A cabin entry light of 10 watts and a dome light of 20 watts are connected in parallel to a 30-volt source. If the voltage across the 10-watt light is measured, it will be _________.

A cabin entry light of 10 watts and a dome light of 20 watts are connected in parallel to a 30-volt source. If the voltage across the 10-watt light is measured, it will be _________.

• A. Equal to the voltage across the 20-watt light
• B. Zero volts
• C. Half the source voltage
• D. Double the source voltage

Answer: (A)

In a parallel circuit, each device is connected directly across the same two points, so every branch has the same voltage as the source. With both lights in parallel across a 30-volt source, the voltage across the 10-watt lamp is 30 volts, just like the voltage across the 20-watt lamp. The wattage only tells you how much current each lamp draws at that voltage (I = P/V), so the 10-watt lamp draws 10/30 = 0.333 A and the 20-watt lamp draws 20/30 = 0.667 A, but the voltage remains the same for both.

In a parallel circuit, each device is connected directly across the same two points, so every branch has the same voltage as the source. With both lights in parallel across a 30-volt source, the voltage across the 10-watt lamp is 30 volts, just like the voltage across the 20-watt lamp. The wattage only tells you how much current each lamp draws at that voltage (I = P/V), so the 10-watt lamp draws 10/30 = 0.333 A and the 20-watt lamp draws 20/30 = 0.667 A, but the voltage remains the same for both.