3
$\begingroup$

An electromagnetic wave is usually illustrated with two sine wave shaped fields, one for the electric field and one for the magnetic field. As in the picture below:

electromagnetic wave

But as how I understand magnetism, it consists of two poles, north and south (alternatively described with field lines, with a direction, where north and south connects). So to focus only on the magnetic field (the red, B) in an electromagnetic wave; where are the poles located in such a sine wave (the red part)? Is only one of the poles illustrated, such that another sine wave could be added for the other pole?

Update: In this video, Maxwell's second equation is described as the field lines in the magnetic field is always connected (north and south pole) - how these field lines are organized in a wave is what I don't understand.

Later in the same video, the fields emitted from an antenna, are illustrated as "bubbles of fields" - quite different from how the "arrows/vectors" are illustrated in my picture above.

Field lines from the video: field lines of wave

$\endgroup$
3

2 Answers 2

4
$\begingroup$

Magnetic fields do not only arise from magnetic poles. For example, they can also be created by electric currents, and the resulting magnetic field lines circle around the wire, without having an "end" at a magnetic pole. See the diagram below for an illustration.

enter image description here

Image from HyperPhysics

Alternately, it is possible to have a changing electric field and magnetic field without any magnetic poles (or electric charges) nearby at all. This is what happens in an electromagnetic wave. Note also that the arrows in your diagram are representatives of the magnetic (and electric) fields at points along the $z$-axis; similar fields exist at all points in a given plane perpendicular to the axis of propagation. (See the diagram below for an illustration of this.) So if you want to think of "field lines", they extend in long horizontal lines from infinity to infinity parallel to the $y$-axis in your diagram.

enter image description here

$\endgroup$
6
  • 1
    $\begingroup$ Thanks that explains it partly for me. So is the red arrows in my picture, what I should understand as "field lines" from origo straight out to nothing? Or is the field lines in an electromagnetic wave more organized as the shape of a sine wave? $\endgroup$
    – Jonas
    Jul 15, 2023 at 20:16
  • $\begingroup$ @Jonas: The field lines don't "start" at the $z$-axis in your diagram; it's just that the $z$-axis is where the field vectors are being drawn. I've edited my answer to clarify this. $\endgroup$ Jul 15, 2023 at 20:57
  • $\begingroup$ I still feel this is counter intuitive. In this video, Maxwell's second equation is presented as the magnetic field always has a north- and south-pole - and not go straight out in the air. youtu.be/K40lNL3KsJ4?t=140 $\endgroup$
    – Jonas
    Jul 17, 2023 at 15:01
  • $\begingroup$ Later in the same video, fields (unclear if electrical or magnetical) in waves are illustrated as circles (makes sense) - but not as vectors straight out in the air. youtu.be/K40lNL3KsJ4?t=351 $\endgroup$
    – Jonas
    Jul 17, 2023 at 15:05
  • 2
    $\begingroup$ "Magnetic fields do not only arise from magnetic poles" [emphasis added]. Actually, I would say that so far as we know (that is, within the standard model), magnetic fields lines never actually arise from magnetic poles! The poles are just a convenient mathematical fiction used when working with $\vec{H}$ instead of $\vec{B}$. But you knew that already. $\endgroup$
    – Buzz
    Jul 17, 2023 at 19:38
0
$\begingroup$

After some more googling and time spent watching different explanations, I think I have got a better understanding.

This one in 2D shows the electric field propagating to the right. Its distance and time on the x-axis. It also shows that the electric field, mostly are not sine waves (as they typically are visualized). The magnetic field is perpendicular to the electric field - but I think it is a bit strange in this illustration, since the electric field probable is a "plane" and not a "thin line". 2D electric field in a wave It is explained here that the electric field can have arbitrary shape, it all depends on how it was created. That the electric field is like a "sine wave" is said to be a misconception in that video. From No, Changing Electric Fields DON'T Cause Magnetic Fields; The Real Origin of Electromagnetic Waves.

That explanation for the shape of the electric field makes sense to me. Especially since an electric field is radiated anytime we e.g. power on a circuit board, as explained in Veritasium - How electricity actually works

But this was only in 2D, and it does not really explain the magnetic field, other than that it is perpendicular to the electric field.

But this picture from wikipedia says little more about the fields in 3D, when radiated from an antenna, especially the magentic field (the red, B). fields from antenna

This is actually not so different from how the fields are organized around a wire carrying electric current. As in this picture: fields from a current carrying wire.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.