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.

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

2 Answers 2


Not all things are resistors, therefore not all things follow Ohm's Law.

Motors, for example, are resistors, inductors, voltage sources all rolled into one. Some of these things read zero when no current is passing through the motor or if the motor is not spinning. Therefore when you try to measure the resistance of the motor at stand-still, you are measuring the resistance because these other things are inactive, but that resistance isn't the only thing in the motor and when you try to measure the resistance while its running, that reading is obfuscated by these other things.

In the same way, other things are resistors, but their resistance changes as they heat up so if you measure their resistance when cold and then measure voltage and current when running, they won't match up (like incandescent bulbs). And some times, being rated for 60W means that you can run it at 60W, not that it you have to run it at 60W.


to enlarge upon DKNguyen's answer:

The electrical power distribution buss (the main "+" cable) in a vehicle possesses only ONE voltage: either the charge setpoint of the regulator (which is about 13.8 volts for a lead-acid battery system) or the battery potential which is between 11.6 and 12 volts for a lead-acid battery, depending on its state of charge.

The buss voltage will read 13.8 when the engine is running and the charging system is ON. It will read between 11.6 and 12 when the engine is OFF or the charging system is OFF.


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