Current
Density "J"
What it is
Current is the flow of electrical charges. Current density is the amount of
current flow per cross-sectional area normal to the direction of current flow.
Current density is a vector which points in the direction of current flow.
The rate of an electrochemical reaction can best be described in terms of
current density. Metal powders can be made by electrodepositing metals at high
current densities, and then breaking up the deposit by milling. Current density
is an important property in chemistry, electro-plating, welding, thermonic
emission, electron tubes, heating elements, etc.
History
Little, or nothing, was known about current density and most other
electrical and magnetic properties until Alessandro Giuseppe Volta(1745-1827)
invented the electric battery in 1794. The electric battery provided man
with his first reliable, high current, source of electrical power. The electric
battery could produce several orders of magnitude more electrical current, than
the electrostatic generators previously used to generate electricity. This allowed
many new kinds of experiments to be conducted, and lead to rapid advances
in many fields, including magnetism, electro-magnetism, electro-plating, and
electro-chemistry.
The concept of current density, introduced by James Clerk Maxwell (1831-1879)
about 1870, was based on the experiments and ideas of Michael Faraday
(1791-1851).
Richardson's law, which is used to compute the rate at which electrons
escape from metal, was proposed by Owen W. Richardson (1879-1959) in 1911.
Common equations
H = dD/dt + J
Units
amperes per meter squared
Editorial comments