# Does a magnetic field have mass?

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42 minutes ago, MPMin said:

As I understand it, a magnetic field can be created by running a current through a wire and an emp can be created by pulsing a current through a wire. Aside from the duration of time between the two I don’t see any other differences.

One is changing and one isn't. Therefore, one creates "just" a (static) magnetic field. The other generates a (changing) magnetic field and therefore also electromagnetic radiation.

This has been explained multiple times already.

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If you consider a cross section view of the wire carrying an alternating current it could be viewed as dropping pebbles (to represent an alternating current) in to a pond from the top down view, where the pebbles impact the water represents the wire and the ripples represent the changing magnetic field emanating outwards from the wire.

It is actually really hard to find an analogy for the behaviour of magnetic fields (or at least, I couldn't come up with one!)

We often use things like water flowing through a pipe as an analogy to electricity. So the rate of flow is current; the width of the pipe is resistance (actually, conductance, as a smaller pipe means tiger resistance) and the pressure of the water is voltage. You also model a capacitor quite easily this way. I'm not sure if it is easy to describe an inductor using this analogy.

And that is because the magnetic field behaves slightly oddly (because its behaviour is so much about changes, rather than the steady state.)

So, your pebble analogy generating waves is wrong. You need an analogy where waves would only be generated if the rate of pebbles changes. The steady dropping of pebbles would just set the water level.

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Could this mean that a magnetic field from an alternating current has mass but a magnetic field from a dc current does not have mass?

As you have been told many times, there is energy associated with the magnetic field. But it isn't "going" anywhere. On the other hand, electromagnetic radiation carries energy away from the source.

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1 hour ago, MPMin said:

On 9/24/2019 at 12:38 PM, swansont said:

It doesn't. The EMP is not the magnetic field. They are not the same thing. This has been pointed out only about twenty times so far.

As I understand it, a magnetic field can be created by running a current through a wire and an emp can be created by pulsing a current through a wire. Aside from the duration of time between the two I don’t see any other differences.

An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagneticenergy. Such a pulse's origin may be a natural occurrence or man-made and can occur as a radiated, electric, or magnetic fieldor a conducted electric current, depending on the source.

If your view that an emp is different to a magnetic field because an emp is a segment of a magnetic field then I can see how you see them as different things an emp is a segment of a magnetic field.

Here is another view. In the scenario of a wire carrying a current, before the current commences let’s assume there is nothing around the wire. When the current commences, wether the current is an alternating current or direct current, at the commencement of the current a magnetic field will populate the space around the wire starting at the wire and moving outwards from the wire at the speed of light.

A direct current will produce a magnetic that doesn’t induce a current in a stationary wire but if you push the wire through that magnetic field then it will induce a current in the wire being pushed. However, On the other hand, the magnetic field from an alternating current will induce an alternating current in a stationary wire in that magnetic field. A current restive to the motion will be induced in either case the, it’s only what’s moving that is the difference. In the direct current’s magnetic field the wire has to move to induce a current and in the alternating current’s magnetic field the it’s the magnetic field that’s moving which leads me to the following analogy.

If you consider a cross section view of the wire carrying an alternating current it could be viewed as dropping pebbles (to represent an alternating current) in to a pond from the top down view, where the pebbles impact the water represents the wire and the ripples represent the changing magnetic field emanating outwards from the wire. Considering that before the current commences there is nothing around the wire, but when the current commences the magnetic field be it from a dc or ac current will emanate from outward from the wire at the speed of light. In this analogy though, the water molecules are the magnetic field and the energy passing through the water molecules can be seen as the ripples. The water molecules do not move away from the centre but the water molecules do oscillate as the energy passes through them. In this analogy the magnetic field is basically a medium to carry energy. In a dc current the water would remain flat. So I can see why you say the magnetic field from a dc current doesn’t carry away any energy.

Could this mean that a magnetic field from an alternating current has mass but a magnetic field from a dc current does not have mass?

appogies for the font size difference I don’t know how to change the font size in this text. [FIXED]

1 hour ago, MPMin said:

As I understand it, a magnetic field can be created by running a current through a wire and an emp can be created by pulsing a current through a wire. Aside from the duration of time between the two I don’t see any other differences.

The difference is right there in your reference and also in th bit of what swansont said that I underlined.

The difference is the electric  field.

An EMP is a combination of two fields, an electric field and a magnetic field.

You have chosen to discuss only a magnetic field and have been answered as such.

Both your reference and swansont  also indicate the different between steady state conditions and changing conditions - the word transient appears in both.

A transient can last forever, it has nothing to do with duration.
A transient is about change of state / condition

A steady state is about, well a steady state ie an unchanging state/condition.

1 hour ago, MPMin said:

a magnetic field can be created by running a current through a wire and an emp can be created by pulsing a current through a wire

So yes both these are true but

1) A magentic field is just that a magnetic field - steady or changing.

2) An EMP is a combination of continually changing  electric and a magnetic fields.

Does this help ?

Edited by studiot

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1 hour ago, MPMin said:

As I understand it, a magnetic field can be created by running a current through a wire and an emp can be created by pulsing a current through a wire. Aside from the duration of time between the two I don’t see any other differences.

Then you should take some physics classes. Or listen to the people who know physics who post here.

The last thing you should be doing is asserting that something is true when it's based on your ignorance.

1 hour ago, MPMin said:

An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagneticenergy. Such a pulse's origin may be a natural occurrence or man-made and can occur as a radiated, electric, or magnetic fieldor a conducted electric current, depending on the source.

If your view that an emp is different to a magnetic field because an emp is a segment of a magnetic field then I can see how you see them as different things an emp is a segment of a magnetic field.

An EMP is not "a segment of a magnetic field"

1 hour ago, MPMin said:

Here is another view. In the scenario of a wire carrying a current, before the current commences let’s assume there is nothing around the wire. When the current commences, wether the current is an alternating current or direct current, at the commencement of the current a magnetic field will populate the space around the wire starting at the wire and moving outwards from the wire at the speed of light.

A direct current will produce a magnetic that doesn’t induce a current in a stationary wire but if you push the wire through that magnetic field then it will induce a current in the wire being pushed. However, On the other hand, the magnetic field from an alternating current will induce an alternating current in a stationary wire in that magnetic field. A current restive to the motion will be induced in either case the, it’s only what’s moving that is the difference. In the direct current’s magnetic field the wire has to move to induce a current and in the alternating current’s magnetic field the it’s the magnetic field that’s moving which leads me to the following analogy.

With an alternating current the wire will be an antenna, and emit EM radiation at the driving frequency. The EMR does not have mass.

34 minutes ago, studiot said:

The difference is right there in your reference and also in th bit of what swansont said that I underlined.

The difference is the electric  field.

An EMP is a combination of two fields, an electric field and a magnetic field.

You have chosen to discuss only a magnetic field and have been answered as such.

And there's even more to it than that, because it's a changing magnetic field

We know (from Maxwell's equations) that a changing magnetic field causes an electric field to exist.  ×E=dB/dt

And similarly, a changing electric field gives rise to a magnetic one. ×B=μ0J+μ0ϵ0E/t

The changing E and B fields satisfy the wave equation: it's electromagnetic radiation.

So you'll end up with a field, proportional to the current (and which will have mass) and EMR (which will not)

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6 hours ago, studiot said:

Does this help ?

Yes it does

thank you

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It sounds like @MPMin wants to have one complete model describing every aspect of magnetism from time t=0 when the power is turned on until some time later when a current have been steady for an extended amount of time and any transients, radiation, photons etc are no longer part of the equations.

I have this feeling that the correct question may not have been asked (yet). I’ll try formulating something, in an attempt to bring the discussion forward:
-Is there a model that describes the mass of a magnetic field, for both steady and transient situations?
-What concepts do one need to study to understand such model(s) of magnetic fields and their physical implications? (Physical implications could for instance be radiation pressure, EMPs, Lorentz force.)
-Are there related concepts one need to study? (For instance mass energy equivalence, wave particle duality, electrical fields, photons, electrons...)

I have some basic knowledge of electrical calculations and transients and steady situations were mostly treated separately using different formulas. There was normally no need to care about transients in a simple DC circuit with a few resistors and batteries in steady state. And an antenna calculation did not normally need to account for the momentum of photons. There were different equations for different applications. Ohms law was for instance not so useful to predict radiation pressure on a solar sail but that did not prevent us from using ohms law to make predictions where it applied. I do not know what model that would answer the original question in this thread but the discussion seems to run in a loop going from steady unchanging concepts to dynamic and transients and then back. MPMin you seems to want one complete (and therefore maybe complicated) model rather than separate answers to each question about different concepts related to magnetism? Members try to give detailed and correct answer of each question and you seem to try to bring the answers over to situations where they do not apply.

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28 minutes ago, Ghideon said:

I do not know what model that would answer the original question in this thread but the discussion seems to run in a loop going from steady unchanging concepts to dynamic and transients and then back.

Nice summary, Ghideon.

I think the one model that applies in all cases is described by Maxwell's equations. The simpler equations that can be applied in specific equations are special cases of the more general model.

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I concur. The model that describes the EM phenomena are Maxwell's equations

The model that describes mass is E2 = p2c2 + m2c

The mass of something is related to its energy, but the energy associated with its CoM motion (its momentum) is accounted for separately.

IOW, if an object has no momentum, the mass and energy are proportional to each other. E = mc^2

If the object's CoM is moving, that changes the total energy, but not the mass. (And there's no wiggle room here for personal interpretation of wanting vibrations to count one way as opposed to another)

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