure 65 Unipolar (electron) current vs. electric field for a 0.1 cm-thick naphthalene single crystal provided with silver electrodes. Several different regimes can be distinguished (i) the low-field linear increase of the current (Ohmic regime) (ii) the SCLC in the presence of shallow traps (AE < kT) (iii) the trap-filled limit at FTFl and (iv) the SCLC with filled traps (no trapping). Adapted from Ref. 350a.

Figure 17 shows how the system determines what the galvanic current, ohmic potential drop, and electrode potentials will be. In Fig. 17(a), the anodic Tafel line for an active metal N and the hydrogen evolution Tafel line on metal M are given with the assumption that the Tafel slopes are equal. Also shown are curves representing 7 X for three different values of R, the solution resistance between the electrodes. The line I x R has the shape of an exponential curve in the semilogarithmic space. The position at which the 7x7 curve for a given resistance intersects the Tafel lines... 

High-current ohmic, where the resistance is controlled by the grain conductivity. 

In tokamaks during the ramp-up phase, the plasma current ohmic heating power heats the plasma to 0.1-1 keV temperature. The value of the toroidal plasma current is set by the magnetic configuration, therefore the ohmic heating power is determined by the plasma resistance R. Unfortunately R oc and thus with increasing electron temperature the... 

Let us imagine a solenoid traversed by an alternating sinusoidal current near a conducting piece. The tension U on the coil is the sum of the tension Rsl due to the ohmic drop of potential in the coil of resistance Rs in the absence of eddy current and of the tension e opposing to the tension e given by the LENZS law ... 

Both ohmic and rectifying behavior are possible, depending on the sign of Unlike the p—n junction the current in a rectifying Schottky barrier... 

Fig. 6. Discharge behavior of a battery where is the open circuit voltage (a) current—potential or power curve showing M activation, ohmic, and M concentration polarization regions where the double headed arrow represents polarization loss and (b) voltage—time profile.

Varistors. Varistors are devices that exhibit nonlinear current—voltage behavior. At low voltages, current flow is minimal and the device behaves as an ohmic insulator. As the voltage approaches a critical value, the breakdown field (Fgj ), current flow increases and the device becomes highly... 

Because some substances may preferentially adsorb onto the surface of the electrode, the composition near the iaterface differs from that ia the bulk solution. If the cell current is 2ero, there is no potential drop from ohmic resistance ia the electrolyte or the electrodes. Yet from the thermodynamic analysis it is seen that there is a measurable cell potential. The question from where this potential arises can be answered by considering the iaterface. 

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

With eveiy change in ion concentration, there is an electrical effect generated by an electrochemical cell. The anion membrane shown in the middle has three cells associated with it, two caused by the concentration differences in the boundaiy layers, and one resulting from the concentration difference across the membrane. In addition, there are ohmic resistances for each step, resulting from the E/I resistance through the solution, boundary layers, and the membrane. In solution, current is carried by ions, and their movement produces a fric tion effect manifested as a resistance. In practical applications, I R losses are more important than the power required to move ions to a compartment wim a higher concentration. 

Ohmic voltage drops of the protection current were exclusively involved in the processes for 7/ -free potential measurements described above. Besides this, other foreign currents can cause potential drops and falsify the potential measurement (e.g., cell currents, equalizing currents and stray currents). 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

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