Where Does the Energy Go When Magnetic Fields Cancel?

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Discussion Overview

The discussion revolves around the behavior of magnetic fields when they cancel each other out in alternating current (AC) circuits. Participants explore the implications of magnetic field cancellation, energy conservation, and the nature of magnetic fields in relation to alternating and direct current. The conversation includes theoretical considerations and analogies, as well as questions about measurement and detection of magnetic fields.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that when magnetic fields from AC wires cancel, they do not disappear but rather become suppressed, similar to the superposition of water waves.
  • Others argue that the fields do not behave like water waves and instead add together as if the other field were not present.
  • There is a question about whether the cancellation of magnetic fields results in a steady magnetic field or if it leads to a complete absence of the field.
  • Some participants inquire about the conservation of energy in the context of magnetic field cancellation and where the energy goes when fields cancel.
  • A participant suggests that the magnetic fields exert forces on the wires, which could lead to a misunderstanding about their interaction.
  • There is a discussion about the nature of the magnetic fields produced by AC currents and how they switch polarity at the frequency of the AC supply.
  • One participant raises a hypothetical scenario involving photons that are out of phase and questions the implications for energy when their fields combine to zero.
  • Participants express uncertainty about the operation of the magnetic field detector and how it measures the fields in the described setup.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of the magnetic fields when they cancel. Multiple competing views remain regarding whether the fields are annihilated, suppressed, or transformed into a different state. The discussion is characterized by ongoing questions and clarifications rather than settled conclusions.

Contextual Notes

Limitations include the lack of clarity on the specific mechanisms of magnetic field detection and the assumptions underlying the analogies used in the discussion. The relationship between AC and DC magnetic fields remains a point of contention, with various interpretations presented.

rodsika
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I got this electromagnetic field detector that can measure magnetic field (B-fields) of Alternating Current of 60 Hz. Supposed after getting the 2 wires together and the magnetic field canceled and reading lowered from 10 milligauss to 1 or 0 milligauss. Does it mean getting a meter that measures steady magnetic field would get reading of the original 10milligauss in the same setup? That is.. when the AC magnetic field cancelled, it becomes DC magnetic field or would the magnetic field turn zero (or close to 0 owing to not all cancelled)?

This is because I heard when magnetic fields canceled in the AC wires of the same circuit. They are not gone. The energy density is still there and only the frequency got cancelled. Thanks.
 
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No, it does not become a DC field. For that you need a DC current in the wire.
 
Antiphon said:
No, it does not become a DC field. For that you need a DC current in the wire.

But isn't it that for the zone where magnetic fields canceled, they were not annihilated.. but simply suppressed... it's like two water waves coming from opposite directions.. when they meet, the zone would be calm, but they would continue at others sides.. much like superposition. Isn't it this is also what happened to magnetic fields cancelled?
 
It doesn't work like water. The fields add each as if the other wasn't there.
 
Antiphon said:
It doesn't work like water. The fields add each as if the other wasn't there.

Can you please elaborate what you meant by "The fields add each as if the other wasn't there"? When magnetic fields from alternating current cancel. Isn't it that the fields still exist only they are not in sine wave anymore but steady? Hope someone with more words and descriptions can explain. Thanks.
 
What is the polarity of each wire? The same or opposite?
 
Drakkith said:
What is the polarity of each wire? The same or opposite?

Just imagine you wire single wire AC cords to a light bulb and plug it into 60 Hertz 110 Volts AC. Separately, the wire would have big magnetic field like 10 milligauss.. but when you put the wires close to each other (the wire before and after the bulb), the magnetic fields cancelled. My question is. Isn't it that energy is conserved. So when two EM waves cancel each other means that the two EM waves are somehow hiding underneath the covers becoming DC magnetic field. If not true, where did the magnetic field go?
 
The fields don't cancel, they exert a force on each wire and try to force the two wires apart. At least that's what I think my basic electronics book was saying. I'll try to get a reference.
 
Actually, I think my first question was wrong. I don't think the field has a polarity between 2 wires. But I definitely remember that two wires in a circuit would be pushed apart.
 
  • #10
Drakkith said:
Actually, I think my first question was wrong. I don't think the field has a polarity between 2 wires. But I definitely remember that two wires in a circuit would be pushed apart.

No this won't happen. The magnetic field of the currents in a power cord would not be strong enough to push the wires apart. What were you thinking? I hope others can comment too of what happened to the magnetic field when they are cancelled. Do they just hide in the zone or completely annhilated.. please elaborate
 
  • #11
rodsika said:
No this won't happen. The magnetic field of the currents in a power cord would not be strong enough to push the wires apart. What were you thinking? I hope others can comment too of what happened to the magnetic field when they are cancelled. Do they just hide in the zone or completely annhilated.. please elaborate

My apologies, I didn't mean to imply the wires would actually be pushed apart in a power cord, obviously this does not happen. I only meant that the wires would feel a force that would want to push them apart.
 
Last edited:
  • #12
Drakkith said:
My apologies, I didn't mean to imply the wires would actually be pushed apart in a power cord, obviously this does not happen. I only meant that the wires would feel a force that would want to push them apart.

But what happened to the magnetic field when the meter reads 0? Please describe what happens. Thanks.
 
  • #13
Here we go, take a look here about current in a loop: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html#c1

As you can see, at the bottom and top of the loop the magnetic field is "around" the wire. This corrosponds to the magnetic field in a single wire as shown here: http://en.wikipedia.org/wiki/File:Electromagnetism.svg

Since the current is traveling opposite in the return line in your power cord the two fields are oriented differently. One would be clockwise and the other counterclockwise if you were looking down the wires. These fields oppose each other in that they exert a force on each wire, attempting to force them apart. However, they also "add" together to produce a basic electromagnet, as the first link shows.

The only difference in your AC circuit would be that the current and orientation of the magnetic fields switch at 60 hz. I believe two wires next to each other instead of in a loop would experience the same effects as the loop. So the fields don't cancel or annihilate, it simply switches poles with the electric frequency.
 
  • #14
rodsika said:
But what happened to the magnetic field when the meter reads 0? Please describe what happens. Thanks.

Do you have a link to the type of detector you have?
 
  • #15
Drakkith said:
Do you have a link to the type of detector you have?

This https://www.amazon.com/dp/B0046HEZMM/?tag=pfamazon01-20
 
  • #16
So are you actually getting a reading near zero? What exactly are you doing when you get this reading?
 
  • #17
Drakkith said:
So are you actually getting a reading near zero? What exactly are you doing when you get this reading?

When the AC line before and after the bulb is split apart, the reading is higher. When they are close together. It got very low (but not zero). What happened to the magnetic field. It got canceled but does it get annihilated or is it just hiding in the cover or what...
 
  • #18
Or let me reformulate the questions in more clearer terms.

Suppose we have two photons that are 180 degrees out of phase. We can assign one unit of energy to each photon. When we superpose the two photons, the combined electric and magnetic field goes to zero as does the Poynting vector and the energy density. So where does the energy go?
 
  • #19
I don't know how your meter detects the magnetic field so I can't really say much on that. I don't think two photons would interact like that though. If anything they would interfere and would act accordingly.
 
  • #20
Drakkith said:
I don't know how your meter detects the magnetic field so I can't really say much on that. I don't think two photons would interact like that though. If anything they would interfere and would act accordingly.

Or just consider a powerline in the street. Some are designed such that the magnetic fields canceled or attenuated a lot so it won't reach the houses below. In such cancellation, what happened to the magnetic field? Or just consider any general scenerio where magnetic field cancel, where does the energy go. Hope others can comment too. Thanks.
 

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