Question T5C09
From subelement T5 - T5C
How much power is delivered by a voltage of 13.8 volts DC and a current of 10 amperes?
Why is this correct?
Power equals voltage multiplied by current: P = E × I. With 13.8 volts and 10 amperes, the calculation is 13.8 V × 10 A = 138 watts. Choice B (0.7 watts) results from incorrectly dividing voltage by current. Choice C (23.8 watts) comes from adding voltage and current. Choice D (3.8 watts) results from subtracting current from voltage. Only multiplication gives the correct power calculation.
Memory tip
Remember 'PIE' as your power formula guide: P (power) equals I (current) times E (voltage). When you see power questions, immediately identify the two given values and multiply them together. This multiplication relationship is fundamental to all DC power calculations.
Learn more
This 13.8-volt, 10-ampere scenario represents typical amateur radio mobile operation — 13.8 volts is standard automotive electrical system voltage, and 138 watts falls within common mobile transceiver power output ranges. Understanding power calculations helps determine proper power supply sizing, antenna system efficiency, and compliance with emission standards for your frequency privileges. Higher power requires more robust cooling and power supply capacity.
Think about it
Why do you think mobile amateur radio equipment is commonly rated around 100-150 watts maximum power output?