Classical Electromagnetism: Question on Ampere's Law and Displacement Current

[jaseh86]

Hi, just out of curiosity...

Ampere's Law describes that an electric current produces a magnetic field. When corrected with Maxwell's displacement current, it describes that a magnetic field is also created by a time-varying electric field.

Does this mean that an electric current produces the magnetic field purely BECAUSE of the changes in electric field associated with moving charges. Or is electric current one of two distinct ways (the other being changing electric field) to produce magnetic field.

A little confused on the matter.

Thanks.

 

[Defennder]

Ampere's circuital law does not say that an electric current produces a magnetic field. That is the Biot-Savart law. I don't see how a changing electric field (without considering the current flow) can induce a B-field.

 

[jaseh86]

Thanks for the reply.

But doesn't Maxwell's Theory of Light explain that changing electric fields induce magnetic fields due to Ampere's Law / Biot-Savart Law. In vacuum for instance, there is no electric current, so why is it that a changing electric field without current flow can't produce magnetic field?

 

[cabraham]

Good question. All we really can say from Maxwell's equations is that under time varying conditions, electric and magnetic fields cannot exist independently. It is futile to even attempt to ascertain which one comes first. Which is the cause and which is the effect is an endless vicious circle. In his 1905 paper "On The Electrodynamics Of Moving Bodies", Einstein states that "questions as to which one (electric or magnetic field) is the "seat" (root, fundamental, canonical, principle, basis) no longer have any point." The parentheses are mine.

 

[Defennder]

Thanks for the reply.

But doesn't Maxwell's Theory of Light explain that changing electric fields induce magnetic fields due to Ampere's Law / Biot-Savart Law. In vacuum for instance, there is no electric current, so why is it that a changing electric field without current flow can't produce magnetic field?

Hmm, actually having taken a closer look at your thread title, I realize that I might have misunderstood your question. The displacement current, which is associated with a changing E-field does itself induce a B-field. The only question that remains if every time-varying E-field is associated with a displacement current.

 

[nicksauce]

The only question that remains if every time-varying E-field is associated with a displacement current.

To which I would definitely say no. The easiest case is solving Maxwell's equation in the absence of sources, and getting a wave equation for E (and B), which is certainly a time-varying field.

 

[rbj]

I don't see how a changing electric field (without considering the current flow) can induce a B-field.

hunh?? that's what one of them 4 Maxwell's Equations tells us. of course a changing electric field induces a magnetic field.

imagine a simple two parallel plate capacitor with axial leads and a steady (DC) current flowing in one lead (and out the other lead). between the plates of the capacitor there is no current, but there is a magnetic field induced.

 

[Defennder]

To which I would definitely say no. The easiest case is solving Maxwell's equation in the absence of sources, and getting a wave equation for E (and B), which is certainly a time-varying field.

Solving which equation?

hunh?? that's what one of them 4 Maxwell's Equations tells us. of course a changing electric field induces a magnetic field.

imagine a simple two parallel plate capacitor with axial leads and a steady (DC) current flowing in one lead (and out the other lead). between the plates of the capacitor there is no current, but there is a magnetic field induced.

That is because of the displacement current. The question I posed was whether every time-varying E-field would be associated with a displacement current. nicksauce said no, so how about those cases whereby we have a time-varying field but no displacement current or current flow? Are there such cases, and is any B-field induced?

 

[cmos]

Ampere's circuital law does not say that an electric current produces a magnetic field. That is the Biot-Savart law. I don't see how a changing electric field (without considering the current flow) can induce a B-field.

Can you defennd this statement a bit (mind the pun )? Ampere's law clearly states that the presence of electric current induces a curling magnetic field. Does this not imply that electric current produces a magnetic field?

The laws of Ampere and Biot and Savart both say that current induces the magnetic field; I would call it a matter of convenience in choosing which law to invoke for a specific problem. Note that you may consider either law as an axiom to EM theory from which you can derive the other.

 

[cmos]

The question I posed was whether every time-varying E-field would be associated with a displacement current. nicksauce said no, so how about those cases whereby we have a time-varying field but no displacement current or current flow? Are there such cases, and is any B-field induced?

The term known as the displacement current (dD/dt) is called that because it is correct to think of it as a current in the traditional (or at least semi-traditional) sense; that is as the movement of charge. You have to remember that the electric displacement (D-field) contains in it two facets of information: 1) the electric field (E-field) and 2) the electric polarization (which is essentially a sum over electric dipole moments).

It is the time-varying realignment of dipole moments, when immersed in a time-varying electric field, that gives rise to the displacement current. Thus we can see the displacement current as a movement of real charge; the charge, however, is a bound charge as opposed to a free charge.

Now in freespace, the polarization vector is identically zero, thus (taking the freespace permittivity to be unity) D=E. Clearly then, what is called the displacement current has nothing to do with an actual current, but is a pure time-varying electric field. But seeing that we have already named Maxwell's correction to the Ampere law as the displacement current, we might as well keep calling it that (or so some would argue). Really, its a matter of taste.

 

[Defennder]

Can you defennd this statement a bit (mind the pun )? Ampere's law clearly states that the presence of electric current induces a curling magnetic field. Does this not imply that electric current produces a magnetic field?

The laws of Ampere and Biot and Savart both say that current induces the magnetic field; I would call it a matter of convenience in choosing which law to invoke for a specific problem. Note that you may consider either law as an axiom to EM theory from which you can derive the other.

I have already answered this part in my reply to rbj with my questions below. I can't edit my old posts so I'll appreciate if you'll read both my later replies as well as the earlier posts. As for the fact that Ampere's and Biot-Savart's law follows from one another, I am aware of that. I was only saying that the Biot-Savart law gives us a more explicit vector equation of B due to I, in comparison to Ampere's.

The term known as the displacement current (dD/dt) is called that because it is correct to think of it as a current in the traditional (or at least semi-traditional) sense; that is as the movement of charge. You have to remember that the electric displacement (D-field) contains in it two facets of information: 1) the electric field (E-field) and 2) the electric polarization (which is essentially a sum over electric dipole moments).

It is the time-varying realignment of dipole moments, when immersed in a time-varying electric field, that gives rise to the displacement current. Thus we can see the displacement current as a movement of real charge; the charge, however, is a bound charge as opposed to a free charge.

Now in freespace, the polarization vector is identically zero, thus (taking the freespace permittivity to be unity) D=E. Clearly then, what is called the displacement current has nothing to do with an actual current, but is a pure time-varying electric field. But seeing that we have already named Maxwell's correction to the Ampere law as the displacement current, we might as well keep calling it that (or so some would argue). Really, its a matter of taste.

How does this answer the questions
1. Is every time-varying E-field would be associated with a displacement current D?
2. Are there any cases whereby you have a varying E-field but no associated displacement current?
3. If the answer to 2 is yes, then would there be a B field induced?

These are just yes/no questions. I don't see how you have answered them.

 

[Defennder]

Anyway, here's what Wikipedia says:

According to Maxwell's equations, a time-varying electric field generates a magnetic field and vice versa. Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form an electromagnetic wave.

But since it's Wikipedia, I'll leave it to the experts to verify it. Otherwise it answers all of my questions.

 

[cmos]

How does this answer the questions
1. Is every time-varying E-field would be associated with a displacement current D?
2. Are there any cases whereby you have a varying E-field but no associated displacement current?
3. If the answer to 2 is yes, then would there be a B field induced?

These are just yes/no questions. I don't see how you have answered them.

I believe I answered all these questions in my previous post. When asking a question, you learn nothing by just getting a yes or no response; hence my lengthy response. But, to be explicit:

1) Well first off D is not the displacement current, it is the electric displacement (sometimes called the flux density). The time-derivative of D is the displacement current. In principle, there is nothing wrong with talking about the D-field in freespace, but since it is equivalent to the E-field ( D=\epsilon_0E ), there really is no point.

As I said before, whether you wish to call the time-derivative of the electric field in freespace a displacement current or not is really just a matter of taste.

2) See above.

3) As you Wikipedia article states, a time-varying electric field induces a time-varying magnetic field and vice-versa. This is the mechanism by which light propagates.

 

[Defennder]

Thanks, got it. Should have read your post more closely. I was just trying to be sure I understood it correctly.

 

[cmos]

I have already answered this part in my reply to rbj with my questions below. I can't edit my old posts so I'll appreciate if you'll read both my later replies as well as the earlier posts. As for the fact that Ampere's and Biot-Savart's law follows from one another, I am aware of that. I was only saying that the Biot-Savart law gives us a more explicit vector equation of B due to I, in comparison to Ampere's.

I think it just may have been your choice of wording in your first post that may lead some to confusion. To make it clear, Ampere's law does, in fact, state that a current induces a magnetic field. In principle, it is a fundamental law of nature. In practice, however, Ampere's law is not always so easy to play with and there are time when deference to the Biot-Savart law gives a simpler way to reach a solution for the magnetic field.

 

[jaseh86]

Hi everyone, thanks for all the input, even though it has kind of deviated from the intention of my original question.

Perhaps the questions I really should have asked are :

1. Does the movement of charges directly result in a displacement current (varying E-field)?
3. Can an electric current induce a magnetic field if there was NO consideration/existence of a displacement current?

Ultimately, from cabraham's post, I assume it becomes pointless to question whether moving charges induce a time-varying magnetic field which then induces a time varying E field (Faraday's), or moving charges have a time-varying electric field which then induce a B field (Maxwell-Ampere).

Thanks.

 

[cmos]

Well now your line of questioning is getting a bit more complex. It comes from the upper-level idea that "there is really no such thing as magnetostatics." What this means is that what is usually taught as "magnetostatics" should strictly be called magneto-quasi-statics. This, as you have said, is because a steady current is really a movement of charge which follows from it time-variances in the fields. But, we can often neglect these effects since they would add only the smallest of corrections (which is good because it is so much easier to write "magnetostatics").

To answer your questions:

1) Yes, the movement of charge implies modification in the electric field, but for a steady current, this can often be neglected. Note, however, that an AC current will induces an AC B-field by Ampere's law which, in turn, will induce an AC E-field by Faraday's which, in-turn, will induce an AC B-field by Maxwell's correction to Ampere's law which, in-turn, will ...

3) Sure. This is just Ampere's law without Maxwell's correction.

 

[Defennder]

Note, however, that an AC current will induces an AC B-field by Ampere's law which, in turn, will induce an AC E-field by Faraday's which, in-turn, will induce an AC B-field by Maxwell's correction to Ampere's law which, in-turn, will ...

I'm getting the impression from some of the sources that I've read (Wikipedia and others) that this results in an electromagnetic wave. Does this explain why wires heat up or something? I guess not, since clearly Joules heating occurs in DC current flowing in wires. But if not, what happens to the EM waves?

 

[merryjman]

I have trouble with the term "displacement current". Maxwell used it as a cheat, and it has no physical meaning. The example I always see is a charging parallel-plate capacitor: if you try Ampere's law with your loop between the plates, you get a nonsensical result because there is no current enclosed. If your loop encloses the wire connected to the plate, you get a sensible answer. But Ampere's law shouldn't depend on what loop you choose. So Maxwell conceived a "displacement current" which flowed between the plates and fixed this discrepancy. Nowadays we realize that it's not just moving particles which create magnetic fields, it's changes in the electric fields which do it, and furthermore, these two fields are inextricably intertwined.

For Defennder: I think that normal DC Joule heating also produces EM waves; they are in the infrared region. In the case of extreme Joule heating, like in a small length of thin nichrome wire (or imagine a DC toaster oven if you want), the energy of the EM waves is high enough to be in the visible spectrum.

 

[cmos]

I have trouble with the term "displacement current". Maxwell used it as a cheat, and it has no physical meaning. The example I always see is a charging parallel-plate capacitor: if you try Ampere's law with your loop between the plates, you get a nonsensical result because there is no current enclosed. If your loop encloses the wire connected to the plate, you get a sensible answer. But Ampere's law shouldn't depend on what loop you choose. So Maxwell conceived a "displacement current" which flowed between the plates and fixed this discrepancy. Nowadays we realize that it's not just moving particles which create magnetic fields, it's changes in the electric fields which do it, and furthermore, these two fields are inextricably intertwined.

You account of the history, to my knowledge, is correct; however, to say the displacement current has no physical meaning is incorrect. I will refer you back to my https://www.physicsforums.com/showpost.php?p=1833860&postcount=10".

 

[cmos]

I'm getting the impression from some of the sources that I've read (Wikipedia and others) that this results in an electromagnetic wave. Does this explain why wires heat up or something? I guess not, since clearly Joules heating occurs in DC current flowing in wires. But if not, what happens to the EM waves?

Yes, direct application of Fariday's law and the Ampere-Maxwell law in a source-free region is sufficient to explain the prorogation of light. This does not directly explain why wires heat up though; Joule heating does (as you brought up).

I want to point out though, Joule heating will occur in both DC and AC conditions (ever use a hair dryer?). Joule heating occurs because you are flowing a current through an element that has a natural resistance to the flow of current. Microscopically, it is due to electron collisions that give up energy to the material lattice; we perceive this as heat.

 

[Defennder]

Yes, direct application of Fariday's law and the Ampere-Maxwell law in a source-free region is sufficient to explain the prorogation of light. This does not directly explain why wires heat up though; Joule heating does (as you brought up).

I want to point out though, Joule heating will occur in both DC and AC conditions (ever use a hair dryer?). Joule heating occurs because you are flowing a current through an element that has a natural resistance to the flow of current. Microscopically, it is due to electron collisions that give up energy to the material lattice; we perceive this as heat.

So is it accurate to say that in AC conditions, heating occurs through both electron collisions with the lattice and EM waves induced by time-varying fields, whereas in DC conditions they only arise due to the former? (I am aware that heat itself would give off EM waves in DC, but this is due to phonons and not EM wave induction).

 

[cmos]

I think you're starting to mix up several concepts. First off, an EM wave is, by definition, a time-variance in the electromagnetic field. The induction of either electric or magnetic field from the other allows us to speak of it simply as a single entity.

Second, remember from thermodynamics that heat is simply energy. Heating occurs because of some mechanism that increases the energy of atoms in a material. Heat itself cannot emit light. What you may be thinking of is that a "hot" body (i.e. one that is hotter than absolute zero) does emit light. This is due to atomic transitions and not directly due to phonons.

 

[Defennder]

Right, so let's drop the term phonons and use "electron collisions" in the lattice instead. But if as I posted earlier, an EM wave is created by an AC current, does such a wave contribute to the heating up of the wire?

 

[cmos]

Well if we're talking about a simple AC circuit, it's the AC voltage that drives the current. In principle, I suppose attenuation of the electric field in the wires and circuit elements could cause heating, but for most practical purposes I would think that this is completely negligible compared to standard Joule heating.

 

[cabraham]

Well if we're talking about a simple AC circuit, it's the AC voltage that drives the current. In principle, I suppose attenuation of the electric field in the wires and circuit elements could cause heating, but for most practical purposes I would think that this is completely negligible compared to standard Joule heating.

Would you please elaborate on "it's the AC voltage that drives the current." Are you referring to the fact that most every day ac power sources are constant voltage, and not constant current sources? If the ac power source is constant current, then what? Just wondering what you're saying. Thanks in advance.

Claude

 

[cmos]

That's pretty much what I meant by saying that the voltage drives the current. I suppose we can get into technicalities or very specific examples. But even then, isn't a current source ultimately controlled by a source voltage?

 

[cabraham]

That's pretty much what I meant by saying that the voltage drives the current. I suppose we can get into technicalities or very specific examples. But even then, isn't a current source ultimately controlled by a source voltage?

I would say no. A current source outputs whatever voltage necessary to sustain a fixed current value, and vice-versa for a voltage source. Neither one "drives " the other. Both are driven by the work being done. An AC generator, for instance, converts mechanical power to electrical. The current and voltage are both driven by the mechanical power, the product of torque and speed. Voltage doesn't drive current, nor the converse. BR.

 

[cmos]

cabraham:

Your example of an AC generator doesn't really follow from the previous discussion nor is your description completely accurate. So that we don't confuse anybody trying to learn from this thread, we'll disregard it for now.

I don't follow your line of thinking in your previous post. Current flows because of the application of a voltage. You place a voltage across a resistor and it will drive the current.

From a more fundamental viewpoint, current is the movement of charge. Charge carriers (e.g. electrons) are compelled to move when immersed in an electric field. Electric field is just the (negative) gradient of the electric potential (i.e. voltage). When you apply a voltage you are, in essence, applying an electric field. Thus, it is the voltage that drives the current; not the other way around.

In a current source, the output current is determined by the source voltage and the circuit elements making up the current source. That is why I said that even in a current source, you are still ultimately driving the current with a voltage.

Sorry to the OP for getting grossly off the original topic. I hope that by now everything makes sense.

 

[cabraham]

cabraham:

Your example of an AC generator doesn't really follow from the previous discussion nor is your description completely accurate. So that we don't confuse anybody trying to learn from this thread, we'll disregard it for now.

I don't follow your line of thinking in your previous post. Current flows because of the application of a voltage. You place a voltage across a resistor and it will drive the current.

From a more fundamental viewpoint, current is the movement of charge. Charge carriers (e.g. electrons) are compelled to move when immersed in an electric field. Electric field is just the (negative) gradient of the electric potential (i.e. voltage). When you apply a voltage you are, in essence, applying an electric field. Thus, it is the voltage that drives the current; not the other way around.

In a current source, the output current is determined by the source voltage and the circuit elements making up the current source. That is why I said that even in a current source, you are still ultimately driving the current with a voltage.

Sorry to the OP for getting grossly off the original topic. I hope that by now everything makes sense.

cmos:

You make statements having no basis at all. E field can be expressed as the negative of grad V, but V can be expressed as the line integral of E. Also, J = sigma*E, where J is current density, sigma is conductivity, and E is electric field intensity. There is a functional relationship between J and E, E and V, etc. Also, an electric field cannot be set up unless current and voltage are both present. To change the E field requires work since E fields store energy. Work takes time. The work divided by the time is power. For non-zero power, both I and V must be non-zero. These equations do not imply "causality". Also, E = -grad V only holds for charged particles. For induction in time changing case, E = -dA/dt, where B = curl A. The relation between E, H, I, and V is mutual inclusion. They cannot exist independently, and neither drives the other. Einstein in his 1905 "On The Electrodynamics Of Moving Bodies" stated E and H as having no pecking order. Also, there is no such thing as "applying a voltage". Read up on transmission lines.

Regarding current sources, a photodiode across the op amp input terminals is a constant current generator proportional to incident light. There is no "voltage" controlling said current. An inductor tends to exhibit constant current behavior. In a switched mode power supply, when the power switch is turned off, the inductor de-energizes through the output rectifier and load as well as current sense resistor. The current in the inductor is the fixed quantity and the voltage developed across the current sense resistor is determined by the current and the resistance. In this case the "current drives the voltage". A resistor can be driven either way. There is no pecking order.

The notion that current is driven by voltage is just a prejudice. It stems from the fact that all batteries and generators in common everyday use are designed and optimized to function in the constant voltage mode. We can get sloppy and easy fall into the "apply a voltage and get a current" misconception.