---
title: "Equilibrium Potentials and What Sets the Resting Voltage"
description: "Each ion has a voltage at which its electrical pull balances its concentration gradient, and that value can be calculated from the two concentrations. The membrane sits near the equilibrium potential "
canonical: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1
source: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1.md
retrieved: 2026-09-12
---

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# Equilibrium Potentials and What Sets the Resting Voltage

Each ion has a voltage at which its electrical pull balances its concentration gradient, and that value can be calculated from the two concentrations. The membrane sits near the equilibrium potential of whichever ion it is currently most permeable to.

Subject: Science · Area: The Human Body · Ages 18 to 19
Page: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1

## Ready when they can

- Calculates an equilibrium potential from inside and outside concentrations.
- Predicts how the resting voltage shifts when external potassium is raised.
- Explains why the resting membrane sits near the potassium value but not exactly at it.

## Lesson: The voltage each ion wants

Each ion has its own equilibrium voltage, where its electrical pull exactly balances its concentration gradient. That value follows from the inside and outside concentrations. Pumps build the gradients: more sodium outside the cell and more potassium inside.

At rest the membrane is most open to potassium, so potassium leaking out leaves the inside negative. That is why rest sits near the potassium equilibrium. Small flows of sodium and other ions drag the real voltage a little way off that ideal.

**Example.** Raise potassium outside the cell and its gradient shrinks, so its equilibrium shifts upward. The resting voltage follows and grows less negative. Lower outside potassium would push rest the other way, more negative.

**Tip.** Treat rest as a compromise weighted by how easily each ion crosses, not a prize won by potassium alone. Change any gradient or any permeability and the compromise moves.

**Recap.** Each ion pulls rest toward its own equilibrium, and potassium pulls hardest because rest lets it through best.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Resting Potential and the Action Potential](https://lightmysky.com/learn/science/the-resting-potential-and-the-action-potential-mt_0sh4w30ocz)
- [Co-transport and Coupled Movement Across Membranes](https://lightmysky.com/learn/science/co-transport-and-coupled-movement-across-membranes-mt_pzQ8n3FWNV)

## Opens up

- [Channel Kinetics and the Ionic Basis of the Spike](https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp)