Add the individual resistances: $R_{\text{eq}, s} = \sum_{i} R_{i}$

Use the reciprocal sum formula: $\frac{1}{R_{\text{eq}, p}} = \sum_{i} \frac{1}{R_{i}}$

Subtract the voltage drop across the internal resistance from the emf: $\Delta V_{\text{terminal}} = \mathcal{E} - Ir$

Add the individual resistances: $R_{\text{eq}, s} = \sum_{i} R_{i}$

Use the reciprocal sum formula: $\frac{1}{R_{\text{eq}, p}} = \sum_{i} \frac{1}{R_{i}}$

Subtract the voltage drop across the internal resistance from the emf: $\Delta V_{\text{terminal}} = \mathcal{E} - Ir$

The total resistance of a group of resistors, treated as a single resistor to simplify circuit analysis.

The potential difference across the terminals of a battery when no current is flowing; the ideal voltage of the battery.

A battery that provides a constant voltage regardless of the current and has zero internal resistance.

An ammeter with zero resistance, so it doesn't affect the current it measures.

A voltmeter with infinite resistance, so it doesn't draw any current from the circuit.

The resistance within a real battery that impacts the voltage it delivers, causing the terminal voltage to be less than the emf when current flows.