# What is A in the following equation

This equation describes the total current in bulk solution in a battery caused by migration in the electrolyte.

Then the source says what A is: In the bulk solution (away from the electrode), concentration gradients are generally small, and the total current is carried mainly by migration. All charged species contribute. For species j in the bulk region of a linear mass transfer system having a cross sectional area, $A$...

$$i = \sum_j i_j = \frac{FA\,\Delta E}{l} \sum_j |z_j| u_j C_j$$

I am confused if $A$ is the electrode area or the solution cross sectional area.

I will describe what the rest of the variables are

1. The $i$ is the current in the bulk solution.
3. ∆E is the change in potential (i.e. Voltage measurement of the battery)
4. l is the distance of how far the electrodes are separated (or at least that is what I think)
5. z is the charge of species j
6. u is mobility of species j
7. C is the concentration of species j
• Can you show us where this equation is from? – Fl.pf. Mar 25 '17 at 8:17
• just giving some random equation doesnt seem such a good idea, could you describe its origin, and/or define what it gets you? ie what is in the LHS of the eqn? – Supernova Mar 25 '17 at 12:41
• sorry for that. I edited the question to hopefully make it more understandable – user510 Mar 25 '17 at 13:20

$A$ is the cross-sectional area in solution. The "bulk" is the relatively homogeneous "mess" of charged particles and solvent -- $A$ is not the area of the electrode.
What this equation tells you in terms of $A$, is that, the larger the area through which some number of charges are passing in a given moment, the greater the possible current, because more area means more room for more charges that can pass through in a moment. If you squeeze the area you start to make a bottleneck wherein less charged particles can pass in a given time. (This is why copper wires with larger diameter (or, smaller US gauge) have higher current capacity).
This makes sense, right? Current is measured as charge per unit time. The implication there, is that it is motion of charge through some point (1D) or cross sectional area (2D, $A$ here) per unit time.