Electronics > Beginners
Isolation transformer and electrons
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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:

--- Quote from: nForce on December 07, 2018, 07:32:23 pm ---Ok, IanB but capacitors are also isolated between plates:


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

So no current with capacitors?

--- End quote ---

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:

--- Quote from: nForce on December 09, 2018, 06:08:43 pm ---Lets freeze the picture in time, when we have 120 V potential on the top and 0 on the bottom tap.
--- End quote ---

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.


--- Quote ---We are standing on the ground which is 0 V.
--- End quote ---

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


--- Quote ---We touch the bottom tap and because it's 0 V
--- End quote ---

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.


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

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


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

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.


--- Quote ---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.

--- End quote ---

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).
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