SOURCES OF EMF

The production of an electrical potential capable of causing a flow of current is generally accomplished by converting another form of energy into electrical energy. This may involve  

The most common rechargeable battery is the one used in automobiles. This has two lead electrodes—an anode coated with Pb02 and a cathode coated with PbO. The electrolyte is a H2SO4 solution. The controlling chemical equation for this battery is  

Current flows through an external circuit from the anode (+) terminal to the cathode (-) of a battery at essentially constant voltage. This is called a direct current (DC). 

The conversion of rotational mechanical energy into electrical energy involves a generator that usually produces current that varies from plus to minus sinusoidally at a relatively high frequency. Such a current is called an 

---

The simplest circuit element is the short circuit. Figure 2-66a illustrates the concept of a short circuit. A source of emf (labeled v ) produces a current that flows relatively unimpeded through the conductor resulting in a nearly zero potential drop and an infinite current. 

The silver oxide and mercuric oxide button cells used in cameras and other devices requiring a miniature source of EMF consist of a zinc disk, which serves as the anode, and, on the other side of a porous separator, a paste of Ag20 or HgO. The reaction products are zinc hydroxide and metallic silver or mercury. Inert metal caps serve as the current collectors. 

A typical thermocouple installation for an industrial application is shown in Fig. 6.23. Instead of placing the reference junction in a temperature controlled environment (which is often inconvenient), an automatic reference junction compensation circuit is fitted. This provides a second source of emf Sj,° in series with the thermocouple emf E. The meter thus measures 0 = E 0 where E%-0... 

The usual and simplest electrolytic separation is carried out by inserting a pair of electrodes, ordinarily of platinum, into a solution and applying an external source of emf. 

If two metals of differing work function are connected electrically, at the same temperature and without a source of emf, the electrostatic potentials just outside the two surfaces are different. This potential difference V12 is known as the contact potential difference and is equal to the difference in the work functions of the two metals and < 2- compensating potential, equal and opposite, is applied... 

To cause a current to flow continuously around a circuit, a driving force is required, just as a circulating pump is required to drive water around a central heating system. This driving force is the electromotive force (emf). Each time an electron passes through the source of emf, more energy is provided to send it on its way around the circuit. 

He reverted to the problem in 1894 in a paper on the seat of the emf of the Daniel 1 cell ( ). He dismissed the metal/metal contact as a source of emf, correctly noting that a transient flow of charge might result from first contact but not an enduring current thus A... 

Reduce the susceptibility of an electronic component, assembly, or system to external sources of EMF, EMI, or RFI. 

Figure 10.23 shows a constant source of emf connected through its fixed internal resistance (r) to an external load of resistance R). It is desired to know how large R should be relative to r such that the greatest energy possible is dissipated at R. The current flowing in the circuit will be ... 

The electrode processes that are reversible provide values for the equilibrium emfs of cells, which are related to the thermodynamic functions. The condition of reversibility is practically obtained by balancing cell emf against an external emf until only an unappreciable current passes through the cell, in order that the cell reactions proceed very slowly. It may, however, be pointed out that for many of the applications of electrometallurgy, it is clearly necessary to consider more rapid reaction rates. In that situation there is necessarily a departure from the equilibrium condition. Either the cell reactions occur spontaneously to produce electric energy, or an external source of electric energy is used to implement chemical reactions (electrolyses). 

Here, an electrochemical cell working under irreversible conditions is considered. Its emf invariably moves away from the equilibrium value, and if the cell is serving as a battery or source of electricity, then its voltage drops below the equilibrium value. If, on the other hand, the cell is in a place where electrolysis is occurring, then the voltage to be applied must exceed the equilibrium value. 

Reversible means counteracting the cell emf by means of an external source whose emf is identical so that no current passes through the cell. 

The above three sources of error all result from the fact we assume that the emf measured in a potentiometric experiment comprises only two terms, i.e. one... 

Up until now, all values of the emf have only comprised two half-cell potentials. The junction potential is an additional source of potential, so our fundamental relationship (equation (3.3)) now becomes ... 

Experimentally, the home-made Ag AgCl electrode and another reference electrode (e.g. an SCE) are immersed in solutions of varying [Cr ] and the emf determined for each concentration, e.g. in the range 0.001-0.1 mol dm. Hydrochloric acid is a suitable source of chloride ion for this experiment. Respective values of EAgcuAg tire then obtained from each emf via the use of equation (3.3). 

The resistance R is variable continuously or in small increments so that the dial reading of R times the multiplier setting equals the unknown resistance R,. The main sources of error in a Wheatstone bridge are the inaccuracies of the three standard resistances R, RA, and RB (as low as 0.001%), establishing the null point, and thermal electromotive force (EMF) values. 

A possible source of difficulty in electrochemical experiments is the presence of an emf when two dissimilar solutions in the cathode and anode regions are juxtaposed or connected by a salt bridge. Different ions in various solutions diffuse at different rates the faster cations moving ahead of the slower ones set up an internal electric field that tends to retard their movement and to accelerate the slower cations. An analogous situation prevails for the anions. After a steady state sets in both types tend to move across the junction in the presence of an internal field this is the origin of the junction emf. 

The thermodynamic analyses used in this chapter make use of the electrochemical potential. In this way the electrical aspects of the interfacial equilibria are clearly defined. Earlier work on this problem, especially that by Volta and Nernst, had led to different conclusions regarding the source of the EMF in an electrochemical cell [12]. This problem was resolved by Frumkin, essentially, by writing out the interfacial equilibria using electrochemical potentials. In this regard, all interfaces in the cell must be considered including those between different metals at the terminals of the cell. This was shown in the discussion of the thermodynamic basis of the Nernst equation. 

Phagocytes are, in vivo, the main source of free radicals and other ROS that are generated in defense against bacteria and in response to various stimuli. For this reason, polymorphonuclear neutrophils (PMNs) are a useful model to study cell activation and the interference of EMF with signalling pathways. 

An electrochemical cell is a source of electricity. Such cells can operate irreversibly when being used as a source of a current, or reversibly, as in emf studies. The terms irreversible and reversible are being used in the thermodynamic sense of the terms (see Section 8.3). 

This has always been an important, though very indirect, source of hydration numbers. Colligative properties and emfs give high precision experimental data from which the activity of the solute can be calculated and stoichiometric mean ionic activity coefficients found. If it is assumed that solvent molecules are bound to the ions, a relation between... 

---