FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A series circuit in a 28-volt DC aircraft system contains a 4-ohm resistor and a 10-ohm resistor. A technician measures the total circuit current at 2 amps. What is the voltage drop across the 10-ohm resistor?
- A20 volts
- B14 volts
- C8 volts
- D28 volts
Show answer & explanationAnswer & explanation
Correct answer: A. 20 volts
In a series circuit, current is the same through every component. Voltage drop equals I × R, so 2 amps × 10 ohms = 20 volts across that resistor.
Why the other options are wrong
- B. 14 ohms is the total resistance, not a voltage value.
- C. This is the drop across the 4-ohm resistor (2A × 4Ω), not the 10-ohm one.
- D. This is the full source voltage, which would only apply if all resistance were in this one resistor.
Series Circuit Voltage Drop
In a series circuit, the same current flows through all components, and each resistor drops a voltage proportional to its resistance.
- Current is constant throughout a series circuit
- Voltage drop = I × R for each resistor
- Sum of all voltage drops equals source voltage
- Total resistance = sum of individual resistances
Memory trick: Think of a single-lane road: the same traffic (current) passes every tollbooth (resistor), each charging its own toll (voltage drop).