FCC Exam Question: 3-61I4

If a marine radiotelephone receiver uses 75 watts of power and a transmitter uses 325 watts, how long can they both operate before discharging a 50 ampere-hour 12 volt battery?

A. 40 minutes.
B. 1 hour.
C. 1 1/2 hours.
D. 6 hours.
Correct Answer: C

Explanation: To determine how long the battery can operate, first calculate the total power consumption. The receiver uses 75 watts and the transmitter uses 325 watts, for a combined total of 75 W + 325 W = 400 W. Next, calculate the total current drawn from the 12-volt battery using the formula P = V * I, which means I = P / V. I = 400 W / 12 V = 33.33 Amperes. Finally, determine the operating time by dividing the battery's capacity in Ampere-hours by the current draw in Amperes. Time = Battery Capacity / Current = 50 Ah / 33.33 A = 1.5 hours. Therefore, the battery can operate both devices for 1.5 hours, or 1 hour and 30 minutes, before discharging.

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