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: $\Delta V_{\text{terminal}} = \mathcal{E} - Ir$

Series: Single path for current, current is the same through each resistor, resistances add directly. | Parallel: Multiple paths for current, voltage is the same across each resistor, reciprocal sum for equivalent resistance.

Ideal: Constant voltage, zero internal resistance. | Non-Ideal: Voltage drops with current, has internal resistance.

Ideal: Zero resistance, no impact on circuit. | Non-Ideal: Small resistance, decreases measured current.

Ideal: Infinite resistance, no current draw. | Non-Ideal: Finite resistance, draws some current, affecting voltage measurement.

It decreases the overall resistance of the circuit, providing more paths for current to flow.

It reduces the terminal voltage of the battery when current is flowing.

The measured current will be lower than the actual current in the circuit.

The measured voltage will be lower than the actual voltage across the component.