[QUOTE=slitterst]
A 9v battery can’t produce 1,000,000 Amps. (Doesn’t that just sound like Dr. Evil should be threatening some city’s water supply? “Surrender before I expose you to {eyebrow lift} One meeeelion Amps!”
Anyhoo, that much current would incinerate you. A 9v battery can produce enough current to make your tongue tingle.
[/QUOTE]
You are making a very common mistake. Re-read all of the previous posts.
[QUOTE=slitterst]
A 9v battery can produce enough current to make your tongue tingle.
[/QUOTE]
O.K., let’s talk about worst case here: you have broken/bleeding skin on each hand. You connect one terminal of a 9V battery to one hand. You connect the other terminal to the other hand. Can it kill you?
According to page 9 of this document, a 9V alkaline battery can source 27 mA into a 270 Ω load and 40 mA into a 180 Ω load. But what’s the internal DC resistance of the body when measured through broken skin? I have no idea. I bet it’s pretty low, though. If we assume it’s around 200 Ω, you’ll get quite a jolt. If it’s much less - like less than 100 Ω - I imagine it could be fatal.
Somewhat related, I once talked a friend with braces into touching a 9V battery to his braces. He used one that was pretty much dead (could only feel the barest of tingles when you touched it to your tongue), he screamed and jerked it out of his mouth, cursing profusely. Apparently, it gave him a helluva jolt, even worse than touching a fully charged one to your tongue.
[QUOTE=groman]
you turn it on and connect it to a 5 uOhm resistor. It’s not going to be true SDMB fashion until somebody tries it and then lives to tell about it. 
[/QUOTE]
I did some calculations on what this resistor might look like.
A 0.5 μΩ resistor could be built using a copper bar with a cross-sectional area of 1 ft[sup]2[/sup] and a length of 90.5 ft. The big issue, of course, is whether or not there is enough surface area to safely dissipate 5 MW of heat via free convection. I don’t know. (I got a “C” in my heat transfer class. ;)) Even if there was, the temperature of the resistor would still be above room temperature, and the resistance would increase due to copper’s PTC. A better design might be a large copper pipe (of the correct dimensions) with chilled water running through it.
[QUOTE=Napier]
>Could you feed off of it with dinky little wires?
Several folks have pointed out that you can’t get that much current through dinky wires. But what I understand the OP to ask is whether you can get SOME useful power using dinky wires, not ALL of it.
And, yes, you could. In a less extreme situation, your car does this all the time. You might have an LED lamp in the dashboard that warns you’ve got a door open, and this draws maybe 0.02 A, and it’s getting that from a 12 V battery that could put out many hundreds of amps. So dinky wires are fine for tapping power, as long as they are heavy enough for the power you are taking (or more specifically for the current you are taking). How much current the source would be able to deliver wouldn’t be the issue here.
[/QUOTE]
Thanks, yeah that’s what I meant.
Like there’s a big, thick, innefficient cable supplying 5vdc at 1,000,000 amps, and I try to power a cellphone off of it with dinky little wires.
[QUOTE=panamajack]
I figured an exposed heart is kind of a problem already
[/QUOTE]
Best thing I’ve read all day. Cheers.
>A 0.5 μΩ resistor could be built using a copper bar with a cross-sectional area of 1 ft2 and a length of 90.5 ft. The big issue, of course, is whether or not there is enough surface area to safely dissipate 5 MW of heat via free convection. I don’t know.
I just got up and haven’t finished a cup of coffee, so I’m a bit fuzzy. But trying to calculate this in my head with a heat transfer coefficient of 10 W/(m K) I conclude free convection will let this thing reach 10,000 °C. Since copper’s resistivity is about proportional to its absolute temperature, this will never do. Let’s not even fool with radiative cooling, which won’t do diddly if we’re trying to keep the temperature close to ambient. I propose a much bigger chunk. How about 270 feet long and 5.5 feet square? That ought to change its resistance by about 3% as it heats up.
[QUOTE=Napier]
I conclude free convection will let this thing reach 10,000 °C.
[/QUOTE]
How conductive is copper vapor? 
[QUOTE=Chronos]
What most people think of as Ohm’s Law (V = IR) is really only of interest when the resistance is constant, finite, and nonzero. While this is often a good approximation, it’s not an absolute law. For instance, the voltage drop across a superconductor will always be zero, no matter what current you push through it. And one could surely design a material or device which would vary resistance such that the voltage across it is constant regardless of current, or even one where the voltage might decrease with increasing current.
[/QUOTE]
To be more precise, the voltage across the superconductor will remain zero only for currents below the critical current. Above the critical current, there will be a voltage and it will not vary linearly with the current. Superconductors are an excellent example of a passive non-Ohmic circuit element.
The above applies only for direct currents. Alternating currents are much more complicated in superconductors (due to the induced magnetic fields).
>How conductive is copper vapor?
Not very. You’re right, the resistor will stop functioning. Good point. No, it isn’t.
The chief danger of high current/low voltage power supplies is due to heating. Getting a wedding band across a 200A supply can burn off all the meat on the ring finger, causing it’s loss. A car battery is more than sufficient for this. Other potential trouble makers are wrist watches, and military dog tags. OSHA regulations refer to this as an “energy hazard”.