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Electrochemistry Dictionary - F
F: Symbol and abbreviation of farad,
and symbol and abbreviation of the Faraday number. (Often a "bold-face" letter
is used for the latter, but this is not a general practice.) It is usually obvious
from the context which meaning is appropriate.
Failure: The state in which
the performance of a cell or battery does not meet the normal specifications.
farad: Measurement unit of capacitance.
Symbol: "F," which is the same as the symbol of the Faraday Number. It is usually
obvious from the context which meaning is appropriate. A capacitor has a capacitance
of one farad when one coulomb charges it to one volt.
shield (cage): a network of parallel wires
connected to a common conductor at one end to provide electrostatic shielding
without affecting electromagnetic waves. The common conductor is usually grounded.
Faraday's Laws: Some of the most fundamental
laws of electrochemistry discovered by Faraday in the 1830's. They are usually
stated as: (1) In any electrolytic process the amount of chemical change produced
is proportional of the total amount of electrical charge passed through the
cell. (2) The mass of the chemicals changed is proportional to the chemicals'
equivalent weight. The proportionality constant being the Faraday Number.
Faraday Number (Faraday constant): The
Faraday Number (or constant) gives the amount of electrical charge needed to
change one gram-equivalent of substance by electrochemical reaction. Its value
is 96,485.34 coulombs or 26.80 ampere-hours. This charge is often simply called
one "Faraday." Symbol: "F," which is the same as the symbol of the farad. It
is usually obvious from the context which meaning is appropriate. The Faraday
number is the product of Avogadro's number and the electrical charge of a single
Faradic current (density): The current
(or current density) that is flowing through an electrochemical cell and is
causing (or is caused by) chemical reactions (charge transfer) occurring at
the electrode surfaces. Contrast with capacitive current.
Faradaic reaction: See heterogeneous
Filling solution: See internal electrolyte.
First kind electrode: See electrode
of the first kind.
Fixed ion: The permanently attached
charged fragment in an ion-exchange resin. Contrast with counterion.
Flade potential: Alternative name for
the passivation potential.
A photovoltaic array in which the incident solar radiation strikes
a flat surface and no concentration of sunlight is involved.
Float charging: A method of maintaining
a rechargeable battery in a fully charged condition by continuous, long-term,
constant-voltage charging, at a level sufficient to balance self-discharge.
See also trickle charging.
Float voltage: The voltage required
for retaining a rechargeable battery in fully charged condition. See float charging.
Flow-through electrode: An electrode
that permits the electrolyte to flow through it, e.g., a porous electrode or
a packed-bed electrode. This type of electrode is especially useful for removing
small traces of impurities from the solution by electrolysis (e.g., waste treatment)
because the solution contacts a large surface of the electrode material.
Fluidized bed electrode: See packed
Formal electrode potential: Similar
to a standard electrode potential except that both the oxidized and the reduced
species are present in unit concentration instead of unit activity. It is not
as well defined as the standard potential but it is useful in cases when the
activities are unknown.
Forming: Electrochemical processing
of a rechargeable battery by repeated charging/discharging that converts the
electrode materials into usable form. This treatment is needed for some batteries
during manufacturing, or when first put into use (and sometime when returned
to service after a long storage). Also called "conditioning."
Formula weight: Essentially the same
as the molecular weight, but it can be used more generally, e.g., also for ions.
Fresnel lens: A concentrating lens,
positioned above and concave to a PV material to concentrate light on the material.
Fuel cell: A device that converts chemical
energy into electrical energy. It is different from a battery in that the energy
conversion continues till fuel and oxidizing agent are fed to the fuel cell;
that is, in principle indefinitely. (A battery is manufactured with a limited
amount of chemicals, and it is exhausted when all the chemicals have reacted.)
It is a galvanic cell, where spontaneous chemical reactions occur at the electrodes.
The fuel is oxidized at the anode, and the oxidizing agent (almost always oxygen
or air) is reduced at the cathode. Presently, the most commonly used fuel is
hydrogen. More conventional fuels (e.g., gasoline or natural gas) must be converted
("reformed") into hydrogen before they can be utilized in a fuel cell. Fuel
cells that can burn hydrocarbon fuels directly are in the development stage.
Some fuel cells employ an aqueous solution as electrolyte, that can be either
acidic or basic (alkaline), or an ion-exchange membrane soaked in aqueous solution
can act as the electrolyte (see PEM) these fuel cells operate at relatively
low temperatures (from room temperature to not much above the boiling point
of water). Some fuel cells employ molten salts (especially carbonates) as electrolytes
and have to operate at many hundreds of oC temperature. Others employ
ionically conductive solids as electrolyte and must operate close to 1000
oC. A single fuel cell has a rather small (typically less then one
volt) cell voltage. For practical applications a large number of them are assembled,
series coupled, in what is called a "fuel cell stack." (A term essentially analogous
to the original meaning of the battery.)
Fugacity: The expression of "activity"
for a component in a mixture of gases. It has the same physical meaning as the
activity for a component in a solution.