Electromotive force, definition

As examples of properties of systems satisfying the conditions of definiteness at a particular temperature and of reversion, we may refer to the electrical resistance of a metal wire the electromotive force of a thermocouple with a fixed temperature at the cold junction the volume of a homogeneous gaseous, liquid, or... 

Starting with this definition the semiconductor diemical sensors can be arbitrary classified with respect to following features the type of electrophysical characteristics diosen for monitoring, such as electric conductivity, thermal-electromotive force, work function of electron, etc. type and nature of semiconductor adsorbent used as an operational element of the sensor and, finally, the detection method used for monitoring the adsorption response of electrophysical characteristics of die sensor. 

In the case of molecules which do not dissociate, the electromotive force must be due to the orientation of the molecules in the surface layer, which molecules must have a definite electric moment. In the few cases where the electromotive force can be accurately compared with the surface concentration such as n-but3rric, ri-valeric and n-caproic acids, the E.M.F. is almost approximately proportional to the surface concentration F, as would be the case if the moment of a molecule were independent of the proximity of its neighbours. 

Electromotive force or voltage, V, is simply the work done in moving a unit electric charge (Q = 1) from a point R to Ri since the voltage or potential is zero when Ri = < , we may write by definition... 

A single electrode potential, if defined as the difference in electrostatic potential between the spaces just outside the metal and the solution, is definite, but it cannot be measured by merely connecting up the phases with wires, and adjusting a potentiometer, until no current flows for this connexion introduces more than one phase boundary. Practically all electrolytic cells consist of at least three phase boundaries and the terminals at which the electromotive force of the cell is measured are, finally, of the same metal. There may, of course, be any. greater number of phase boundaries. A simple type of cell consists of two metals, M and M, dipping into a solution 8 containing the ions of each metal. 

The term electromotive force is misleading and undesirable. However, it is so firmly entrenched that we will continue to use it here. Note that the definition involves the electrochemical potential of the electrons on the product side of the equation as written, minus that for the electrons on the reagent side for the reaction as written. 

The electromotive force of a galvanic cell is a measure of the electrical work which can be obtained from the reaction in the cell. The total or maximum work which can be obtained from the cell reaction includes also the work which is done against the external forces owing to the changes in volume (formation of gas, etc.) of the reacting substances. From the definition of affinity (p. 318) it follows, therefore, that the electromotive... 

According to its definition, the standard (reduction) potential of the A/A couple is the standard electromotive force of a cell in which an A/A electrode (where the activities of A and A are made unity) is opposed to an NHE (normal hydrogen electrode) whose potential is assigned to zero by convention. 

Reproducibility of Couples.—When thermocouples are employed in the laboratory for scientific purposes, although desirable, it is not of serious importance that the calibration or temperature-electromotive force relation of couples of the same type be exactly similar. However, in the industrial plant this question of reproducibility is of considerable moment. The indicating instruments are usually graduated in degrees of temperature and the graduation applies for one definite temperature-electromotive force relation only. If the temperature electromotive force relations of various couples of the same type are not similar, corrections must be applied to the readings of the indicator, and these corrections will be different for each couple. When several couples are operated with one indicator and when the process is such as to require a frequent renewal of couples, the applying of these corrections becomes very troublesome. 

As already stated, the measured electromotive force of a galvanic cell does not necessarily correspond to a single definite reaction. For example, a powerful primary cell may be constructed by placing a porous cup containing a platinum or carbon electrode, surrounded by nitric acid, into another vessel containing an amalgamated zinc electrode in an electrolyte of sulphuric acid. This cell may be represented by... 

As concerns the electromotive force, a definition of its quadrature and inphase components can be done in two ways. In fact, we can compare either a phase shift of the electromotive force with the current in the transmitter or with the primary electromotive force t Q. In the future, the latter approach will be used and correspondingly one can write (Fig. 2.3b) ... 

Resistance (R) - Electric potential difference divided by current when there is no electromotive force in the conductor. This definition applies to direct current. More generally, resistance is defined as the real part of impedance. [1]... 

We have previously used the term electromotive force but have not rigorously defined it. The definition of the electromotive force can be given in terms of the equation... 

Equation (5) or (11) can be applied directly to half-cell reactions such as (6) and (7) and the resulting potentials obtained will be identical to those obtained from the overall reactions (9) and (10) because of the definition of the SHE as the universal standard. A selection of standard potentials of half-cell reactions is shown in Table 1 [5]. By international convention, electrode reactions in thermodynamic tables are always written as reduction reactions, so the more noble metals have a positive standard potential. Lists such as that in Table 1 are also called electromotive force series or tables of standard reduction potentials. 

Ohm om, [Georg Simon Ohn (1867) (Q) n. The SI unit of electrical resistance, equal to IV divided by 1 A. The SI definition for 1Q is the resistance between two points of a conductor when a constant difference of potential of 1V, apphed between these two points, produce in this conductor a current of 1 A, this conductor not being the source of any electromotive force. 

Practically electricians have been compelled to define electromotive forces by comparison with those of the Grove s or Daniell s cell, the copper and zinc cell, or other electromotive sources . . . not one has been hitherto found which could be relied upon to give a definite electromotive force however pure the materials, and however skilful the manipulation, differences varying from four to five percent upwards constantly occur without any assignable cause and different observers using different materials of course meet with still larger discrepancies. ... 

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