# Two electical questions

by mearvk
Tags: electical
 P: 133 1. I have a car that has some electrical/electronic weirdness; the basic issue is apparently parasitic draw of about 2.5amps @ 12v. When I connect the (problematic) component the chassis ground (connected to battery ground) what I observe is that the battery chassis jumps from 0v to something like 1.1v and I see the 2.5 amp draw. I connected the related chassis ground connection directly to battery ground and saw the amp draw go to 0.5 amps. This tells me there is some sort of grounding weirdness. My basic question is what kinds of things could/would cause chassis ground and battery ground voltage disparity? I imagine that a bad diode would let current go back into a circuit creating a potential loop where the voltage never got back to battery ground. Ideas? 2. Unrelated to the first question. V = I/R is Ohm's law but if you take a 12w fluorescent high efficiency bulb and run 120v into it the math says there's a 1200 ohm resistor sitting in the bulb given by 120v * (120v/1200ohms = 0.10 amps) = 120v*0.10 amps. Likewise you might have a 100w bulb that would have a smaller resistance. My question is does wattage take into account heat generated at the resistor(s) in the circuit or just I*V? If wattage doesn't define energy but power then what formula defines total energy consumed as opposed to power? Forum looks great. Sorry about undescriptive title.
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P: 12,359
A lot of what you wrote does not make a lot of sense, i.e.
 When I connect the ... component the chassis ground ... what I observe is...
Can you see how someone would have trouble seeing what you are describing? What did you do with the component and the chassis-ground? Presumably you connected it - but how? Where else is it connected? For that matter - what is the component?

 I connected the related chassis ground connection directly to battery ground and saw the amp draw go to 0.5 amps.
Does this mean that an ammeter read 0.5A or that an amplifier drew 0.5A? Is the "related chassis ground" not the same thing as the "chassis ground" above? Must be because you said, before, that it was connected to the battery ground and now you are connecting it!

i.e. please post a clear description.
It just sounds like what you said - there is an electrical connection someplace you have not accounted for. I'm guessing you have not been careful to isolate the variables - perhaps you are unclear about what the different "ground"s represent?

Part 2.
V=IR and P=VI ... P=ΔE/ΔT

R is a constant for ohmic conductors only - the resistance of a bulb filament depends on it's temperature.

Light bulbs are usually rated with a power level in Watts as well as a voltage or a current level.
These are ratings not actual values. If you turned down the voltage, the bulb dissipates less power (it gets dimmer).

Relationship between power and energy: 1W = 1J/s
1kWhr = 1000J/s drawn for 3600s or 3600kJ

This help?