Volt, defined

The e.m. and e.s. units described above are not all of a convenient magnitude for experimental purposes, and so a set of practical units have been defined. The practical unit of current, the ampere, often abbreviated to amp., is one-tenth the e.m. (c.g.s.) unit, and the corresponding unit of charge or quantity of electricity is the coulomb the latter is the quantity of electricity passing when one ampere flows for one second. The practical unit of potential or e.m.f. is the volt, defined as 10 e.m. units. Corresponding to these practical units of current and e.m.f. there is a unit of electrical resistance this is called the ohm, and it is the resistance of a conductor through which a current of one ampere passes when the potential difference between the ends is one volt. With these units of current, e.m.f. and resistance it is possible to write Ohm s law in the form... 

Where ceii is the cell potential in volts, defined to be positive F is the Faraday constant, 96485 C mol and n is the number of moles of electrons that migrate from anode to cathode in the cell reaction. Thus a galvanic cell is also a Gibbs energy meter. 

Electrical measurement, like all measurement, is a comparative process. The unit of potential difference—called the volt—defines the electrical force required to move a current of one ampere through a resistance of one ohm. Devices that measure voltage are calibrated against this standard definition. This definition similarly defines the ohm but not the ampere. The ampere is defined in terms of electron flow, such that a current of one ampere represents... 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

The numerical results for argon are a = 0.66 X 10 22 and b = 0.4. Since b = ki2kib/knki, we must make some further assumptions to obtain values of interest. Considering the nature of Reactions 15 and 16, we conclude that 15/ 16 1 which leads to ki2/kn 0.4. Substituting this value into the equation which defines a, we obtain kn/kis 4.75 X 10 23. In a separate study (31) with a mass spectrometer for conventional ion-molecule reaction work we could determine the cross-section for Reaction 15 as Q/ = 208 X 10 16 sq. cm. at Erl = 0.55 volt, where Er is repeller field strength and l the ion path length in the ion source. 

The value of p defined by Eq. (29.6) is sometimes called the absolute electrode potential measured against vacuum. We must remember here that we are concerned with electrochemical potentials stated in electron volts rather than with electrostatic potentials stated in volts. Hence, this absolute potential, which can be determined... 

The activation energy Ea - defined as Ec - Ey for the conduction band (and analogously for the valence band), can be used to assess the presence of impurities. Due to their presence, either intentional (B or P dopant atoms) or unintentional (O or N), the Fermi level shifts several tenths of an electron volt towards the conduction or the valence band. The activation energy is determined from plots of logafT) versus 1/7, with 50 < 7 < 160°C. For undoped material Ea is about 0.8 eV. The Fermi level is at midgap position, as typically Eg is around 1.6 eV. 

Electronic energies are often expressed in the unit electron volt (eV). An electron volt is defined as the kinetic energy of an electron accelerated through a potential difference of 1 volt. Thus, we have... 

The quantum of magnetic flux is only 2.07 x 10-15 Wb, which is approximately equal to the amount of the earth s magnetic field enclosed by a ring of 10p,m in diameter. The Josephson effect is observed when two superconductors are separated by a very thin insulating layer (about 20 nm). Single electrons and Cooper pairs can tunnel through such a layer. The characteristics of the Josephson junction are now used to define the volt and have enabled the uncertainty in the maintained standard to be reduced to 0.1 p,V. 

Electron Volt (eV) A non-SI unit of energy defined as the energy acquired by a particle containing one unit of charge through a potential difference of one volt,... 

In the discussion of the Daniell cell we indicated that this cell produces 1.10 volts. This voltage is really the difference in potential between the two half-cells. There are half-cell potentials associated with all half-cells A list of all possible combinations of half-cells would be tremendously long. Therefore, a way of combining desired half-cells has been developed. The cell potential (really the half-cell potentials) depends on concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard temperature of 298 K (25°C) and all components in their standard states (1 M concentration of all solutions, 1 atmosphere pressure for any gases, and pure solid electrodes). All the half-cell potentials are tabulated as the reduction potentials, that is, the potentials associated with the reduction reaction. The hydrogen half-reaction has been defined as the standard and has been given a value of exactly 0.00 V. All the other half-reactions have been measured relative to it, some positive and some negative. The table of standard reduction potentials provided on the AP exam is shown in Table 16.1 and in the back of this book. 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

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