Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
A technician is troubleshooting a series circuit containing three resistors: R1 = 10 Ω, R2 = 15 Ω, and R3 = 25 Ω. If the total voltage applied to the circuit is 100 V, what is the voltage drop across resistor R2?
- A20 V
- B60 V
- C30 V
- D50 V
Show answer & explanationAnswer & explanation
Correct answer: C. 30 V
First, find the total resistance (R_total) in a series circuit: R_total = R1 + R2 + R3 = 10 Ω + 15 Ω + 25 Ω = 50 Ω. Next, find the total current (I_total) using Ohm's Law: I_total = V_total / R_total = 100 V / 50 Ω = 2 A. Finally, calculate the voltage drop across R2 using Ohm's Law again: V2 = I_total × R2 = 2 A × 15 Ω = 30 V.
Why the other options are wrong
- A. This would be the voltage drop across R1 (2A * 10Ω = 20V).
- B. This is an incorrect calculation, possibly from using an incorrect total current or resistance.
- D. This would be the voltage drop across R3 (2A * 25Ω = 50V).
Series Circuit Analysis
In a series circuit, components are connected end-to-end, so the current is the same through each component, and voltages add up.
- Total Resistance (R_total) = R1 + R2 + R3 + ...
- Current (I) is constant throughout the circuit.
- Total Voltage (V_total) = V1 + V2 + V3 + ... (Kirchhoff's Voltage Law).
Memory trick: Series: Current's the same, Resistance sums up, Voltages drop.