Direction of current

By time/direction Protective devices responsive to the direction of current flow. These are necessary for parallel feeders or closed ring-main supplies. 

Draw a diagram for a voltaic cell, labeling electrodes and direction of current flow. (Example 18.1 Problems 3-8) Questions and Problems assignable in OWL 2,6... 

Draw a diagram for a salt bridge cell for each of die following reactions. Label the anode and cathode, and indicate die direction of current flow throughout the circuit. 

Conductivity is a very important parameter for any conductor. It is intimately related to other physical properties of the conductor, such as thermal conductivity (in the case of metals) and viscosity (in the case of liquid solutions). The strength of the electric current I in conductors is measured in amperes, and depends on the conductor, on the electrostatic field strengtfi E in tfie conductor, and on the conductor s cross section S perpendicular to the direction of current flow. As a convenient parameter that is independent of conductor dimensions, the current density is used, which is the fraction of current associated with the unit area of the conductor s cross section i = I/S (units A/cnF). 

Electrodes with Invertible and Noninvertible Electrode Reactions Most electrode reactions are invertible in the sense that they will occur in the opposite direction when the direction of current is inverted. Two types of reactions exist that are noninvertible in this sense. 

Two directions of current flow in galvanic cells are possible a spontaneous direction and an imposed direction. When the cell circuit is closed with the aid of electronic conductors, current will flow from the cell s positive electrode to its negative electrode in the external part of the circuit, and from the negative to the positive electrode within the cell (Fig. 2.2a). In this case the current arises from the cell s own voltage, and the cell acts as a chemical source of electric current or battery. But when a power source of higher voltage, connected so as to oppose the cell, is present in the external circuit, it will cause current to flow in the opposite direction (Fig. 2.2b), and the cell works as an electrolyzer. 

It follows that in batteries, the negative electrode is the anode and the positive electrode is the cathode. In an electrolyzer, to the contrary, the negative electrode is the cathode and the positive electrode is the anode. Therefore, attention must be paid to the fact that the concepts of anode and cathode are related only to the direction of current flow, not to the polarity of the electrodes in galvanic cells. 

FIGURE 2.2 Directions of current flow when the galvanic cell functions as a battery (a) and... 

After an intermission of 4 years, the editor of the electrochemistry series in Topics in Current Chemistry has felt the necessity to cover recent important developments in the field of organic and bioorganic electrochemistry in a fifth volume. The four contributions demonstrate typical directions of current research in organic electrochemistry which can be described as ... 

Fig. 3.13 Semiconductor-electrolyte interface (a) at equilibrium, (b) under reverse bias (c) under forward bias. Arrows denote direction of current flow [reduction reaction ox + e red], (d) Electron transfer mediated through surface states.

Surface treatments of CD CdSe films deposited from selenosulphate/NTA solutions have a pronounced effect on various optical, electrical, and optoelectronic properties of the films, due to interaction with or modification of such surface states. Mild etching (dilute HCl) of the films reverses the direction of current flow both in CdSe/polysulphide photoelectrochemical cells [108] and in Kelvin probe surface photovoltage (SPV) measurements in air [109], These studies are discussed in more detail in Chapter 9, in Section 9.2 on photoelectrochemical cells. At this point, it is sufficient to state that the effect is believed to be due to preferential trapping of either electrons or holes at surface states that are modified by the etching process. 

Figure 8.2 Schematic electrochemical cell of potential , showing directions of current /, electron e, cation Cz+, and anion Az flow as the cell redox reaction proceeds, with z electrons transferred to/from the ions at each electrode.

This p term does not occur in the conventional formalism, so that is conventionally aligned with the direction of current, Jo. Similarly, in the case of a steady current, conservation of charge requires n Jo = 0, so that p = 0 automatically, which again leads to the alignment of with the current direction. Again as B = V x A, so B L (aj - 0 + pn). 

Students will construct an electrochemical cell and identify the anode, cathode, electrolyte, direction of current flow, and direction of ion flow. 

Diodes are circuit elements that have a large conductance in one direction and a smaller conductance in the reverse direction. Thus, they can be used to control the direction of current. Effective diodes can be made by having p-type and n-type regions next to each other within a single crystal of silicon. Such a junction between the two regions is called a pn junction. 

Fig. 7. Schematic of a gradiometer coil. The arrows indicate the direction of current flow.

For an electrical current to flow through the 2-level system, the holes have to move in the direction of current flow and the electrons, due to their negative charge, in the opposite direction. We consider some sort of valve or semi-permeable membrane which allows the electrons to enter or leave the 2-level system at an energy of 2 only on one side and the holes at an energy of... 

Conventionally the direction of the current flow B (+) — A — C — B (—) is considered as positive. The direction of negative current which is identical with the movement of electrons is opposite, thus B (—) — C — A —>- B (-)-). From the theoretical point of view it is more correct to consider the direction of current according to the flow of electrons. 

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