# EM propagation after leaving a wire having an alternating current

Initially, the changing electric field generates a magnetic field around a wire having an alternating current. The electric field and magnetic field travel outward from the wire at the speed on light.

-After leaving the wire, do the fields actually self generate each other in a continuously cycle?

-Cannot the fields remain fixed and travel together as a unit after leaving the wire?

-Is there an experimental observation that shows the fields are actually self generating and not traveling as an unchanging unit?

Edit:

-If I understand correctly from Maxwell's equations, a time varying electric field generating a magnetic field has not been experimentally determined (in capacitors, I do not know about electromagnetic radiation). A time varying magnetic field generating an induced electric field can be observed with a solenoid.

-I am not aware if the induced electric field has been experimentally shown to be able to produce a magnetic field in empty space. An induced electric field can drive a current in a conductor and the current charges in turn generate a magnetic field.

-An unqualified observation can be: The electric field from charges can generate a magnetic field. An induced electric field cannot generate a magnetic field.

-The key question is: in EM radiation, is a time varying electric field generating a magnetic field based upon the prediction of Maxwell's equation, but not on experimental observation?

-Could the electric field component of the EM radiation be obtained from source charges and the magnetic field component from the time varying electric field of those source charges and both fields move as an unchanging unit?

• The field pattern moves radially away from the wire. For a simple wave (plane, spherical), which is a good model if we observe field far enough from the wire, maximum of electric field is always at the same point of space as maximum of magnetic field, so in a sense they move together. – Ján Lalinský May 6 '19 at 19:03
• I have no idea what this even means: “ Is there an experimental observation that shows the fields are actually self generating and not traveling as an unchanging unit?” What would constitute such an observation? – Dale Dec 23 '19 at 18:26

The propagation itself consists of electric field being generated by alternating magnetic field and magnetic field being generated by alternating electric field. This is a result stemming from Maxwell equations governing electromagnetic fields. There is no such thing as a solely self propagated electric disturbance or solely self propagated magnetic disturbance.

It might be useful for you to look at the Lienard/Wiechert equations and their derived electric and magnetic formulas.

For two charges, this describes the force presented by one charge on the other, after a delay.

The magnetic force is special. It is only present in frames where both charges are moving. If the source charge is not moving, we say there's no force. If the target charge is not moving, we say the magnetic force does not affect it.

In this context, magnetic force is entirely an artifact of frames. And not just any frames.

When the source charge is moving directly toward or away from the target, there is no magnetism.

When you make a plane composed of the direction to the target and the source charge's velocity, and the target is moving perpendicular to that plane, there is no magnetism.

In all those frames, you get the same amount of force. But when the source is moving sideways relative to the direction to the target, and the target is moving on that same plane, then the magnetic correction is needed.

In that case, and only in that one case, the formulas don't add up correctly and we get a different answer unless we apply a correction factor of size $$(1-v_{target}/c)$$ applied perpendicular to the velocity of the target, in the same plane as everything else.

We have a description of the amount of electromagnetic force applied to a target charge from a source charge, after a time delay. The direction and amount of force depend only on:

the direction from source at sending time to target at receiving time, the distance from source at sending time to target at receiving time, velocity and acceleration of source at sending time, and velocity of target at receiving time.

And those depend on the frame you choose.

In vacuum, the force depends on the source and the target, and nothing else. We don't need to imagine parts of the force affecting other parts of the force during transit. Just knowing the charge positions, velocities, acceleration, and frame is enough.

• Thank you for your answer. My question needs improvement. – MarkJanus1 Dec 29 '19 at 7:44