Imagine a cell has a higher concentration of Na inside than outside. Imagine the membrane voltage on this cell is negative (Vm < 0). What direction is the electrical force on Na

Answer and explanation:

If a cell has a higher concentration of sodium inside than the sodium concentration that there is outside of it, there will be a chemical gradient that will direct the sodium to the outside, to equate the concentrations between the inside of the cell and the outside of the cell.

The membrane voltage on cells depends on the conductance of each ion. The more conductant an ion is, the more likely is that it will go through the membrane from the inside to the outside or vice versa. This conductivity relies on the number of active protein channels specific to that ion that can be found in the membrane. If the membrane voltage is negative when the cell is at resting potential, it means that the most conductant ion is one whose equilibrium potential is negative as well. Sodium's equilibrium potential is positive, which suggests that there is another ion inside this cell that is more conductant and whose equilibrium potential is negative - probably potassium.

Given that the membrane voltage is negative, which means that there are more positively charged particles outside than inside, the sodium will most likely flow from the outside to the inside - the electrical force on sodium will be from the outside to the inside.

The determining direction of the sodium will be given by the electrochemical gradient and will depend on the magnitude of the chemical gradient and the electrical one.