One circuit. Different routes.
Before you begin: You should recognize a source, a load, a switch, an open circuit, and a voltage measurement from Unit 1.
After this unit, you can: trace current paths in series, parallel, and combination circuits; explain how voltage and current are shared; calculate values for ideal resistors; and predict how an open affects the remaining circuit.
Series: the same current through every load
In a series circuit, loads are connected end to end along one conducting path. With a closed switch and an ideal source, the same current passes through every series element. Each resistor develops part of the supply's voltage drop.
What adds?
Total resistance: RT = R₁ + R₂. For 6 Ω and 12 Ω, RT = 18 Ω.
What stays the same?
Current: I = V ÷ RT. With 12 V, I = 12 ÷ 18 ≈ 0.667 A through both resistors.
Voltage drops are I × R. The 6 Ω resistor drops 4 V, and the 12 Ω resistor drops 8 V. Together, the drops equal 12 V.
Reveal the reasoning
There is no longer a complete path. In this ideal DC model, current through both resistors becomes zero.Parallel: the same voltage across each branch
In a parallel circuit, each branch connects between the same two nodes. Both branches have the same voltage across them. The current in each branch depends on that branch's resistance, and the source supplies the sum of branch currents.
Branch A: 6 Ω
At 12 V: IA = 12 ÷ 6 = 2 A.
Branch B: 12 Ω
At 12 V: IB = 12 ÷ 12 = 1 A.
Source current
IT = 2 + 1 = 3 A. Req = 12 ÷ 3 = 4 Ω.
Opening Branch A interrupts that path, but Branch B remains supplied in this ideal circuit. Its 1 A branch current does not change when the source stays at 12 V.
Reveal the reasoning
Adding another conducting parallel path lowers the equivalent resistance, so the ideal source supplies more total current at unchanged voltage.Series-parallel: trace the common path first
A combination circuit contains both series and parallel relationships. In our example, resistor A (6 Ω) sits in series with two parallel resistors: B (6 Ω) and C (12 Ω).
The source current passes through A, then splits: B carries 4.8 ÷ 6 = 0.80 A; C carries 4.8 ÷ 12 = 0.40 A. After the branches rejoin, their currents add back to 1.20 A.
An open circuit changes what can still operate
Don't diagnose from a dead load alone. Trace which components still have a complete path.
Series circuit
One load opens → current stops everywhere in that single path.
Parallel circuit
One branch opens → that branch stops; intact branches can continue operating.
Series-parallel circuit
Common series resistor A opens → all branches stop. Branch B opens → current can still flow through A and C.
A fuse that opens may remove supply to several branches. A damaged branch wire may affect only one load. Distinguish shared components from independent paths before choosing a test point.
Change the connections. Open a path. Read the evidence.
Select a circuit, then open a component to observe how current distributes. All components below are ideal resistors, not real vehicle lamps.
The Challenger's lab missions
Explore all three circuit types, then create at least one open-circuit fault and compare the readings. Use the evidence to explain which loads can still operate.
Explored: 0 of 3 types; fault demonstrated: no.
Eight questions. Trace the path before guessing.
Select one answer for each question. A score of 7 out of 8 meets this unit's on-device knowledge target. Review feedback and retry whenever you like.
Show it on a real low-voltage trainer
Use a properly protected 12 V training board with a known current limit, safe resistor power ratings, and instructor-approved procedures. Turn power off before rewiring or changing components. A virtual score does not certify physical competence.
These checkboxes are a personal preparation checklist, not instructor sign-off or a credential. A qualified instructor must verify hands-on work independently.
What have you demonstrated?
Complete four lessons, compare the circuit types in the lab, and meet the assessment target.
Next suggested unit: Electrical Measurements & Ohm's Law — connect what happens in these paths to voltage, current, resistance, and power. This is a planning direction, not a link to a completed unit.