Understanding intra and extracelullar concentrations (membrane potential)
Afaik you can describe the potentials between the two sides of the cell membrane using the Nernst-equation. Yepp wikipedia writes the same here.

Membrane potential is a very important thing, because human cells require stable internal concentrations of Na+, K+, Ca2+, etc... in order to work properly. These concentration can depend on species, for example plants cells work completely different than animal cells, they have higher Na+ than K+ in their cytoplasm. So every human cell has to maintain a membrane potential if they don't want to die. It requires a lot of ATP (energy) to do so... Human cells typically maintain this potential against the blood plasma. Cytoplasm has around 120mM K+ and 15mM Na+, while the blood plasma has about 5mM K+ and 140mM Na+. You can count the membrane potential from these concentrations using the Nernst equation (a short presentation about how). I don't remember the exact mV value, does not matter. So human cells are working hardly on keeping the Na+ outside and K+ inside, and they can possibly die because of membrane potential changes (e.g. when they require too much energy to maintain the internal concentrations and/or osmolarity).

Excitable cells, e.g. nerve cells or muscle cells are very special because they can change their membrane potential rapidly by sending out K+, Ca+ or sending in Na+. This can create small electric discharges, which can be used to send electric signals or contract muscles, etc... Different cells have different discharge diagrams, e.g. axons have something like this:




Figure 1 - axon electric discharge diagram - source


and muscles have something like this:




Figure 2 - muscle electric discharge diagrams - source


Summa summarum it can depend on the cell type what happens, but basicly you can count the new potential using the Nernst equation by the concentration changes you mentioned...