Every living cell is a tiny battery. Pump more potassium inside than out, let it leak, and the charge imbalance builds a VOLTAGE across the membrane — the Nernst equation says exactly how much: about −90 millivolts for potassium. That voltage is the resting state a nerve fires FROM. Concentration on one side, electricity on the other, balanced. Slide the ion gradient.
The Nernst equation gives the equilibrium potential where an ion’s diffusion (down its concentration gradient) exactly balances the electrical force: E = (RT/zF)·ln([out]/[in]). At body temperature RT/F ≈ 26.7 mV, so for potassium (z=+1, ~5 mM out / 140 mM in) Eₖ ≈ −89 mV — close to a neuron’s resting potential, which the cell holds by leaking mostly K⁺. Every action potential is a controlled departure from these Nernst voltages. A fail-loud self-check throws unless K⁺ at 5/140 gives about −90 mV. ◆ real electrophysiology, node-verified.
Nernst gives ONE ion’s equilibrium; the real resting potential (Goldman equation) weights several ions by permeability, so it sits near but not exactly at Eₖ. The concentration↔voltage balance is the exact content.