Question T5C11
From subelement T5 - T5C
How much current is required to deliver 120 watts at a voltage of 12 volts DC?
Why is this correct?
The correct answer is B (10 amperes). Using the power formula I = P/E, we calculate: I = 120 watts ÷ 12 volts = 10 amperes. Choice A (0.1 amperes) would only deliver 1.2 watts at 12 volts. Choice C (12 amperes) would deliver 144 watts, which is too much power. Choice D (132 amperes) would deliver an enormous 1,584 watts at 12 volts.
Memory tip
Remember the power circle: cover what you want to find, and the remaining values show the formula. For current when you know power and voltage, divide power by voltage. This pattern works for any power calculation problem.
Learn more
In practical amateur radio circuits, this calculation helps determine proper fusing and wire sizing. A 120-watt transmitter at 12 volts DC draws exactly 10 amperes, so you'd need at least 12-15 amp fusing and appropriately rated conductors. Mobile installations commonly operate at 13.8 volts, where the same 120-watt radio would draw slightly less current due to the higher supply voltage.
Think about it
Why do you think higher voltage systems require less current to deliver the same power, and how does this principle benefit amateur radio mobile installations?