External circuit

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

Ia early telephoaes, souad (voice) waves caused a carboa microphone s resistance to vary, thus varyiag the current flowing ia a series external circuit. This d-c curreat could thea be used to regeaerate voice waves ia a receiver. Two wires were required to carry a single coaversatioa. With time, telecommunications traffic was eacoded oa a-c carriers, at first usiag ampHtude or frequeacy modulatioa, and more recently pulse code modulation. 

The electrons, Hberated at the anode, travel by electrical cable through the external load, such as an electric motor, to the cathode. If the external circuit is open the reaction is stopped, no fuel is consumed, and no power is generated. The electrolytic reaction, then, is controlled by the load connected to the cell. The overall fuel cell reaction is... 

Cathode is the positive electrode of a primary cell associated with chemical reactions that gain electrons from the external circuit. 

Closed circuit voltage is the voltage of a cell or battery when the battery is producing current into the external circuit. 

In a battery, the anode and cathode reactions occur ia different compartments, kept apart by a separator that allows only ionic, not electronic conduction. The only way for the cell reactions to occur is to mn the electrons through an external circuit so that electrons travel from the anode to the cathode. But ia the corrosion reaction the anode and cathode reactions, equations 8 and 12 respectively, occur at different locations within the anode. Because the anode is a single, electrically conductive mass, the electrons produced ia the anode reaction travel easily to the site of the cathode reaction and the 2iac acts like a battery where the positive and negative terminals are shorted together. 

Electrodes. AH of the finished silver electrodes have certain common characteristics the grids or substrates used in the electrodes are exclusively made of silver, although in some particular cases silver-plated copper is used. Material can be in the form of expanded silver sheet, silver wire mesh, or perforated silver sheet. In any case, the intent is to provide electronic contact of the external circuit of the battery or cell and the active material of the positive plate. Silver is necessary to avoid any possible oxidation at this junction and the increased resistance that would result. 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

Equation 14 is a form of the Nemst equation. The overall chemical reaction for the passage of 2 Earadays of charge in the external circuit from right to left... 

Figure 16.1 Simple dry cell battery. Electrons are conducted along the external circuit (4), which physically connects the active (2) and noble (1) materials. An equivalent ionic counter-current is conducted through the electrolyte (3), thereby completing the circuit.

In the first two types a brake shoe, operated by an external auxiliary supply, is mounted on the extended shaft at the NDE (non-driving end) of the motor. These brakes are normally operated after the motor is switched OFF. The heat of braking appears in the external circuit and the motor windings are not affected. For motors with this braking, only the starting heat need be considered, depending upon the frequency of starts and not the heat of braking. 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

In the absence of free charge in the disk, the electric displacement will be independent of position, although it will vary with time D = [D(t),0,0]. The current induced in the external circuit is attributable to changes in electric displacement within the disk and is given by... 

Nonlinear properties of normal dielectrics can be studied in the elastic regime by the method of shock compression in much the same way nonlinear piezoelectric properties have been studied. In the earlier analysis it was shown that the shape of the current pulse delivered to a short circuit by a shock-compressed piezoelectric disk was influenced by strain-induced changes in permittivity. When a normal dielectric disk is biased by an electric field and is subjected to shock compression, a current pulse is also delivered into an external circuit. In the short-circuit approximation, the amplitude of this current pulse provides a direct measure of the shock-induced change in permittivity of the dielectric. 

All flashlight batteries, button batteries, compact rechargeable batteries and vehicle storage batteries operate under the same basic principles. An electrochemical cell is constructed of two chemicals with different electron-attracting capabilities. Called an electrochemical couple, these two chemicals, itntncrscd in an electrolyte (material that carries the flow of energy between electrodes), are connected to each other through an external circuit. 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

The external circuit provides a path for the excess electrons at the negative electrode to move toward the positive electrode. Although the flow of electrons would seem to cancel out the two charges, chemical processes in the battery build the charges up as fast as the depletion rate. When the electrodes can no longer pass electrons between them, the battery dies. ... 

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