Internal resistance

What is internal resistance?

Internal resistance (r) is the resistance inside a cell or battery that opposes current flow. The terminal potential difference is lower than the cell's electromotive force (EMF, ε) when current flows: V = ε − Ir. As current draw increases, terminal V drops. A-Level Physics examines this via plotting terminal V against I — the y-intercept is the EMF, the negative gradient is the internal resistance. The cell is also a closed loop in Kirchhoff analysis.