Voltage electromotive force

Cell voltages (electromotive force of a complete cell)... 

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... 

Thermoconples nnfortnnately develop only relatively small voltage (electromotive force, or emf) changes for a temperatnre difference of 1°C. Hence errors of predicted temperatures are often relatively large. The first three thermoconples often have errors of 2°C to 3°C or 0.5% to 0.75%. The latter three (and more expensive) thermocouples typically have smaller errors. With specially constructed thermocouples having purer metals, better connections, etc., errors are reduced by perhaps 50% as compared with the values reported above. 

According to this equation, voltage (electromotive force) between atoms-electrodes is determined by the sum of two addends. 

Define the following terms anode, cathode, cell voltage, electromotive force, standard reduction potential. 

Voltage Electromotive force of electricity also defined as the work required to move a charged object through an electric field. 

It is possible to obtain work from many reactions by forming an electrical ceU in which the reaction occurs. The familiar storage battery and dry cell are examples. In many cases it is possible to measure the maximum work available from the reaction quite precisely by measuring the voltage (electromotive force, E) of the cell. E is proportional to the maximum work per electron transferred, and is related to AF by the equation... 

Fig. 5. Energy requirements of the HaH-Hfiroult cell (23—25). E, decomposition of alumina Eg, depolarization by carbon E, anode overvoltage E, counter electromotive force E, bath voltage drop E, bath bubble voltage F/, anode voltage drop Eg, cathode voltage drop E, external voltage drop ...

FIG. 24-14 Dissolved-oxygen electrodes a) polarographic (impress breakdown voltage for oxygen measure current) (h) voltametric (measure electromotive force). 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Since the electrolyte membrane only allows the conduction of ions, the electrons are forced through an exterior circuit, creating an electromotive force. The voltage generated by such a cell is given by the Nernst equation. For the hydrogen-oxygen reaction we can write ... 

Such switches have a drawback when the magnet current is sweeping, a back electromotive force (e.m.f.) is present equal to L-dl/dt. This voltage is also present across the switch and so a current flows through the switch. 

The Seebeck Effect The production of an electromotive force in a thermocouple under conditions of zero electric current. Thermoelectric power is the change in voltage at a thermocouple as a function of temperature. 

While the voltage of the cell represents the potential difference between the two terminals of the battery, in reality it relates to the separation in energy between the two half-cells. We call this separation the emf where the initials derive from the archaic phrase electromotive force. An emf is defined as always being positive. 

The mechanism of action of defensins is largely unknown. Incubation with defensins results in the formation of voltage-regulated ion channels that permeabilise the outer and inner membranes of metabolically-active E. coli. Because the target bacteria must be metabolically active for defensins to exert their effects, it may be that the transmembrane electromotive force is involved in the mechanism of action. 

The strength of an electric current varies directly with the electromotive force (voltage) and inversely with the resistance. 

The cell voltage is sometimes called the electromotive force, abbreviated emf. However, this term can be misleading. 

La photovoltaic cells, the same redox reaction, OX + e = KED, may be used for both the anode and the cathode. Figure 10-33 shows an eneigy diagram of an operating photovoltaic cell this cell consists of a metallic cathode and a photoexcited n-type semiconductor anode. The electromotive force (the open cell voltage), ph > approximately equals the difference between the flat band potential of... 

From the energy diagram shown in Fig. 10-33, the operating cell voltage, V,, is obtained, as expressed in Eqn. 10-60, by subtracting from the electromotive force AEph the potential barrier of the space charge layer, the cathodic overvoltage t h, and the iR drop in the electrolyte ... 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

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