# Current and Resistance

The headlights of a certain moving car require 11 A from the 12 V alternator, which is driven by the engine. Assume the alternator is 84% efficient (its output electrical power is 84% of its input mechanical power), and calculate the power (in hp) the engine must supply to run the lights.

I plugged the equation into P = iV and multiplied it by .84 efficiency, then got my answer in watts, which I converted to hp. When I checked in my book, however, the answer was not correct. I'm pretty much stumped on how else to approach it.

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G01
Homework Helper
Gold Member
The headlights of a certain moving car require 11 A from the 12 V alternator, which is driven by the engine. Assume the alternator is 84% efficient (its output electrical power is 84% of its input mechanical power), and calculate the power (in hp) the engine must supply to run the lights.

I plugged the equation into P = iV and multiplied it by .84 efficiency, then got my answer in watts, which I converted to hp. When I checked in my book, however, the answer was not correct. I'm pretty much stumped on how else to approach it.
You should be dividing by .84, not multiplying by it:

$$P_{out}=.84P_{in}$$

Therefore,

$$P_{in}=\frac{P_{out}}{.84}=\frac{iV}{.84}$$

See where you went wrong?

Next time, please post homework type questions in the appropriate homework help forum.

Ah! Yes!! Thank you so much!!

G01
Homework Helper
Gold Member
Anytime. 