The operating resistance of a 30-watt light bulb designed for a 28-volt system is approximately what?

The operating resistance of a 30-watt light bulb designed for a 28-volt system is approximately what?

• A. 28 ohms
• B. 24 ohms
• C. 30 ohms
• D. 26 ohms

Answer: (D)

When a device is rated to operate at a certain power with a given voltage, its resistance at operating temperature can be found from P = V^2 / R, so R = V^2 / P. For a 28-volt system and a 30-watt bulb, R = 28^2 / 30 = 784 / 30 ≈ 26.1 ohms. So the operating resistance is about 26 ohms. This reflects the hot, running resistance of the filament; cold resistance is lower, but the rating uses the resistance at operating temperature. The computed value around 26 ohms is the closest match to the options.

When a device is rated to operate at a certain power with a given voltage, its resistance at operating temperature can be found from P = V^2 / R, so R = V^2 / P. For a 28-volt system and a 30-watt bulb, R = 28^2 / 30 = 784 / 30 ≈ 26.1 ohms. So the operating resistance is about 26 ohms. This reflects the hot, running resistance of the filament; cold resistance is lower, but the rating uses the resistance at operating temperature. The computed value around 26 ohms is the closest match to the options.