In a circuit with an applied voltage of 30 volts and a load consisting of a 10-ohm resistor in series with a 20-ohm resistor, what is the voltage drop across the 10-ohm resistor?

• A. 30 volts.
• B. 5 volts.
• C. 10 volts.
• D. 20 volts.

Answer: (C)

When resistors are in series, the same current flows through each, and the voltage divides in proportion to each resistance. The total resistance is 10 Ω + 20 Ω = 30 Ω, so the current is I = 30 V / 30 Ω = 1 A. The voltage across the 10-ohm resistor is V = I × R = 1 A × 10 Ω = 10 V. You can also see this as V across 10 Ω = 30 V × (10/30) = 10 V. The remaining drop goes across the 20-Ω resistor (1 A × 20 Ω = 20 V). So the voltage drop across the 10-ohm resistor is 10 volts.