In a parallel circuit with two lamps rated 10W and 20W connected to a 30V source, the voltage across each lamp is equal to the source voltage.

In a parallel circuit with two lamps rated 10W and 20W connected to a 30V source, the voltage across each lamp is equal to the source voltage.

• A. 10 volts
• B. 20 volts
• C. 0 volts
• D. 30 volts

Answer: (D)

In a parallel circuit, each device is connected directly across the same two terminals, so they all share the same voltage as the source. With two lamps in parallel across a 30V supply, each lamp has 30V across it. The wattage ratings tell you how much power each lamp draws at that voltage, not the voltage they experience. For a 30V supply, the 10W lamp has a resistance of 90 ohms (R = V^2/P = 900/10), and the 20W lamp has 45 ohms (R = 900/20). The currents are 0.333 A and 0.667 A respectively, but the voltage across both remains 30 volts. Therefore, the voltage across each lamp is 30 volts.

In a parallel circuit, each device is connected directly across the same two terminals, so they all share the same voltage as the source. With two lamps in parallel across a 30V supply, each lamp has 30V across it. The wattage ratings tell you how much power each lamp draws at that voltage, not the voltage they experience. For a 30V supply, the 10W lamp has a resistance of 90 ohms (R = V^2/P = 900/10), and the 20W lamp has 45 ohms (R = 900/20). The currents are 0.333 A and 0.667 A respectively, but the voltage across both remains 30 volts. Therefore, the voltage across each lamp is 30 volts.