Except in post #34.
Sorry, I’m lost. Isn’t voltage directly proportional to current? Increase voltage and you increase the current?
OK, for a given amount of power…
Yes, when powering a resistor. But you’re not always powering a resistor - sometimes it’s a non-linear component. And you sometimes have the luxury of controlling the load resistance.
When designing a power transmission system, you have the luxury of controlling most of the parameters. I can, for example, design a system wherein there is a high voltage between the two transmission wires, and relatively low current in each wire. Conversely, I can design a system wherein there is a high current in the wires, and a relatively low voltage between the wires. In either case, the load impedance will be designed for maximum efficiently over the power range.
Clear as mud, right?
Let me try again.
Everyone knows V = IR, right? That’s Ohm’s Law. But I don’t like calling it Ohm’s Law. I prefer to call it the Ideal Resistor Equation. It is an equation that describes what’s going on with an ideal resistor. It also works well (though not perfectly) with practical resistors (as long as you stay within their temperature and power ratings). But it does not work at all with non-linear things. And there are LOTS of those. Examples include diodes, LEDs, batteries, and incandescent light bulbs. Those things are more complicated than resistors, and a simple application of Ohm’s Law will lead you down a wrong path. You also don’t want to use it when looking at the primary of a transformer, since its effective impedance is a function of the load impedance.
There’s an excellent online demonstration site that visually shows how certain circuits work:
Two light switches for a single bulb
Impedances of same magnitude and different phase
I highly recommend playing with these. Each of them is basically a preloaded circuit in a circuit simulator. You can erase it and draw your own and simulate that (there’s no easy way of saving it, unfortunately).
The lowest reading I’ve been able to get from sweaty palm to sweaty palm is 145K[symbol]W[/symbol], and that’s by jamming the sharp probes into my skin as hard as my pain threshold will allow. That’s only low enough to draw 80[symbol]m[/symbol]A from 12V, or 1/1500th of the estimated lethal draw.
I then pricked a thumb on each hand and mashed the bloody spot into the probes and the reading only dropped a few more K[symbol]W[/symbol]. It would have to drop all the way down to 100[symbol]W[/symbol] in order to draw the estimated 100mA needed to bother your heart.
Yes, I bled for this message board today.
Sounds like you need some electrolytes 
It seems to me that both of you misunderstand what Cold Cranking Amps is (are?)
It is a standard that allows you to compare the relative power that can be delivered by a car battery.
In this Wiki article on car batteries there is a good explanation of what the term means
So you decide to go into the car battery business. You start building batteries. You send the to a testing lab, where they take a fully charged battery, chill it to 0F and then see how many amps it will deliver for 30 seconds and still have 7.2Volts left at the end of the test. What ever number they come up with is what you print on your labels.
A few things I would note: You car won’t start if the battery is only providing 7.2Volts, and since chemical reactions slow down as things get colder, this is a very tough test of a battery’s ability to provide power.
Bottom line is this a standard that allows you the consumer* to compare different batteries to determine which is best for your needs.
One last thing, when shopping for batteries, do not confuse CCA with CA which is done at 32F and therefore yields a much higher number.
*CCA is also the basis for battery testing. When using a carbon pile tester, we apply a load equal to 1/2 of the CCA for 15 seconds and read the voltage at the end of the test to determine if the battery is still serviceable.
beowulff Getting back to Ohm’s law for a moment. As a fellow instructor of mine used to explain it:
If you drink too much coffee in class, it is voltage that makes you want to visit the men’s room. Amperage is what you leave in the urinal, and if you squeeze it in the middle, that is resistance.
Thanks for the great link groman, I have a trainee right now and that will be an excellent teaching aid.
Someone already posted a link to this site Articles on "Electricity" WJ Beaty and I want to second the recommendation, lots of good information there.