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. Greater than the voltage across the 20 watt light
• B. Less than the voltage across the 20 watt light
• C. Equal to the voltage across the 20 watt light
• D. Not determinable

Answer: (C)

In a parallel circuit, the voltage across each component is the same because both are connected directly across the same two nodes. Here, both lights are wired across a 30-volt source, so each lamp experiences the full 30 volts. The wattage ratings simply tell you how much power each lamp would dissipate at that voltage (and relate to their resistances). Using P = V^2 / R, the 10 W lamp has a resistance of 90 ohms and the 20 W lamp 45 ohms, so currents are 30 V / 90 Ω = 0.333 A and 30 V / 45 Ω = 0.667 A, respectively. Despite different currents, the voltage across the 10 W light equals the voltage across the 20 W light: both are 30 V.