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

Answer: (B)

In a series circuit, the same current flows through every component, and the voltage divides in proportion to each component’s resistance. The total resistance is 10 Ω + 20 Ω = 30 Ω, and with a 30-volt source, the current is I = V / R = 30 V / 30 Ω = 1 A. The voltage drop across a resistor is V = I × R, so across the 10-Ω resistor the drop is 1 A × 10 Ω = 10 volts. The other resistor drops 1 A × 20 Ω = 20 volts, and 10 V + 20 V = 30 V. Therefore, the voltage drop across the 10-Ω resistor is 10 volts.

In a series circuit, the same current flows through every component, and the voltage divides in proportion to each component’s resistance. The total resistance is 10 Ω + 20 Ω = 30 Ω, and with a 30-volt source, the current is I = V / R = 30 V / 30 Ω = 1 A. The voltage drop across a resistor is V = I × R, so across the 10-Ω resistor the drop is 1 A × 10 Ω = 10 volts. The other resistor drops 1 A × 20 Ω = 20 volts, and 10 V + 20 V = 30 V. Therefore, the voltage drop across the 10-Ω resistor is 10 volts.