What happens when magnetic fields cancel

[seazal]

What happens when magnetic field cancel. Where do they go. Do they just vanish or return to the vacuum?

In a GFCI receptacle we have in most of our homes. The sense coil can output zero when there is deviation from net magnetic field between the line and neutral. This means the cancellation when the load appliance doesn't leak current is perfect. So there is perfect cancellation of the magnetic field within the sense toroid. So the magnetic flux just vanish, just like that? But energy can't be created or destroyed. Is it not?

 

[Cryo]

Energy can be moved. When magnetic fields cancel in one place they can reappear somewhere else. Also there is radiation, reflection due to impedance mismatch and heating to worry about, when you start talking about real world.

 

[seazal]

Energy can be moved. When magnetic fields cancel in one place they can reappear somewhere else. Also there is radiation, reflection due to impedance mismatch and heating to worry about, when you start talking about real world.

In GFCI, where does the canceled field go in the sense coil?

 

[anorlunda]

Forces and fields can have directions as well as magnitude. Perhaps you can visualize it better with forces. What happens to the forces from the left and right hands in this exercise device? What happened to the energy needed to stretch the spring? The answer when combining forces and fields is to pay attention to both the magnitude and the direction.

 

[Cryo]

In GFCI, where does the canceled field go in the sense coil?

Can you provide a diagram to support your question?

 

[seazal]

Can you provide a diagram to support your question?

Most of us use GFCI in our kitchen or bathroom so it is natural to ask this.

see https://www.powerelectronictips.com/teardown-leviton-ground-fault-circuit-interrupter/

When magnetic fields of the two input cancel, where do they "reappear somewhere else"?

 

[Cryo]

My field is electromagnetic radiation and optics, so I could well make errors when looking at this circuit, but I'll give it a go.

You did not label the inputs or where the "magnetic fields cancel". I presume you mean the field due to two line wires. Under normal operation, all the current that flows into one wire of the line, must flow out of the other line wire, so the net current through the sense transformer ring should be zero. If you get a leak to the ground, this will change.

I therefore presume you are asking where does the magnetic field go where there is no leak. I think it gets localized between the wires.

 

[seazal]

My field is electromagnetic radiation and optics, so I could well make errors when looking at this circuit, but I'll give it a go.

You did not label the inputs or where the "magnetic fields cancel". I presume you mean the field due to two line wires. Under normal operation, all the current that flows into one wire of the line, must flow out of the other line wire, so the net current through the sense transformer ring should be zero. If you get a leak to the ground, this will change.

Yes.

I therefore presume you are asking where does the magnetic field go where there is no leak. I think it gets localized between the wires.

Localized in the sense that the field is still there.. only suppressed by each other polarity? Do you have any illustration that shows this?

 

[anorlunda]

Look at that schematic that @seazal posted in #6. The operative part is the sense transformer at the upper left.

There are two wires, line and neutral passing through that transformer core. In principle, we are trying to measure the line current minus the neutral current. If those two currents are equal in magnitude and opposite in direction, then the net is zero. No flux is induced in the transform and the output is zero.

There is no need to ask about fluxes cancelling. The flux is induced by current, and if current is zero there is no flux.

 

[jartsa]

What happens when magnetic field cancel. Where do they go. Do they just vanish or return to the vacuum?

In a GFCI receptacle we have in most of our homes. The sense coil can output zero when there is deviation from net magnetic field between the line and neutral. This means the cancellation when the load appliance doesn't leak current is perfect. So there is perfect cancellation of the magnetic field within the sense toroid. So the magnetic flux just vanish, just like that? But energy can't be created or destroyed. Is it not?

1: A long straight wire has some inductance.

2: That same wire bent into a round loop has more inductance than the straight wire.

3: That loop squeezed to a very narrow loop has less inductance than the straight wire.

4: When we make current to go through a wire that has large inductance, then a magnetic field that contains a large amount of energy is created.(When we make a current to go through a circuit with low inductance, we don't use much energy, in other words we don't send much energy to the circuit.

So, when we make a current to go through wires that don't create magnetic fields, we don't deposit any energy to the wires. This solves the problem "where does the energy go", doesn't it?)

 

[seazal]

Look at that schematic that @seazal posted in #6. The operative part is the sense transformer at the upper left.

There are two wires, line and neutral passing through that transformer core. In principle, we are trying to measure the line current minus the neutral current. If those two currents are equal in magnitude and opposite in direction, then the net is zero. No flux is induced in the transform and the output is zero.

There is no need to ask about fluxes cancelling. The flux is induced by current, and if current is zero there is no flux.

But remember the current induced its own magnetic field. So with opposite current flowing, the net magnetic field of the two wires is zero? In the portion where there is perfect cancellation, the magnetic field energy just disappear or get destroyed?

 

[Nugatory]

So the magnetic flux just vanish, just like that? But energy can't be created or destroyed. Is it not?

A magnetic field stores energy, so it takes a smidgen more energy to send current through an arrangement of conductors in a way that increases the magnetic field. This manifests itself as a larger impedance across that part of the circuit (google for the equations describing an ideal inductor) compared with what you'd get from just the resistance of the wires, hence a transient increase in the energy delivered by the current source - this is how energy is conserved. Conversely if the current changes in a way that decreases the magnetic field, the energy stored in the field is released and for a moment the current source has to supply less energy to keep the current going. (Note that if the magnetic field isn't changing, there's no effect at all and you can analyze the power, voltage, and current flows just using Ohm's law. This is a lot simpler, which is why intro classes tend to focus on steady-state DC circuits).

In the GFCI... when the ground fault is created, the magnetic field becomes non-zero. The energy for doing this comes from the household wiring doing a bit more work to create the magnetic field by pushing current through the two coils and out through the ground fault. The total amount of energy required to increase the magnetic field from zero to non-zero is very small, but the resulting magnetic field is easily detectable and triggers the device to open the circuit.

 

[seazal]

1: A long straight wire has some inductance.

2: That same wire bent into a round loop has more inductance than the straight wire.

3: That loop squeezed to a very narrow loop has less inductance than the straight wire.

4: When we make current to go through a wire that has large inductance, then a magnetic field that contains a large amount of energy is created.(When we make a current to go through a circuit with low inductance, we don't use much energy, in other words we don't send much energy to the circuit.

So, when we make a current to go through wires that don't create magnetic fields, we don't deposit any energy to the wires. This solves the problem "where does the energy go", doesn't it?)

Could you give an example of other dynamics that can do this too? If we make two matter, one normal matter, and two antimatter meet. They annihilate releasing energy instead of nulling themselves. Why does magnetic fields not follow this and instead null themselves. And in the act of the magnetic field disappearing (nulling) in balance pair of wires with opposite current, is not the act of disappearing destroying of energy? Visualize the current moving in the wire with magnetic field around it. When it meets an opposite current of the same value, then it withdraws its magnetic field just like that?

 

[anorlunda]

But remember the current induced its own magnetic field. So with opposite current flowing, the net magnetic field of the two wires is zero? In the portion where there is perfect cancellation, the magnetic field energy just disappear or get destroyed?

OK, your question is not about GFCI at all. You are asking about energy stored in magnetic fields. So let us make the question simpler and more basic, with one current and one coil.

Put the coil in a circuit where current flows, and we will induce a voltage across the coil $V=L\frac{dI}{dt}$. Instantaneously power P=VI will add to the energy stored in the coil's magnetic field. Integrate P over time and you have the energy stored. Note that the energy is static unless the current is changing in time.

Now consider a coil with an established field, and we want to reduce the current to zero. But we will not be able to do that instantaneously. If we open a switch, $\frac{dI}{dt}$ will have a large negative value. That causes a large voltage; so large that it causes a spark to jump across the contacts of the opening switch. All the energy in that coil will be dissipated in that spark (plus resistive losses in the rest of the circuit). What I just described, is found in everyday life in the ignition circuits of an old fashioned car. It has a coil that stores energy. When we attempt to open a switch to break the current, that causes a spark in the spark plug. So the coil's energy goes to the spark.

Energy is conserved at every moment in this process. The hard part is to do the accounting to figure out where the energy comes from and goes to. But rest assured, if we do our job correctly energy will be conserved, not just approximately conserved but exactly.

So in the above example, if I make the current go to zero not by opening a switch, but by a second wire carrying current in the opposite direction (as in that GFCI case), the physics remains unchanged, but the accounting of where energy comes from and goes to becomes more complicated because now we have the circuit of the second wire to include in the accounting. In your OP, you tried to think about the physics in the coil while ignoring the details of the circuits of those two wires. I think that is what led to your confusion.

In understanding concepts, it is usually best to begin with the simplest case, then add complications later.

 

[seazal]

OK, your question is not about GFCI at all. You are asking about energy stored in magnetic fields. So let us make the question simpler and more basic, with one current and one coil.

Put the coil in a circuit where current flows, and we will induce a voltage across the coil $V=L\frac{dI}{dt}$. Instantaneously power P=VI will add to the energy stored in the coil's magnetic field. Integrate P over time and you have the energy stored. Note that the energy is static unless the current is changing in time.

Now consider a coil with an established field, and we want to reduce the current to zero. But we will not be able to do that instantaneously. If we open a switch, $\frac{dI}{dt}$ will have a large negative value. That causes a large voltage; so large that it causes a spark to jump across the contacts of the opening switch. All the energy in that coil will be dissipated in that spark (plus resistive losses in the rest of the circuit). What I just described, is found in everyday life in the ignition circuits of an old fashioned car. It has a coil that stores energy. When we attempt to open a switch to break the current, that causes a spark in the spark plug. So the coil's energy goes to the spark.View attachment 235874

Energy is conserved at every moment in this process. The hard part is to do the accounting to figure out where the energy comes from and goes to. But rest assured, if we do our job correctly energy will be conserved, not just approximately conserved but exactly.

So in the above example, if I make the current go to zero not by opening a switch, but by a second wire carrying current in the opposite direction (as in that GFCI case), the physics remains unchanged, but the accounting of where energy comes from and goes to becomes more complicated because now we have the circuit of the second wire to include in the accounting. In your OP, you tried to think about the physics in the coil while ignoring the details of the circuits of those two wires. I think that is what led to your confusion.

In understanding concepts, it is usually best to begin with the simplest case, then add complications later.

So how does the accounting of the energy occurs in the more complicated case where we have circuit of the second wire to include in the accounting producing apparently net zero magnetic field with the first one having opposite current?

 

[anorlunda]

So how does the accounting of the energy occurs in the more complicated case where we have circuit of the second wire to include in the accounting producing apparently net zero magnetic field with the first one having opposite current?

The energy in the field goes somewhere into the external circuits. Exactly where depends on the details.

 

[jartsa]

Could you give an example of other dynamics that can do this too? If we make two matter, one normal matter, and two antimatter meet. They annihilate releasing energy instead of nulling themselves. Why does magnetic fields not follow this and instead null themselves. And in the act of the magnetic field disappearing (nulling) in balance pair of wires with opposite current, is not the act of disappearing destroying of energy? Visualize the current moving in the wire with magnetic field around it. When it meets an opposite current of the same value, then it withdraws its magnetic field just like that?

Well, I could copy-paste my post #10 here, then change words "inductance" to "capacitance", "current" to "voltage" and "magnetic field" to "electric field".

The dynamics in that edited story would be such that a wire is mangled when electricity is turned off - just like in the original story.Now, let's say we first have a wire loop, then we turn the electricity on, a very large amount of electricity. What happens is that first strong energetic fields are created, then the wire moves so that the fields get weaker and less energetic. I mean the wire breaks into pieces that fly apart. Does this sound a little bit like the annihilation?

(Let's say the wire loop is a spring bent into a loop shape, it works better that way, I forgot that wire stops being a conductor if it breaks to pieces :) )

 

[seazal]

How about in bar magnets. When you get two magnet bars of opposite pole and attach them. Does the canceled field energy stay there or is it also diverted elsewhere?

 

[Nugatory]

When you get two magnet bars of opposite pole and attach them. Does the canceled field energy stay there or is it also diverted elsewhere?

Work is done (just plain ordinary $W=Fd$) to move the magnets. This will balance the energy gained or lost by the magnets as they move from their initial configuration to their final configuration.

 

[seazal]

Work is done (just plain ordinary $W=Fd$) to move the magnets. This will balance the energy gained or lost by the magnets as they move from their initial configuration to their final configuration.

Let's just focus on magnets as it's simpliest and also focus on the final configuration. So when the two bar magnets are binded together with opposite poles at each end. What exactly is happening to their magnetic field? Can you still separately detect a north and south pole in each magnet? Or are they gone (cancelled)?
Better if there are illustrations about it.

Are the magnets behavior similar to the two wires in the sense coil of the GFCI?

 

[seazal]

Ive been studying all your excellent answers for a week. I noticed we were focusing on energy accounting of magnetic field only. But magnetic field is separate from the concept of energy. You can have thermal energy, cellular energy, etc. Although magnetic field have energy. Magnetic field is not energy. So the question become. 1) when there is zero energy, where is the identity of magnetic field kept. Remember if u pull the wires apart. They regain the energy from your arms. But why doesn't it become nuclear energy. So somehow the magnetic field with zero energy content is still magnetic field and it may be lurking in its potential form. What is the potential form called?

 

[Herman Trivilino]

What happens when magnetic field cancel. Where do they go. Do they just vanish or return to the vacuum?

You are misunderstanding what is meant by "cancel" in this context. Here are two sources, and we can calculate the effects of each source so that we know, for example, the magnitude and direction of the magnetic field inside your GFCI transformer. We are not not saying that the magnetic field at various locations due to each source alone is the magnetic field that exists at those locations.

But we do go on to use the superposition principle, meaning that the field at each location is the sum of the fields that each source alone would produce. Inside your transformer those two fields add up to zero. Saying they "cancel" is jargon.

 

[anorlunda]

We are going in circles. After all those replies, you are back where you started. That means you did not understand or did not learn from any of the answers.

I suggest that you learn from a textbook or from Wikipedia. This Internet forum is not helping you.

Thread closed.