In response to a stimulus, the membrane can become permeable to Na ions. As Na ions rush into the cell, the interior of the cell reaches an electric potential of about 40 rm mV. This process is termed depolarization. In response to depolarization, the membrane again becomes impermeable to Na ions, and the K ions flow out of the interior of the cell (driven by the positive electric potential inside of the cell), reestablishing the resting potential. This is termed repolarization. Only a small percentage of the available Na and K ions participate in each depolarization/repolarization cycle, so the cell can respond to many stimuli in succession without depleting its “stock” of available Na and K ions. During the resting phase, what is the electric potential energy of a typical Na ion outside of the cell?