alleryStart learning →A cheap extension cord made with thinner wire has a resistance of 0.300 ohm. How much power does it waste as heat when carrying 5.00 A?
A1.5 W
B7.5 W
C75 W
D0.75 W
Answer & Solution
Correct answer: B. 7.5 W
1. The power dissipated in a resistance carrying current is P = I squared times R.
2. Square the current: (5.00 A) x (5.00 A) = 25.0 A squared.
3. Multiply by the resistance: P = 25.0 x 0.300 ohm.
4. This gives P = 7.5 W, all of it turned to heat inside the cord.
5. Thinner wire means higher resistance, which is why cheap cords run warm at currents a heavier cord handles easily.
6. 1.5 W is the power a good 0.0600-ohm cord would waste at the same current, and 0.75 W is the trap of multiplying 5.00 x 0.300 divided by 2 without squaring the current at all.
_Source: OpenStax College Physics (CC BY 4.0), Ch 20 "Electric Current, Resistance, and Ohm's Law", section 20.4 Electric Power and Energy_
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