Electrical emf Work

The flow of electric charge Q through a difference in voltage ( electromotive force, emf, E) gives rise to electrical emf work wemf, whose differential form can be expressed as 

Parallel to other work forms, this can be expressed as follows  

Electrical (emf) work The transfer of a quantity of electric charge (extensive) through a difference in electromotive force (intensive). 

Electrical work is usually performed in an electrical circuit, as shown schematically in Fig. 3.7. The circuit consists of a voltage source (e.g., a battery) E connected through a resistance R with circulating current I. These quantities are related by Ohm s law, 

The electrical work (3.16) can therefore be expressed in alternative form as 

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Figure 6.1 Schematic depiction of Gibbs equilibration for three driving forces (a) thermal (T difference), (b) electrical ( emf difference), (c) chemical (pq difference), showing the transported quantity Xt and available work RffXi for each driving field / /.

In Section 3.3.4, we discussed the inclusion of electrical (emf) contributions to the general expression for work,... 

Electrons participating in the intercalation/deintercalation reaction (Equation (5.1)) can be represented by a current-producing system. Second, it is characteristic that the current-producing system reversibly operated by a self-driven (galvanic) cell (discharging the battery) performs the electrical useful work AG = —zFE (where E is the EMF of the cell), because electrical potential difference is spontaneously developed between two electrodes. By contrast, when the cell is short-circuited - that is, when the two electrodes are not separated from each other but are directly in electrical contact - electrons do not appear explicitly but rather participate in corrosion (or permeation in the case of solid electrolyte cells). They perform no electrical useful work because the two electrodes have the same electrical potential. 

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]. 

This equation links the EMF of a galvanic cell to the Gibbs energy change of the overall current-producing reaction. It is one of the most important equations in the thermodynamics of electrochemical systems. It follows directly from the first law of thermodynamics, since nF% is the maximum value of useful (electrical) work of the system in which the reaction considered takes place. According to the basic laws of thermodynamics, this work is equal to -AG . 

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. 

The concentration overpotential i/c is the component of the overpotential due to concentration gradients in the electrolyte solution near the electrode, not including the electric double layer. The concentration overpotential is usually identified with the Nernst potential of the working electrode with respect to the reference electrode that is, the thermodynamic electromotive force (emf) of a concentration cell formed between the working electrode (immersed in electrolyte depleted of reacting species) and the reference electrode (of the same kind but immersed in bulk electrolyte solution) ... 

Living on less than five hundred dollars a month from Social Security income, Susan recently became the owner of a small home built to meet her environmental needs. The kitchen, living and work areas are all part of one open room. Electrical equipment is limited to a computer and television, both carefully sealed and shielded to minimize EMF exposures. 

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

With regard to points in a distributor, these are opened mechanically with the resulting bounce, lag and dwell, however electrically operated relays rely solely on the induced magnetism of the operating coil. In this instance are we expecting each relay to operate, lets say, assuming 6000 rpm, that the relay will operate a hundred times per second How is the back emf of the relay coil accounted for Or does it just work anyway I must admit I am confused but not too doubtful especially if someone claims that it works, you never know ... 

If we choose a set of standard conditions (cf. Section 2.3) and one convenient half-cell to serve as a reference for all others, then a set of standard half-cell EMFs or standard electrode potentials E° (Appendix D)1-9 can be measured while drawing a negligible electrical current, that is, with the cell working reversibly so that the equations of reversible thermodynamics... 

In order to derive the relation between EMF and the chemical potential difference probed at different surfaces of the stressed solid, we formulate the reversible work and its electrical equivalent. If zAF-dnA electric charges are transported across the electrolyte between the two surfaces labeled 1 and 2 in Figure 8-8, the electrical work is... 

Some experiments on rats and mice show that for continuous exposure at high levels of EMF (400 mG) some physiological changes occur. These EMF levels are well above what humans are normally exposed to at home or at work. One study that exposed humans to high levels of electrical and magnetic fields (greater than 100 times normal) for a short duration found a slowing of heart rate and inhibition of other human response systems. 

As already mentioned, the EMF of a reversibly operating cell may be ealcu lated from the thermodynamic properties of the system, i. e. from the equivalency between the maximum electric work which can be obtained when the cell is operating at constant temperature and pressure, and the change in free energy accompanying the corresponding chemical reaction. 

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... 

The amount of work required to add or remove electrons is called the electromotive potential or force (emf) and is designated E. It is measured in volts (joules per coulomb, where a coulomb is a unit of electric charge or a quantity of electrons). 

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. 

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