Current and Resistance

1. Jul 27, 2008

4eleven

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.

2. Jul 27, 2008

G01

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.

3. Jul 28, 2008

4eleven

Ah! Yes!! Thank you so much!!

4. Jul 28, 2008

Anytime.