I used to work around electric resistance welders: 5V AC at 20kA. The bulk of the welding machine was a transformer, and the secondary was a single loop that included the welding tips. There was all kinds of trickery involved in chopping the input AC waveform to get various heating effects and duty cycles.
The weld current was only on for a short time though. The magnetic fields as the current rose and fell were something wicked, and my friend the field troubleshooter had all his magnetic bank cards erased more than once.
So if you had megamp currents forming and unforming, with total powers in the 5-megawatt range, I suspect the magnetic effects would be unhealthy.
No, my statement was correct. A current source uses a closed-loop controller to regulate the load current at a constant value. Most do it by measuring the current (via an internal series resistance) and adjusting the bias on a series transistor. The voltage is “free” to be anything, as long as it’s below a maximum value, a.k.a. the compliance voltage.
Of course, when the current ramps up, the voltage will (usually) also ramp up as a result.
I believe you, but you’re also measuring the voltage when the DVM (set to measure ohms) is looking at essentially an open circuit. Have you tried it when the DVM is measuring 10,000 ohms? I may be wrong, but I think you’ll find the voltage will decrease.
We have a quite a variety of DVMs in the lab. If I get bored over the next few days I’ll make some measurements and report back.
Of course, we’re talking about run-of-the-mill DVMs here. For completeness sake, however, I should mention that we have some instruments that measure resistance up to 100,000 GΩ in our metrology lab. These instruments must produce up to 1000 VDC in order to measure resistance up in that range.
Could? Yes, it could? Would it? It all depends on how healthy your skin is. A healthy skin will be able to resist well more than 5v. I wouldn’t advise touching your tongue to it, though.
You could not feed off it with dinky little wires. Wire ampacity is entirely dependent upon cross-sectional area. The jacketing/insulation of the wires could probably handle the 5v, but even 4/0 cable (about an inch in diameter) is only rated for 400A, and even that load heats up the cable.
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.
>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.
While we’re getting far afield, remember that Ohm’s law only holds for passive circuits. Any circuit that has active devices (transistors, tubes, SCRs, etc.) does not necessarily need to obey the law.
That’s interesting.
I think that Superconductors are considered non-Ohmic, even though with R=0, Ohm’s law still holds. Of course, for all current superconductors, R != 0 for large values of I, and the change from zero to non-zero is a step function (which would make them non-Ohmic).
I think it hasn’t been clearly established whether the OP was talking about a source specification (and if that, whether it’s a voltage or current source) or a circuit that’s pulling that current at that voltage.
In SDMB fashion, I think all bases have eventually been covered, though.
Looking up lightning on Wiki, natural lightning averages 40K amp, with up to 120K. Voltage is a little harder to determine, Wiki says 3 million volts per meter of length.
Folks have survived lightning strikes, possibly because the shock didnt go directly through the heart on its way to ground…
For what it’s worth. (Which aint much, I know. Sorry.)
The difference between the two scenarios, is that in the former you have a 5V voltage source that can provide 1,000,000 amps (or a 1megamp current source, whichever), and in the latter 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.
If by “Ohm’s Law”, you mean “V = IR”, then nothing violates Ohm’s Law, since that equation is the definition of resistance. But if the resistance isn’t constant, then V = IR isn’t a very useful or interesting definition. And if R = 0, as it is for a superconductor, then V won’t change with varying I (since it’ll be constantly zero).
And thanks for the links, casdave. I figured that such things would be possible; I just didn’t know what they would be called. I wouldn’t say that either of those has a “negative resistance”, though, since V and I still have the same sign. A negative resistance would have to be a sort of power source (and a very weird sort of power source, at that).
If you use a current source to power a load, and you linearly increase the current, there is no guarantee the voltage will increase in a linear manner. If, for example, you were to stick a silicon diode across the current source, the voltage will increase linearly up to about 0.6 VDC. After that, the voltage will rise much slower with current.
A better example - and **casdave ** beat me to it - is negative resistance. If you stick a device that exhibits negative resistance across a current source, the voltage will actually *decrease * with increasing current. Though it should be kept in mind that, for a passive device, this can only occur after the device is already biased with a voltage. In other words, with a passive negative resistor there will *always * be a positive resistance region around zero, and a negative resistance region at some bias level.
Lots of passive devices have regions of negative resistance. Tunnel diodes and neon bulbs come to mind. You can also build an active negative resistor using op amps. The cool thing about it is that it has negative resistance around zero.
Yep. One example is semiconductors. Let’s say I apply 100 mA through a diode using a current source, and the resultant voltage across the diode is 0.65 V. By definition, the absolute resistance is 6.5 Ω. But this number is pretty much useless, because the resistance will greatly change if I change the current. For semiconductors, it is more useful to talk about the *incremental * resistance rather than the absolute resistance. It is defined as dV/dI.
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.
