Can magnetic field exist without electric field?

[amitSingh95]

Recently I bought - Introducing Relativity A Graphic Guide, in the book, after introducing Maxwell's theory of electromagnetism a line is written, "Simple magnetic fields can occur when there is no electric field (and vice versa)".
I understand that electric field can exist without any magnetic field, although it is also generated by varying magnetic field, but as much I know, magnetic fields don't exist independently, they are generated by varying electric field or moving charge. Even in any magnetic substance, magnetic field is generated due to electrons orbiting the nucleus, and electrons have their own electric field, so electric field is present their too.
How is it possible that magnetic field exists without any presence of electric field?

 

[davenn]

Even in any magnetic substance, magnetic field is generated due to electrons orbiting the nucleus, and electrons have their own electric field, so electric field is present their too.
How is it possible that magnetic field exists without any presence of electric field?

Hi
I couldn't find any direct references to support the idea that a permanent magnet also has an electric field, everything I read said there was only a magnetic field, some one else might.
I think you will find that any electric field produced by the electrons probably doesn't radiate outside of the permanent magnet. for all purposes a permanent magnet only has a magnetic fieldDave

 

[amitSingh95]

Hi
I couldn't find any direct references to support the idea that a permanent magnet also has an electric field, everything I read said there was only a magnetic field, some one else might.
I think you will find that any electric field produced by the electrons probably doesn't radiate outside of the permanent magnet. for all purposes a permanent magnet only has a magnetic field


Dave

Even if electron's electric field is negligible, electric field IS present there, that's what I don't want, I might be misinterpreting the line but I think according to the book, magnetic field can exist even if there is no electric field at all.

 

[KatamariDamacy]

Even if electron's electric field is negligible, electric field IS present there, that's what I don't want, I might be misinterpreting the line but I think according to the book, magnetic field can exist even if there is no electric field at all.

Neutron has no apparent electric charge and yet it has magnetic moment. But there really shouldn't be any magnetic fields where there are no electric fields, so it is believed neutrons are actually composed of positively and negatively charged particles.

 

[Nugatory]

How is it possible that magnetic field exists without any presence of electric field?

There are two ways of creating an electrical field: Introduce an electrically charged particle, or introduce a time-varying magnetic field.

However, there are no magnetically charged particles (if they existed they'd be called "magnetic monopoles") so the only way of creating a magnetic field involves some form of electric current or time-varying electric field.

In that sense, you can't have a magnetic field without an electric field somewhere. However it is possible and indeed fairly common to find a magnetic field at points in space where the electric field is zero, or close enough to zero as not to matter. That's likely what your book meant.

 

[Vanadium 50]

You have to decide if you want to think like a lawyer or like a physicist. A lawyer can complain that there aren't any frictionless surfaces, stretchless ropes, massless pulleys or ideal gasses. And he'd be right. He'd also fail to understand a great deal of physics where these are good approximations.

You can certainly quibble that nothing is neutral enough for your tastes. All matter is made of atoms and atoms have electrically charged components. But again, you would fail to understand something important.

 

[amitSingh95]

You have to decide if you want to think like a lawyer or like a physicist. A lawyer can complain that there aren't any frictionless surfaces, stretchless ropes, massless pulleys or ideal gasses. And he'd be right. He'd also fail to understand a great deal of physics where these are good approximations.

You can certainly quibble that nothing is neutral enough for your tastes. All matter is made of atoms and atoms have electrically charged components. But again, you would fail to understand something important.

Well I prefer to think like someone curious enough to switch to any point of view necessary because no outlook is useless, I just wanted to know if this was practically possible by some method I was unaware of, but turns out that this too is one of those ideal conditions which are practically probably impossible to achieve, and when I learned that, I accepted that without any question.

 

[mfb]

Every known charged particle has a magnetic moment, so there are no electric fields without magnetic fields either. There are extremely good approximations to both cases.

 

[Malverin]

When we have ideal inductor or ideal capacitor, the current and voltage are out of phase, shifted with +/- 90 degrees.
So there will be moments in time, when current will be zero at maximum voltage and moments when voltage will be zero at maximum current.
Without current there is no magnetic field and without voltage - no electric field.

http://www.electronics-tutorials.ws/inductor/ac-inductors.html

 

[cabraham]

Under static conditions, either can exist w/o the other. A superconducting loop w/ a steady dc current has magnetic field, but no electric field. The counterpart of this setup is a charged capacitor open circuited, as there is an electric field w/o a magnetic field.

Again these are static conditions, i.e. not changing with respect to time. Under dynamic conditions, i.e. changing wrt time, neither can exist w/o the other. If one field is time-changing, the other must be non-zero.

Claude

 

[unter71]

Without current there is no magnetic field and without voltage - no electric field.

There is plenty of electric and magnetic fields in wires regardless of voltage. About as much positive as that of negative polarity, so they superimpose and cancel out on average or macroscopically, but microscopically fields are already there.

I don't think the question is about complex systems, fields cancellation and net result, but really about single elementary particle. I'm not sure whether such particle is necessarily supposed to be a "magnetic monopole" or not.

 

[Dale]

How is it possible that magnetic field exists without any presence of electric field?

For any EM field there is a quantity defined at each location: E²-B² which is invariant (a scalar field). If that quantity is negative at some location then there exists a reference frame where the E field is 0 at that location and the B field is non-0.

I suspect that this is what the book is referring to. I don't think that it is claiming that there is no E field anywhere in the universe, simply that you can have an E field without a B field at a given location, or vice versa.

Also, in classical EM we typically treat charge distributions as continuous, in which case we certainly can have a neutrally charged wire carrying a current and thereby producing a B field with no E field.

 

[Mallika]

Yes, neutrons have a magnetic field even in absence of electric field.

 

[Sagar98]

Yep, you can make magnetic field exist without electric field. Example, any normal bar/horse shoe magnet

 

[Drakkith]

FYI, several posts that were better suited to a new thread have been moved.

 

[Pythagorean]

Hmm... isn't there always a reference frame in which you can find 0 electric field or 0 magnetic field? My special relativity is rusty, but I remember something like that.

 

[Delta2]

Magnetic field can be generated either by a current (moving charge) or by time varying electric field.

If the magnetic field is generated by a current, it is possible that the electric field is zero or very close to zero. For example the magnetic field from bar magnets is generated by the electrons orbiting the nucleus, while the electric field of those electrons is practically canceled by the electric field of the protons in the nucleus.

In the case of a DC current the magnetic field is generated by the free electrons of a conductor which have a net drift velocity, the electric field of the free electrons is again practically canceled by the electric field of protons of the nucleus of the conductor atoms. What is left is only the electric field of the DC voltage source which also is very small in the region outside the conductor , where in the same region the magnetic field is not zero at all.

 

[Dale]

Hmm... isn't there always a reference frame in which you can find 0 electric field or 0 magnetic field? My special relativity is rusty, but I remember something like that.

Yes. In natural units E²-B² is an invariant. So if it is positive then there exists some reference frame where B=0 and E is minimum, and if it is negative then there exists some reference frame where E=0 and B is minimum.

 

[mfb]

Hmm... isn't there always a reference frame in which you can find 0 electric field or 0 magnetic field? My special relativity is rusty, but I remember something like that.

For a specific point in spacetime, yes - but not for all locations at the same time, at least not in general.

 

[wolfekeeper]

The standard answer taught to undergraduates is that yes, you can have a magnetic field without an electric field.

However, my understanding of the equations and what they really mean is that that's not strictly correct.

Although you can have a magnetic field without there being any average electric field; you have to consider what a magnetic field really is.

A magnetic field is not actually a separate field to the electric field; they are one thing, electromagnetism.

So how does that work?

Well, when you have (say) electrons moving along a wire, they mostly cancel out the positive charges in the positive wire.

But, there's a slight wobble in the electric field due to those charges moving; and the more charges that are moving, they on average cancel out over time, but that wobble does NOT cancel out! In fact the more charges there are, the more overall wobble there is. You would think it would be irrelevant, but the electric charge force is so very strong, so even microscopic wobbles in it can have a very big effect when summed up.

That wobble IS the magnetic field!

As a pretty good analogy, the equation for angular momentum in gyroscopes has a cross product in it. However, if you look at what that equation is really telling you, you discover that when a gyroscope is turned by a torque there's a slight motion, called a nutation, that invisible wobble is how the gyroscope turns at 90 degrees to the applied force, the wobble moves the gyroscope imperceptibly every cycle; and the result is precession.

Similarly, that slight wobble in the electric field IS the magnetic field; and that's why there's those funny cross products in the equations relating electric fields and magnetic fields in Maxwell's equations.

Anyway, that's how I understand it, others will understand it a different way, but to the best of my knowledge, that's what's really going on at bottom, and why it's called electromagnetism, and why Maxwell's equations has all those funky cross products in it.

 

[davenn]

The standard answer taught to undergraduates is that yes, you can have a magnetic field without an electric field.

However, my understanding of the equations and what they really mean is that that's not strictly correct.

Although you can have a magnetic field without there being any average electric field; you have to consider what a magnetic field really is.

A magnetic field is not actually a separate field to the electric field; they are one thing, electromagnetism.

So how does that work?

Well, when you have (say) electrons moving along a wire, they mostly cancel out the positive charges in the positive wire.

But, there's a slight wobble in the electric field due to those charges moving; and the more charges that are moving, they on average cancel out over time, but that wobble does NOT cancel out! In fact the more charges there are, the more overall wobble there is. You would think it would be irrelevant, but the electric charge force is so very strong, so even microscopic wobbles in it can have a very big effect when summed up......

.

Yes, the moving current causing an EM field has already been well discussed and understood.

But you didn't address the case of the permanent magnet as commented on at the beginning of the thread


Dave

 

[Orodruin]

Yes. In natural units E²-B² is an invariant. So if it is positive then there exists some reference frame where B=0 and E is minimum, and if it is negative then there exists some reference frame where E=0 and B is minimum.

No, this is not true. While $E^2 - B^2$ is an invariant, so is $\vec E \cdot \vec B$. If this second invariant is non-zero, then both fields must be non-zero in all frames. Not even when the second invariant is zero can you guarantee the existence of a frame where either electric or magnetic field is zero, just think of a plane wave in vacuum. The fields are orthogonal and of the same magnitude (i.e., both invariants are zero!).

 

[Jabbu]

But you didn't address the case of the permanent magnet as commented on at the beginning of the thread

Permanent magnet is not without electric fields, they just cancel out.

 

[WhatIsGravity]

If you attach the leads of a voltmeter to the poles of a bar magnet, and then introduce something that 'feels' that magnetic field, you'll see a voltage change- thus a current, or electric field. But didn't someone discover a magnetic monopole a bit ago?

 

[Jabbu]

In fact the more charges there are, the more overall wobble there is.

It's more rotation than wobble. Direction of this rotation for negative charge is always clockwise relative to movement direction, for positive charge it's in anticlockwise direction. So the more charges the more "wobble", but what does their speed have to do with anything and what is it relative to?

 

[wolfekeeper]

But you didn't address the case of the permanent magnet as commented on at the beginning of the thread

Dave

It's the same thing. It's just electric current. Each atom is like a little mini electromagnet.

The unpaired electrons in the orbitals around an iron atom are stuck; they're just endlessly going around in lossless loops, forming an electromagnet. It's very like a superconducting loop, but even more so. In a macroscopic superconducting loop the current will go down over a long timescale (weeks). But with an atomic current, the energy is fixed, and nothing much short of ionising the atom can change that.

It's more rotation than wobble. Direction of this rotation for negative charge is always clockwise relative to movement direction, for positive charge it's in anticlockwise direction. So the more charges the more "wobble", but what does their speed have to do with anything and what is it relative to?

Because of the propagation delay from when you change an electric charge it turns out that that reference frame doesn't matter; any reference frame gives you the same answer, and it's the relative movement that matters for what wobbles the charges see.

If you allow for the propagation delay and what that does to the shape of objects when they move, you end up with relativity and constancy of the speed of light. Provided the propagation delay is fixed in any reference frame, it's effectively fixed in all reference frames.

The faster the charges move, the more wobbles, and the quicker the wobbles happen.

And note, I'm not saying that the wobbles create the magnetic field; I'm saying that the electric field wobbles ARE the magnetic field; I think that this is slightly different from what is normally claimed in undergraduate texts, it may be even be wrong, but that's how I think about it, and I believe it to be true.

 

[Dublious]

I do not believe ferromagnetic magnets have electric fields. It is the alignment of single electrons in their orbits that create the effect of magnetism and since it generates a constant magnetic field, no electric field is created. Again, a lot of this depends on the frame of reference that you view things through though...

 

[wolfekeeper]

Because the electrons and the nucleus are not laid out in their orbitals as concentric, spherical charges, there is going to be a dipole/quadrupole/higher order electric field anyway in addition to the magnetic one; the field is oriented by the chemical bonds between neighbouring atoms.

 

[thegreenlaser]

A lot of points made in this thread can be easily seen by looking at Maxwell's equations with $\vec{E}$ set to zero:

$$ \frac{\partial \vec{B}}{\partial t} = 0 $$
$$ \vec{\nabla} \times \vec{B} = \frac{\vec{J}}{\mu_0} $$
$$ \vec{\nabla} \cdot \vec{B} = 0 $$
$$ \rho = 0 $$

A few things to note when there is no electric field:
1. Gauss's law says the charge density is zero everywhere.
2. The magnetic field must be static.
3. The current density cannot be zero if we want the magnetic field to be non-zero.

So, if we want a magnetic field without an electric field, we need to have current without charge. With continuous charge distributions, that's possible. For example, current in a wire can be approximated as a stationary mass of positive charge and a moving mass of negative charge. The positive and negative charges cancel out to make the net charge density zero, but the motion of the negative charge constitutes a current.

However, if all you have is discrete charges which can't occupy the same location (i.e., reality at the microscopic scale), then it's impossible to have $\rho=0$ with $\vec{J} \neq 0$. At a macroscopic scale, you can probably make the electric field effectively zero, but if you "zoom in" and look at the fields created by each individual electron and proton, Maxwell's equations say that the electric field can't be zero.

 

[Orodruin]

However, if all you have is discrete charges which can't occupy the same location (i.e., reality at the microscopic scale), then it's impossible to have $\rho=0$ with $\vec{J} \neq 0$. At a macroscopic scale, you can probably make the electric field effectively zero, but if you "zoom in" and look at the fields created by each individual electron and proton, Maxwell's equations say that the electric field can't be zero.

If you zoom in enough your discrete charges will start behaving as quantum objects. I imagine some composite neutral particles in their ground state would tend to give no electric field at the classical level. The problem with those would be that they are not stable. A Dirac neutrino would also have a non-zero magnetic dipole moment proportional to its mass, resulting in a magnetic field without an electric one. (Again at the classical level. On a quantum level this would result from propagator corrections involving charged particles.)

 

[Dale]

Classically you can certainly have an electric field without a magnetic field or a magnetic field without an electric field. On the quantum level, since all fundamental charged particles also have spin you cannot have either without the other.

In either case the answer to the question is the same for the magnetic field as it is for the electric field, but the answer differs between classical and quantum EM.

 

[Orodruin]

Classically you can certainly have an electric field without a magnetic field or a magnetic field without an electric field. On the quantum level, since all fundamental charged particles also have spin you cannot have either without the other.

It is possible also for neutral particles to have a non-zero magnetic moment. Of course, if you are moving relative to such a particle an electric field still appears ...

Does it really make sense to talk about electric and magnetic fields on the quantum level other than as a semi-classical approximation?

 

[Jabbu]

It is possible also for neutral particles to have a non-zero magnetic moment. Of course, if you are moving relative to such a particle an electric field still appears ...

I don't think we would call them neutrons if they could acquire electric fields every time something is moving somewhere.

 

[Orodruin]

Just as moving charges give you magnetic fields, anything giving rise to a magnetic field that moves is going to give you an electric one. This is due to how the electromagnetic field transforms under Lorentz transformations. The divergence of the electric field from the moving magnetic source would still be zero and so also the charge density.

 

[Thierry]

Under static conditions, either can exist w/o the other. A superconducting loop w/ a steady dc current has magnetic field, but no electric field. The counterpart of this setup is a charged capacitor open circuited, as there is an electric field w/o a magnetic field.

Again these are static conditions, i.e. not changing with respect to time. Under dynamic conditions, i.e. changing wrt time, neither can exist w/o the other. If one field is time-changing, the other must be non-zero.

Claude

What about transformer cores? the magnetic field oscillates at 50 hz but there's no electric field in a metal. There's eddy currents but no electric field, or am I missing something?