Series vs. parallel circuits
CERT 1 · DC/AC Theory & Safety · ~11 min
In a series circuit there is one path: the same current flows through every component, and voltages divide across them. Total resistance is the simple sum, R_T = R₁ + R₂ + …. Open any point and the whole circuit dies — old holiday lights taught everyone this.
In a parallel circuit each component gets the full source voltage, currents divide among branches, and total resistance is always lower than the smallest branch: 1/R_T = 1/R₁ + 1/R₂ + …. Branch circuits in a home are parallel — every receptacle sees 120 V regardless of what its neighbors do.
Exam trap: adding a branch in parallel lowers total resistance and therefore raises total current. That's why plugging too many loads into one circuit trips the breaker — the breaker is watching total current, not the number of devices.
Exam focus: expect three question styles — compute R_T, identify what happens when a component opens, and the 'equal resistors' shortcut (two equal resistors in parallel = half of one).
Quiz — 3 questions
1. Two 10 Ω resistors in series total:
2. Two 10 Ω resistors in parallel total:
3. One lamp in a parallel string burns open. The others: