# When to use Ohm's law as opposed to using power formula?

I am a mechanic trying to gain a better understanding of electrical theory on vehicles. I have a sound understanding of Ohm's law and also the power formula (Power = Voltage x Current)

However, I am just trying to understand when to use each one in the case of diagnosing the electricals on a vehicle.

When assessing a light globe circuit on a vehicle I have a globe that is rated at 60 watts. When the circuit is turned on, it has 11.9 volts of potential to it, it draws 2 amps of current and it has a resistance of 1 ohm. Therefore it isn't following Ohm's law. But if I use the power formula I can find that it is only using 23.8 watts of power. Am I correct in this case? Does the globe not count as a conductor in this case so therefore it isn't following Ohm's law?

When assessing a fuel pump circuit, the fuel pump doesn't have a rating for power but it is a DC motor so I am assuming it has some sort of power rating. The fuel pump has 13 volts of potential to it, it draws 3.5 amps and it has 1 ohm of resistance, again this doesn't seem to follow Ohm's law.

So my confusion is when should I be using Ohm's law and when should I opt to use the power formula instead? And why aren't these 2 components following Ohm's law or am I not using the formula correctly?

I appreciate any help, thanks for taking the time to read through.

• The resistance of a cold incandescent lamp filament is significantly less than the resistance when the filament is white hot. Jul 30, 2020 at 13:15