Compute the total current flowing between two electrodes in an Electro Quasistatic LF simulation

Assume we have an LF simulation with two electrodes, at + and - 1V, respectively, as shown in the voxel view below. We want to compute the total current flowing between the two electrodes.


We start by creating a plane that cut through the computational domain, with each electrode in either part of the plane:


We then use the Flux Evaluator to compute the total current flowing through the plane:

Is there a disadvantage to calculating the current by using the power loss in the simulation domain and avoiding extra geometry? (i.e. R = (V^2)/P and then I = V/R using the "Total Loss" in "All Regions" from the SAR statistics evaluator?)

The "Total Loss" approach works too. Be aware that it assumes that all the power dissipated by the system occurs via the Joule effect in conductive materials. In other words, it only accounts for Ohmic currents and neglects the contribution of Displacement current (like the current that flows between the two plates separated by air in a capacitor).
This means you can use this approach if you could have used the Ohmic Quasi-Static solver instead of the Quasi-Static one (since the former neglects displacement currents, while the latter does not).
For currents flowing between electrodes placed on a human body, displacement currents are typically negligible.

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Can I consider the current in the body during electrode stimulation like this? First, the simulation result can get the current density. Its unit is A/m². Assuming that the current current density is 100A/m², I want to calculate the current at a certain point in the muscle Intensity, I choose 1mm², then the current density at this time becomes 0.1mA/mm², can I consider the current intensity at the current position as 0.1mA?

Current intensity is defined for a given cross-section. Usually, it is used for wires, where the cross-section is obvious, and it relates to the "amount of current" passing through the wire. A bit like the amount of water passing under a bridge, for a river (referred to as volumetric flow rate, or "discharge").
For an infinitesimal point, it doesn't make much sense to speak of current intensity (or of flow rate, for the river analogy). The relevant quantity, in that case, is the flux of current (or water...), expressed in A/m2 (or volume/s/m2).

I hope this helps.