I. CURRENT DENSITY. 7
P2, which are a cm. apart, is V volts, then at each point a force is acting against the displacement of 1 coulomb of positive electricity from the lower to the higher potential. The total work which is done in moving 1 positive coulomb from P2 to P1 is, therefore, , expressed in 10.2 kilogram-centimetres as unit. Usage has led to the adoption of electromotive force as synonymous with potential difference, although the former expression is not quite exact.
Current Density. — The processes which take place at
the poles P1 and P2 depend to a great extent on how much
gas, or substance in general, is deposited on each square
centimetre of the plates per second. If the current strength
is A amperes, and if plate P1 has an area of y sq. cms., then
the quantity deposited on 1 sq. cm. per second is given by
. The value of this expression is termed the "current
density," which obviously is measured in amperes per square
centimetre. In the above example, must only be regarded
as an average value of the current density; but in those cases
where the fall of potential in the solution is the same throughout, the current density has the same value at all parts of the
plate.
Electrochemical Equivalents. — It has already been
mentioned that 1 coulomb can bring about the deposition
of 1.118 mgram. of silver, 0.3284 mgram. of copper, or
0.0104 mgram. of hydrogen. On this account, therefore, we
say that 1.118 mgram. of silver, 0.3284 mgram. of copper, and
0.0104 mgram. of hydrogen are electrochemically equivalent.
The electrochemical equivalents correspond exactly with
the chemical equivalents, which represent the weights of two
substances capable of replacing each other in chemical
compounds (Faraday's law). Thus, for instance, 31.8 grams of
copper can replace 1 gram of hydrogen from 49 grams of
sulphuric acid, and produce 79.8 grams of copper sulphate.
