What the ASVAB is actually testing
Circuit-type questions are some of the most calculation-heavy on EI. They give you a circuit configuration and ask about total resistance, current, or voltage. The key is knowing which of the two rule sets applies — series or parallel — before you start calculating.
Series circuits: everything in one loop
In a series circuit, components are connected end-to-end in a single loop. There's only one path for current.
Rules for series:
- Current is the same everywhere — one loop means identical flow throughout
- Voltage divides — each component gets a share of the total voltage; they add up to the source
- Resistance adds — R_total = R1 + R2 + R3 + ...
Consequence: if any component fails (opens), the entire circuit goes dark — like old-style Christmas lights.
Parallel circuits: multiple branches
In a parallel circuit, components are connected side-by-side, sharing the same two connection points. Current has multiple paths to choose from.
Rules for parallel:
- Voltage is the same across every branch — each branch sees the full source voltage
- Current divides — more current flows through lower-resistance branches; branch currents add to the total
- Total resistance decreases — adding more branches always lowers total resistance
This is how household wiring works — each outlet gets full voltage, and appliances operate independently.
Two-resistor parallel shortcut
For exactly two resistors in parallel, skip the reciprocal formula:
R_total = (R1 × R2) / (R1 + R2)
Two 6 Ω resistors in parallel: (6 × 6) / (6 + 6) = 36/12 = 3 Ω
That's always exactly half of either resistor when they're equal.
How to attack circuit questions
- Identify: series (single loop) or parallel (multiple branches)?
- Write down the appropriate rule set
- Solve for total resistance first, then use V = IR for current or voltage
If the question mentions a short circuit or open circuit: an open (broken wire) stops current; a short (zero resistance path) causes maximum current and often damages the circuit.
Study approach
Draw a simple series and parallel circuit diagram from memory. Then practice each rule set with two and three resistors until you can execute the calculation in under 30 seconds. Speed matters on EI.