A parallel circuit consists of R1 = 30 ohms, R2 = 15 ohms, R3 = 10 ohms. If the current through R2 = 4 amps, what is the source voltage?

A parallel circuit consists of R1 = 30 ohms, R2 = 15 ohms, R3 = 10 ohms. If the current through R2 = 4 amps, what is the source voltage?

• A. 120 volts
• B. 30 volts
• C. 60 volts
• D. 15 volts

Answer: (C)

In a parallel circuit, the voltage across every branch is the same as the source voltage. Use Ohm’s law on the known branch: with a current of 4 A through a 15 Ω resistor, the voltage across that branch is V = I × R = 4 × 15 = 60 V. Since all branches share the same voltage in parallel, the source voltage is 60 V. (If you check the other branches, they would carry 60/30 = 2 A and 60/10 = 6 A, respectively, but the given data already fixes the source voltage at 60 V.)

In a parallel circuit, the voltage across every branch is the same as the source voltage. Use Ohm’s law on the known branch: with a current of 4 A through a 15 Ω resistor, the voltage across that branch is V = I × R = 4 × 15 = 60 V. Since all branches share the same voltage in parallel, the source voltage is 60 V. (If you check the other branches, they would carry 60/30 = 2 A and 60/10 = 6 A, respectively, but the given data already fixes the source voltage at 60 V.)