What Is Nernst equation?
The Nernst equation relates the actual voltage of an electrochemical cell to temperature and the current balance of reactants and products. In common form, E = E0 – (RT / nF) ln(Q), where the logarithmic term captures how far the system is from standard conditions. It explains why battery voltage changes as concentration and state of charge change during real operation.
In a working cell, the equation connects measured voltage to the underlying Electrochemical Potential difference between electrodes. As ions are redistributed and species in the Electrolyte change their effective concentrations, the reaction quotient Q shifts and the cell potential follows. Temperature also matters, which is why open-circuit voltage is never just a fixed chemistry label.
The concept matters because voltage curves are one of the most visible outputs of battery chemistry. In battery voltage modeling, engineers use Nernst-style reasoning to interpret equilibrium voltage, estimate state of charge, and separate thermodynamic limits from extra losses caused by resistance, polarization, or reaction kinetics. It is most accurate near equilibrium, but it still provides a thermodynamic baseline for interpreting measured voltage under load.
Example:
A lithium-ion cell shows a different open-circuit voltage after partial discharge because the reaction quotient has changed from its fully charged value.
Related Concepts:
- Reaction Quotient
- Open-Circuit Voltage
- Faraday Constant
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