Actually, superconductors are not almost zero ohms, they are zero ohms. The nature of quantum mechanics allows for zero ohms. It is not a mathematical abstraction. There is another class of conductors that are not superconducting, but are considered ultra low resistance, that is very useful.

Superconductors do exist, this is why the high temperature variations using liquid nitrogen was a big deal. The holy grail is a room temperature variety, maybe it can exist, maybe it can't, there are about three models to explain this behavior, and the book is still very open on it. The limiting factor of superconductors is magnetic fields, exceed a certain number of Tesla and it stops being a superconductor, abruptly.

In the realm of quantum mechanics there are several other oddities that would be great if we could make them at normal temperatures. Things like super thermal conductivity and superfluidity (zero viscosity), but they require temperatures near absolute zero.

Current without voltage is an unstable condition in both antennas and LC circuits, tapping their energy off another source such as magnetic fields. Unless you have a superconductor it still doesn't occur, as I mentioned earlier.

Another point, as with the superconducting coil, the antenna and LC circuit still need voltage to excite them, they don't just start arbitrarily. So again, this condition can not exist without voltage being used in the beginning of the cycle. It can not exist by itself.

 

What if your pendulum was charged?

Go on then, develop this train of thought. The pendulum can either carry an isolated charge or it can carry a charge under the influence of a potential field from another charge.

If you define current as the movement of charges, I was just showing that other things apart from electric fields can make charges move, hence cause currents.

Of course if you adopt a definition of currents as the net movement of charge in an electrically neutral conductor then it is hard to make currents flow without an electric field.

 

If you have a LC circuit (which I assume you are equating with a pendulum) charged in this case also means voltage, unless you couple to the coil with a magnetic field.

 

I'm glad folks are beginning to think more deeply about the question.

Yes Bill, it may be unstable but still theoretically possible, just as balancing a pencil on its point is unstable but theoretically possible.

Take a circuit containing a series battery, open switch, resistor and capacitor.

At the moment of closure of the switch the current through the capacitor is large, but the voltage across it is zero.

As I earlier commented this situation is only momentary, but it does exist.

If, instead, you have an oscillatory circuit you can repeat this situation many times. Again the circumstances of finite current but zero voltage exist momentarily in each cycle.

In an earlier post I specifically excluded thermal effects. This is because you can cause electron drift by thermal means, without a voltage.
You can also cause charge carrier movement by mechanical means (pressure) in solution via a semi permeable membrane.

All this simply shows that the forces of nature are linked so we can transpose one effect into another.

 

Take a circuit containing a series battery, open switch, resistor and capacitor.

At the moment of closure of the switch the current through the capacitor is large, but the voltage across it is zero.

This is not true - in any physical implementation the capacitor maintains it's voltage when the switch is closed. You can draw a circuit that suggests this, but you can't make it.

 

As I have said, it is only zero if there is no resistance though. The moment you have any resistance, there will be voltage, however slight. The EMF is coming from collapsing magnetic field.

 

As I have said, it is only zero if there is no resistance though. The moment you have any resistance, there will be voltage, however slight. The EMF is coming from collapsing magnetic field.

It's not an issue of conductivity, even using perfect conductors you cannot make something that causes the voltage across a capacitor to vanish by closing a perfectly conducting switch.

 

in any physical implementation the capacitor maintains it's voltage when the switch is closed

Exactly!

So if the capacitor is initially uncharged as my statement about an intitial high current implies.

 

It's not an issue of conductivity, even using perfect conductors you cannot make something that causes the voltage across a capacitor to vanish by closing a perfectly conducting switch.

Hmmm, if something has zero resistance (a superconductor) then you will have zero volts. This is inherent in Ohm's Law. This is part of the reason superconductors are such a sought after goal.

With real world conductors (say 0.001Ω) that cap will be in the picovolts or less in less than a millisecond. The discharge will be very fast.

Personally I'm looking forward to having superconducting components across the board, coils, caps, wire, electronic switches (doubt they'll be semiconductors). I suspect things will get interesting. All the stuff we talk about being theory starts becoming reality.

At this time though, I would still argue that you can not have current without a voltage inducing it somewhere.

Magnetic fields can induce current without voltage, but if there is any resistance involved then voltage pops up.

You want to see a mind bender check this thread out.

http://forum.allaboutcircuits.com/showthread.php?t=16150

 

Hmmm, if something has zero resistance (a superconductor) then you will have zero volts. This is inherent in Ohm's Law. This is part of the reason superconductors are such a sought after goal.

With real world conductors (say 0.001Ω) that cap will be in the picovolts or less in less than a millisecond. The discharge will be very fast.

Personally I'm looking forward to having superconducting components across the board, coils, caps, wire, electronic switches (doubt they'll be semiconductors). I suspect things will get interesting. All the stuff we talk about being theory starts becoming reality.

At this time though, I would still argue that you can not have current without a voltage inducing it somewhere.

Magnetic fields can induce current without voltage, but if there is any resistance involved then voltage pops up.

It's not an issue of circuit theory, I understand that if R=0 then V=0. The issue is that you can't build the circuit that only has an R and C in it. There is inevitably some L which stops the current rising instantly. Your challenge is to describe a capacitor that can be discharged in a way that the discharge current does not induce a magnetic field. If you get a magnetic field then you need some sort of L in your circuit.

 

Stray effects can be minimized pretty well, but the point is valid. I built an identical filter using SMT vs. through hole components. The roll off was the same, but the rejection was 10db deeper with the SMT version due to the negligible factors being even more negligible.

R is a value that actually can be eliminated eventually, and if you can minimized unwanted effects they really can be negligible.

I will concede that you can have current without voltage, but you will have to have some other source of energy to start things moving, such as magnetic fields or voltage (AKA EMF). Even then it requires some really specific circumstances to be completely pure, such as the superconductor examples I keep mentioning. It isn't that common, and almost has to be cultivated as an effect.

The current can not just create itself, it must be excited somehow.

 

Again I urge you to delve deeper.

As I previously noted, current at zero voltage is very common and occurs 50 times per second in any home in Europe when the potential field at any section of the house wiring reverses and in doing so passes through zero.. Edit I should have said 100, but nobody's perfect.
This is a momentary condition.

If you want a sustained one without silly arguments as to whether a superconductor resistance is actually zero or just very low I also posted previously and you contained the seeds of the idea in your comment

some other source of energy

Tesla thought I meant to compare kinetic energy with charge in motion in my pendulum example - I meant no such thing.
I simply meant that an oscillatory system repeatedly exchanges energy between two forms. In this case both are electric in nature.
I also noted other forms of energy interchange - viz thermal and osmotic.
If one of the forms is not electric then you have a driving 'force' for a continued current with or without oscillation. All you need is a bunch of charge carriers and thermal or concentration effects will cause a net movement for you. Another word for this net movement is - current.

 

Again I urge you to delve deeper.

As I previously noted, current at zero voltage is very common and occurs 50 times per second in any home in Europe when the potential field at any section of the house wiring reverses and in doing so passes through zero..

This is a momentary condition.

I strongly oppose the idea that there is zero volts. True, the potential from the incoming line does go through zero, but the potential that results in the current flow during that time is the result of stored energy in reactive devices connected to the line, i.e. motors, transformers, etc. At that instant, the loads become the source. That is the exact reason that ELI THE ICE MAN works.

 

I strongly oppose the idea that there is zero volts

Anyone who opposes this opposes the intermediate value theorem in maths, upon which most of engineering mathematical theory is based.

Of course the voltage is momentarily exactly zero, just as is is momentarily every other value between the positive maximum and the negative minumum during its cycle.

For some circuits this zero occurs coincident with the zero in the current cycle.
For some it does not.

What is so difficult about this?

 

Again I urge you to delve deeper.

Tesla thought I meant to compare kinetic energy with charge in motion in my pendulum example - I meant no such thing.
I simply meant that an oscillatory system repeatedly exchanges energy between two forms. In this case both are electric in nature.

I didn't. All I did was use your pendulum (ignoring whatever you were illustrating with it) and suggest that if it was charged then you would have a current that was not caused by an electric field, but that was driven by gravity. You would in fact have a simple dipole that would radiate (eventually stopping the pendulum).

 

What is so difficult about this?

Excuse me for intruding on this extremely complex subject, I am in no means at the calibur of y'all, but I see studiot's point. It is indeed true that at that moment of absolute zero, crossing between negative and positive oscillation, there is no voltage. There is also no current. There is nothing. All stops, even time. You are on the verge of researching infinity.

 

No, time doesn't stop. There are circumstances with phase shift (especially resonance) where current and voltage are out of phase. I don't think it applies to an ideal AC source though. Like a battery, there is internal resistance, but it is low as they can make it.

 

No, time doesn't stop.

My personal opinion is that without Man, there is no time. Movement will indeed continue, such as planets, water, air etc. but the interpretation of time would not be there. That's just me.
Resonance, in it's cycle, crosses the absolute threshold where negative and positive do not exist. At that moment (if you were to stop time) there would be nothing, absolutely nothing. (Quick, take a picture of God) The theories based on no voltage and absolute current, I believe are wrong.

 

If a tree falls in the forest and there is no one around to hear it, is there sound??

 

My personal opinion is that without Man, there is no time. Movement will indeed continue, such as planets, water, air etc. but the interpretation of time would not be there. That's just me.
Resonance, in it's cycle, crosses the absolute threshold where negative and positive do not exist. At that moment (if you were to stop time) there would be nothing, absolutely nothing. (Quick, take a picture of God) The theories based on no voltage and absolute current, I believe are wrong.

That is philosophy, which has it's place, but not concerning electronics or science. Mathematically time exists, just like space, energy, and matter. They are here whether we are or not. In the scheme of things we aren't that important, except to ourselves.

Yes, there is sound when there isn't an observer, it being a form of energy.

I still maintain without voltage current can not exist. They don't have to be at the same time, but if you have electrons moving then there will be a voltage before or after the fact. Electrons need something to make them move.