Series and Parallel Circuits

The idea

Components can be wired in a single loop (series) or on separate branches (parallel), and the two behave completely differently.

graph LR
    subgraph Series
        B1["Battery"] --> L1["Lamp 1"] --> L2["Lamp 2"] --> B1
    end

graph LR
    subgraph Parallel
        B2["Battery"] --> N1(( ))
        N1 --> P1["Lamp 1"] --> N2(( ))
        N1 --> P2["Lamp 2"] --> N2
        N2 --> B2
    end

	Series	Parallel
Current	Same everywhere	Splits between branches
Potential difference	Divides among components	Same across each branch
Total resistance	$R_T=R_1+R_2$	$\frac{1}{R_T}=\frac{1}{R_1}+\frac{1}{R_2}$
One component fails	Everything stops	The others keep going

Why your house is wired in parallel

Every outlet gets the full $120\mathrm{V}$, and unplugging a lamp does not switch off the fridge. Old Christmas lights were series — one dead bulb, and the whole string went dark.

Predict before you build

Two identical lamps in series, then the same two in parallel, on the same battery. Which arrangement is brighter, and why? Commit to an answer, then test it in Building Series and Parallel Circuits.

Curriculum connection

D2.6

construct series and parallel circuits to compare electric current, potential difference, and resistance in both types of circuits

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D2.4

investigate the relationships between electric current, potential difference, and resistance in electrical circuits, and develop a mathematical model to represent the relationships

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