Isolation transformer and electrons

[nForce]

I have a basic question:

When we use isolation transformer we have a physical electric isolation between two systems. If we connect a wire between a system on the other side of an isolation transformer and ground then the current would not flow.

But how do electrons know on the other side of an isolation transformer that we don't have a current loop? They just feel a potential difference, so they should flow because they don't know nothing about isolation.

Thank you.

[bson]

An electric potential causes charge to shift around, it's the force acting on electrons.  A transformer is coupled through a magnetic field, so there is no potential between the two sides.  (Capacitors are coupled through an electric field, hence they do have a potential.)  The potential is between the individual taps on each side, not between the sides.  Since there is no potential between the sides you can connect one of the taps on the secondary to ground or any other primary-side reference.  The electrons simply have no force acting on them, hence no current.

[Zero999]

As far as the secondary coil is concerned, the electrons are being moved around by the changing magnetic flux. The primary coil could be replaced with a spinning magnet, and it would behave in the same manner.

[IanB]

Actually the electrons do flow, however the size of the current is too small to be of significance. The electrical potential on the secondary side of the transformer is fluctuating at the AC frequency and the ground potential is not. So a small AC current can flow due to the capacitance between the ground and the secondary.

For normal low voltages the current is too small to matter, but for large electrical transmission transformers where the secondary voltage is (say) 500 kV this capacitance effect is very significant and considerable precautions have to be taken to prevent arc over.

[nForce]

An electric potential causes charge to shift around, it's the force acting on electrons.  A transformer is coupled through a magnetic field, so there is no potential between the two sides.  (Capacitors are coupled through an electric field, hence they do have a potential.)  The potential is between the individual taps on each side, not between the sides.  Since there is no potential between the sides you can connect one of the taps on the secondary to ground or any other primary-side reference.  The electrons simply have no force acting on them, hence no current.

Ok, but the magnetic field also moves charges (qv x B), the Lorentz force? And there is also a magnetic potential not only electrical one.

[nForce]

Sorry, but I am still waiting for an answer on my last question.

[SG-1]

The impedance of the secondary side is reflected back across to the primary side with the same ratio as the transformer turns.

With a perfect transformer:

If you place a dead short on the secondary that impedance is reflected back across the magnetic circuit to the primary side as a dead short & you get a lot of electric current.

If the secondary is open that impedance is reflected back across to the primary side as an open circuit & you get no electric current in the primary side.

In the real world: 

Because of internal losses, etc... some small primary current is always present when the transformer is energized.

[nForce]

An electric potential causes charge to shift around, it's the force acting on electrons.  A transformer is coupled through a magnetic field, so there is no potential between the two sides.  (Capacitors are coupled through an electric field, hence they do have a potential.)  The potential is between the individual taps on each side, not between the sides.  Since there is no potential between the sides you can connect one of the taps on the secondary to ground or any other primary-side reference.  The electrons simply have no force acting on them, hence no current.

Ok, but the magnetic field also moves charges (qv x B), the Lorentz force? And there is also a magnetic potential not only electrical one.

I meant by what  bson wrote: "A transformer is coupled through a magnetic field". But I don't understand why is there no current, what about Lorentz force? Electrons also feel a force, because of a magnetic field.

[SG-1]

The magnetic force on the electron is perpendicular to the conductor, not axial.

[IanB]

I meant by what  bson wrote: "A transformer is coupled through a magnetic field". But I don't understand why is there no current, what about Lorentz force? Electrons also feel a force, because of a magnetic field.

I don't think any of us understand your question. Where should there be current, due to the magnetic field?

[lordvader88]

I'll add some questions

If u have a cloud of free electrons, and in that cloud u put a transformer with AC source on one side, and a closed loop on the other side, what does the electron cloud do ? Or would they just all repel due to Coulumbs law.

Ok what if it was a plasma of hydrogen, so just all free electrons and protons, what would they do ?

In an ideal transformer with a magnetic core, is the magnetic flux literally confined only inside the core ?

[boB]

The charges flow in the secondary due to the will of Faraday's law

[nForce]

Can someone explain in detail this:

A transformer is coupled through a magnetic field, so there is no potential between the two sides.  (Capacitors are coupled through an electric field, hence they do have a potential.)

What about magnetic potential? So magnetic potential does not move charge around?

[IanB]

What about magnetic potential? So magnetic potential does not move charge around?

A changing magnetic field will cause charges to move along a conductive path. In a transformer the conductive paths are the wires that make up the windings. There is no conductive path between primary and secondary so the magnetic influence cannot cause charges to move in that direction.

[nForce]

Ok, IanB but capacitors are also isolated between plates:

(Capacitors are coupled through an electric field, hence they do have a potential.)

So no current with capacitors?

[GeoffreyF]

Electrons have no feelings. In fact when there are a lot of them and they have a good running start they can be downright heartless, even taking joy in one's death or burning.

[IanB]

Ok, IanB but capacitors are also isolated between plates:

(Capacitors are coupled through an electric field, hence they do have a potential.)

So no current with capacitors?

Correct. There is no net current through a capacitor other than leakage. The average current between the two poles of a capacitor is zero in steady state or repetitive cycles assuming there is no dielectric breakdown.

Maybe what you are trying to think of is eddy currents? Any conductive objects in a changing magnetic field will experience induced eddy currents due to the influence of the magnetic field. So if you as a human were exposed to a changing magnetic field of sufficient intensity you could be electrocuted by the eddy currents induced in your body.

[SG-1]

The same principals apply to each side of the transformer.  If the circuit is closed then you have current.  If the circuit is open no current.

To know why requires a look from physics, not engineering.

With an open circuit their is no internal e-field axial to the conductor, or along it's length.  There is no emf to move the charge.

Here are some notes I made from some IEEE papers.

[nForce]

OK, now I know what means coupled through a magnetic field. It's just like with optocouplers, which circuits are also isolated.

I have made a drawing, where we have a transformer which has 1:1 turns. So the magnetic flux is changing in the core of a transformer, because we are changing the AC voltage on the primary. Because of that the voltage between one tap and another tap of a transformer on the secondary side is also changing. Lets freeze the picture in time, when we have 120 V potential on the top and 0 on the bottom tap. We are standing on the ground which is 0 V. We touch the bottom tap and because it's 0 V, and we are standing on the ground which is also 0 V, there is no current. Now we touch the top tap which has 120 V potential and we are standing on the ground which is 0 V. Now what?

You are going to say, yes there will be no current, because we have isolated system and we don't have a current loop to flow. Ok, but electrons do not know nothing about isolated system, they just see a voltage difference. So we are touching the top tap 120 V potential, and we are standing on the ground 0 V potential.

[IanB]

Lets freeze the picture in time, when we have 120 V potential on the top and 0 on the bottom tap.

But this picture is wrong. The transformer output winding is isolated so we don't know the potential of any part of it. It is undefined. All we know is that one end of the winding is at a 120 V higher potential than the other.

We are standing on the ground which is 0 V.

To be more precise, the ground is at 0 V because we have decided to use this as a reference.

We touch the bottom tap and because it's 0 V

As noted above, before we touch the bottom tap its potential is undefined. We do not know what it is. After we touch the bottom tap its potential becomes 0 V, because we have connected it to ground and all connected parts of the system equalize to the same potential.

and we are standing on the ground which is also 0 V, there is no current.

Naturally there is no current because potentials have equalized and there is no driving force.

Now we touch the top tap which has 120 V potential and we are standing on the ground which is 0 V. Now what?

Before we touched the top tap its potential was undefined. After we touched the top tap its potential became 0 V, because we have connected it to ground and it will equalize its potential through the connection.

You are going to say, yes there will be no current, because we have isolated system and we don't have a current loop to flow. Ok, but electrons do not know nothing about isolated system, they just see a voltage difference. So we are touching the top tap 120 V potential, and we are standing on the ground 0 V potential.

You are correct, there is no current because there is no circuit. But also there is no current because the top tap is at 0 V potential after we touch it, the same as the ground, so there is no voltage difference to produce any current.

Note that to say there is "no current" is not 100% accurate. In order for potentials to equalize there will be a small movement of charge. This will manifest as a current, but it will be a transient effect. The current will flow for a very short time and usually will be very tiny (depending on the capacitance of the system and the magnitude of the voltages concerned).

[ArthurDent]

GeoffreyF – “Electrons have no feelings.” GeoffreyF is correct and I have never met an electron that was hell bent on doing me harm and in a one-on-one interaction they cause no problems at all. However, in a large crowd they can do a great deal of harm if you cross them.
 

All these metaphysical descriptions of electrons with ‘free will’ is just confusing the issue. There also seems to be some mystique about isolation transformers where there is no similar concern about step-down and other transformer ratios. You might guess that an isolation transformer does just what it says-isolates. There are also some things that just have to be taken as real and ohms law is one of those things.

Simply put, if there isn’t a complete circuit there will be no current flow. The following modified drawing displays it pictorially.

[Zero999]

Oh I see, it seems this is about the common misconception of what ground is, rather than how transformers work.

In order for current to flow, there needs to be a complete circuit. Any electrons coming out of the negative terminal of a battery, must go back into the positive. The same is true for the transformer, except the direction of the current flow is continuously changing.

Another interesting fact is that within a closed circuit, the net charge remains constant. When a capacitor discharges through a resistor, the number of electrons and thus charge in the entire system doesn't actually change. All that happens is electrons are transferred from the negative plate of the capacitor, to the positive plate, via the resistor. The number of electrons inside the capacitor stays the same.

Ground is just a reference point, from where all voltages are measured. It makes no sense to measure the voltage between a stake in the ground and a floating transformer secondary. All you'll read is a ghost voltage, leaking through the transformer's inter-winding capacitance. The primary coil is acting as one plate of the capacitor with the neutral of the mains connected to the soil and the secondary is the other plate of the capacitor. If the voltage source powering the transformer was to be completely isolated from earth, which is actually impossible, as there will always be some capacitive coupling,  then you'd read zero between earth and any part of the circuit, because no current would flow.

[Shock]

when we have 120 V potential on the top and 0 on the bottom tap. We are standing on the ground which is 0 V. We touch the bottom tap and because it's 0 V, and we are standing on the ground which is also 0 V, there is no current. Now we touch the top tap which has 120 V potential and we are standing on the ground which is 0 V. Now what?

You are going to say, yes there will be no current, because we have isolated system and we don't have a current loop to flow. Ok, but electrons do not know nothing about isolated system, they just see a voltage difference. So we are touching the top tap 120 V potential, and we are standing on the ground 0 V potential.

The problem with the question you are asking is you are defining a 0V that does not exist on the secondary side in the circuit shown.

Between L1 and L2 (the secondaries) is the 120VAC difference in potential. Those two connections are no longer Live/Neutral/Earth (or what you want to call them) as they are galvanically isolated.

Ground/Earth to the secondary is just like any other conductor, it might as well be a piece of wire. It's not 0V to the secondary, it's nothing while not connected. While not connected the only situation it would be called it 0V is in reference to the live side if the Neutral is at the same potential as Ground/Earth.

The electrons won't see a voltage difference between the secondary and Ground/Earth because there is none. There has to be some kind of completed circuit in order to induce the flow of electrons.

[Shock]

You can find how electrons flow in a circuit in any textbook but going back to your original question of how do they know. A simplified analogy would be how evaporation and precipitation works. This is a completed circuit and has flow (of moisture/water in this case) aided by the warming and cooling of the earth.

If we introduce an insulator and isolate the earth from the sky no complete circuit exists and no flow occurs.  There might be plenty of potential to flow in the oceans but without the loop nothing will occur.



 

[nForce]

You can find how electrons flow in a circuit in any textbook but going back to your original question of how do they know. A simplified analogy would be how evaporation and precipitation works. This is a completed circuit and has flow (of moisture/water in this case) aided by the warming and cooling of the earth.

If we introduce an insulator and isolate the earth from the sky no complete circuit exists and no flow occurs.  There might be plenty of potential to flow in the oceans but without the loop nothing will occur.

If I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer? If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?