## What are the Adavantages of High Amperage?

I just knew that some one would say superconductors.... we are all here !!!!

 Quote by Pharrahnox Thanks for the fast replies. Why is it dangerous to have high voltage in electronic welding? Is it so that the arc cannot travel far, or doesn't want to go through your body?
One advantage of high amperage with low voltage is that long electric arcs are suppressed. If the voltage is small, unwanted arcs over large distances of air are suppressed. In fact, long arcs through any sort of insulator is suppressed by small voltages. The waste caused by heating of the wires is offset by the safety from long electric arcs.
A high voltage difference between any two points in the air can result in an electric arc between the two points. If the electric field strength (volts/meter) is very large, then a large electric current can be generated between the two points. If a person is between the two points, he can be electrocuted or burnt alive by the electric current. Since the distance between two points can't always be increased, electric engineers choose to decrease the voltage.
The issue concerns the electric field strength necessary to produce an arc in the air. The magnitude of an electric field is the electrical potential difference between two points divided by the distance between the two points. A commonly used unit for electric field is volts per meter.
Air is an insulator for electric fields with small magnitude. However, air becomes a conductor for electric fields with high magnitude. Ordinarily, air is an electric insulator. The current density through air is very small if the electric field is very small.
Air has a threshold electric field where the neutral molecules break down into ions. When the electric field in air exceeds this threshold, the air becomes an electrical conductor within a few milliseconds. Before it becomes an electrical conductor, the electric current can be very small. When air becomes an electrical conductor, the electric current density through the air can suddenly become very large. This produces what is called an electric arc. The electric current through the air becomes very large.
In fact, any insulator becomes a conductor when the electric field in the material becomes large enough. If the voltage divided by the distance is large enough, an electric current can go through any material.
Note that a fixed voltage difference can produce a small arc instead of a big one. In arc welding, the user wants an arc between two electrodes placed a small distance apart (say 0.01 meter). However, the user does not want an arc to pass from one electrode to the plumber pipe (ground) on the other side of his head (about 5 meters). So he wants the average voltage small enough so the electric field strength between the two electrodes is above breakdown threshold but small enough so the electric field through his head is small.
The electric current through the small gap between electrodes has to be very large in order for the arc to get hot enough to melt steel. So here is an advantage of high amperage with small voltage.

 Quote by truesearch As I understand it you cannot have a current without an emf but you can have an emf without a current. I think that power supplies are 'intentionally designed' to generate an emf rather than a current, a current needs a complete circuit.... what if no one connects!!...It then seems natural to design electrical appliances to operate on a constant voltage rather than a constant current. Could a power company generate a 'constant current', waiting for consumers to connect? Are their any examples of such a thing? I think I am also correct in saying that a constant current source has a very high (infinite) effective output resistance. A constant voltage source has a very low (zero) effective output resistance. Look forward to enlightenment
Yes, you can buy electric current sources. There are power controllers (sometimes called power supplies) that keep the electric current constant while changing the voltage.
Photomultiplier tubes are a type of light detector that often acts as a constant current source. When a photon hits the cathode of a photomultiplier tube, an electron is released. The electron is copied through a cascade process. So a sizable electric current can be caused by one electron hitting the photocathode. Typically, a million photons get released for every photon that hits the cathode. The number changes, but is not important.
For a fixed flux of light, the electric current generated by the photomultiplier is constant. The electric current in electrons per second is proportional to the flux of the light in photons per second. So for constant power from a light source, the electric current from the photomultiplier is constant.
Therefore, the photomultiplier is effectively a source of constant electric current. No matter what resistors that you place in the electric circuit, the photomultiplier in a steady light produces the same amount of electric current. The voltage may change as you change the resistance, but the electric current will remain the same.

There are off course caveats to what I just said about photomultiplier tubes. They do have an upper limit to the amount of current they can produce. The current is linear with light flux only over a specific range. PMTs can burn out! However, a PMT under steady light conditions should be considered as a constant current source to first order. One can make serious mistakes by assuming that the PMT to first order acts as a constant voltage source.
I want to go slightly off the topic, since you are interested in constant current sources. "Bare" photoemission devices in steady light act as sources of constant electric current. However, electrical engineers and scientists are more comfortable with constant voltage. So scientists use terminating resistors to force the photoemission devices to act as constant voltage sources.
One makes the PMT act like a constant voltage source by adding a low terminating resistance. In other words, a low resistance should be placed in parallel to the PMT if you want it to act as a constant voltage source. However, such a resistor effectively makes any constant current source into a constant voltage source.
It took some time to learn how the terminating resistor of a PMT works. It also is useful in suppressing the capacitance of the PMT, which is a related problem.
Photovoltaic devices act as constant voltage sources and photoemission devices act as constant current sources.

 As I understand it you cannot have a current without an emf
Thermionic current?

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 Quote by cabraham Dude, I hate to tell you, but you are the one in need of studying. I am in the final stage of the Ph.D. in EE, & I am in my 34th year as a pro EE.
Sorry about that cabraham. I was thinking I was responding to the OP who is admittedly not familiar with Ohm's law. My mistake.

AM
 No problem, apology accepted. Best regards. Claude
 Thermionic current?.......OK.. I go with that and I would even add photoelectric current

 Quote by truesearch As I understand it you cannot have a current without an emf but you can have an emf without a current. I think that power supplies are 'intentionally designed' to generate an emf rather than a current, a current needs a complete circuit.... what if no one connects!!...It then seems natural to design electrical appliances to operate on a constant voltage rather than a constant current. Could a power company generate a 'constant current', waiting for consumers to connect? Are their any examples of such a thing? I think I am also correct in saying that a constant current source has a very high (infinite) effective output resistance. A constant voltage source has a very low (zero) effective output resistance. Look forward to enlightenment
If the power company delivered CCS instead of CVS, the customers are always "connected". The loads are stacked in a series loop, and the "circuit" is always complete, i.e. a closed loop. Each load has a shunt switch across it. To turn on, the switch is open, to turn off, switch gets shorted.

With CVS a short is a fault, so a series circuit detects overcurrent and opens. With CCS, an open is a fault resulting in overvoltage. A shunt circuit "maker" closes when this happens.

With CCS, you have to flip your thinking. Loads are in series instead of parallel, switches are in parallel instead of series, switches and circuit makers open for ON, closed for OFF. These are the opposite of that with CVS. They are analagous.

Again, CCS works just fine with any load. We just have to make mental adjustments. At stall, a motor is a near short. With CCS, the synchronous motor would not work. A CCS system has variable frequency, so other means, such as servo control using a frequency reference, would be needed to maintain constant speed. Likewise induction motors would run at varying speed.

If speed does not need to be fixed, CCS works. Like I said, we have to adjust our thinking from a Thevenin system to Norton. That gives many trouble.

Claude

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