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. The same current flows through both lamps.
• C. Twice the supply voltage.
• D. Zero volts.

Answer: (A)

In a parallel circuit, every branch is connected across the same two points, so each component experiences the same voltage as the source. That means the 10-watt lamp has the full supply voltage, and the 20-watt lamp has the same voltage as well. With a 30-volt source, the voltage across the 10-watt light is 30 volts, just like across the 20-watt light. The wattage rating determines how much current each lamp draws at that voltage (I = V/R, or R = V^2/P). For 30 V, the 10-watt lamp has R = 30^2/10 = 90 Ω and the 20-watt lamp has R = 30^2/20 = 45 Ω, so currents are 0.333 A and 0.667 A respectively, but the voltage remains 30 V on both.