ohms law?

The 12.5vdc is on the input of the amp.

The 600W and 1 ohm are on the output - there is not direct correlation as it isn't a straight forward circuit.

Amplifiers aren't 100% efficient anyway, so you can't tell how much power the amp will output even if you knew the voltage in and the current in, UNLESS you know the exact efficiency of the amp.

Ohm's law does have a purpose - to show the relationship between voltage, current, and resistance in a circuit.

The trick is you have to have all the right information to apply Ohm's law.  You don't have that information here.

Again, the voltage going in to an amp can not be used to calculate the power out of the amp, as they arn't technically the same circuit.  There is a gain involved, and there are also efficiency issues that need to be accounted for.

Ohm's Law applies in every case in every electrical circuit.  That's why it's a LAW.  You simply can't use it all by itself to figure out the kind of issue you describe because you are talking about a power supply.  Power supplies can be designed to do all sorts of things (like regulate their output no matter what the input might be) which is why you simply can't do an Ohm's Law calculation to figure out what you want to figure out.

I recommend you do some reading (or go take a class) on basic electricity and basic electrical circuits.  A good understanding of the basics is necessary for anyone who wants to deal with automobile electrical systems, including sound systems, alarms, etc.  Google the term "Basic electricity" and also try the links on the left column of this site under "Mobile Electronics Basics."

If you have a simple LED circuit connected directly to a car electrical system you can use Ohm's Law to determine the proper size of the resistor needed to control the current to the LED.

Say you have a forward current of 25mA for the LED.  We want to designe for a worst case voltage scenario so lets say our battery voltage is 15vdc.  Resistors restrict the amount of current that flows in the circuit so we can use Ohm's Law to determine what size resistor will net us 25mA.

V=IR
R=V/I
R=15V/0.025A

R=600 ohms

You can see in this circuit that it is a simple circuit with basically only a resistive device.  Once you start throwing inductive/capacitive/discreet/ect. components in to the circuit Ohm's Law may or may not hold "true" as it will depend on what exactly you are trying to do.  Many discreet components, for example a basic transistor, have amplification values that need to be factored in.  If you use Ohm's Law to determine the base current going to a 3904 NPN transistor you may find that you have 20mA of current.  However, coming out of the collector you will have a total current capacity of 600mA or so (based on a beta of 30).  Since discreet components, such as transistors and diodes, don't have a resistance Ohm's Law doesn't account for them.

Kevin,

You forgot to allow for Vf of the LED.. If we assume the LED has a foward voltage of 2.4v, to calculate resistor size you first take your 15v, subtract the 2.4v for a remainder of 12.6v. The you use the LED current spec and divide your 12.6 by the 0.025a to come up with 504 ohms. You can also calculate the power rating required by taking 12.6v and multiplying it by 0.025 to come up with 0.315 watts, so use a 1/2 watt resistor around 500 ohms.

Jim

The forward voltage and power calculations don't pertain to Ohm's Law.  He wanted it simple, so he got simple!  :)

Also, by not accounting for the forward voltage you'll get a small buffer in the max current available to the LED.