Nernst equation
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
E = E° - (RT/nF) × ln(Q) = E° - (0.0592/n) × log(Q) at 25°C; relates cell potential to non-standard concentrations.
Nernst equation in plain English
A battery's voltage isn't a fixed number — it drops as the battery gets used up, and the Nernst equation is the formula that predicts that voltage for any given moment. The idea: a chemical reaction pushes electrons harder (higher voltage) when it has plenty of fresh reactants, and pushes weaker as products pile up and reactants run low — a dead battery is one where the push has fallen to zero. In E = E° - (0.0592/n)log(Q), E° is the voltage under ideal 'standard' conditions, Q measures how far the reaction has drifted from those conditions, and n is how many electrons the reaction moves. So the equation just adjusts the ideal voltage up or down depending on the actual concentrations in front of you.
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