# Electric and Magnetic Fields, Photons, Charges

• I
I know that magnetic fields create moving charges (an electric current) normal to the plane of the magnetic force lines. I also have heard that magnetic and electric fields create each other in a perpendicular direction to the other (badly worded). Electric currents are moving charges (usually electrons), but electric fields are propagated through lines of force, not in one direction. How are these equivalent? Also, if there are no charges in the area of the magnetic field (it is emitted from a ferromagnetic substance), then how does the magnetic field create moving charges?

I know that oscillating magnetic and electric fields is basically what EM radiation is. However, I cannot visualize light (a transverse wave with a finite amplitude, frequency and wavelength which travels in a single direction) as two fields that turn into each other rapidly. If that were true, then EM waves would be projected in all directions in a sphere from the source. If a photon is equivalent to a standing wave probability function, and photons move in a single direction, how does this make sense (as these two things are equivalent, but seem different)?

## Answers and Replies

Drakkith
Staff Emeritus
Science Advisor
I know that magnetic fields create moving charges (an electric current) normal to the plane of the magnetic force lines.

A changing magnetic field creates an electric field, not an electric current.

Electric currents are moving charges (usually electrons), but electric fields are propagated through lines of force, not in one direction. How are these equivalent?

They are not. An electric charge is a piece of matter with the property that it generates/affects the EM field around it. An electric field is the electric portion of the EM field.

I know that oscillating magnetic and electric fields is basically what EM radiation is. However, I cannot visualize light (a transverse wave with a finite amplitude, frequency and wavelength which travels in a single direction) as two fields that turn into each other rapidly. If that were true, then EM waves would be projected in all directions in a sphere from the source.

This isn't true, but unfortunately I lack the knowledge to really explain why. In any case, neither the electric nor magnetic portion of the EM radiation (EM wave) turns into the other. If that were true then the two portions would be 90 degrees out of phase with one another, such that when one is at maximum amplitude the other is at minimum. This is not the case. Both portions are in phase with one another and reach maximum and minimum amplitude at the same time.

If a photon is equivalent to a standing wave probability function, and photons move in a single direction, how does this make sense (as these two things are equivalent, but seem different)?

Honestly I wouldn't even begin to try to understand the details of how photons work until you get a better understanding of classical EM theory first. The idea that you can even say which path a photon took is problematic in quantum theory (problematic in that you can't!).

• Surya97
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
but electric fields are propagated through lines of force, not in one direction.
This statement is nonsense. Electric and magnetic fields satisfy Maxwell's equations.

Also, if there are no charges in the area of the magnetic field (it is emitted from a ferromagnetic substance), then how does the magnetic field create moving charges?
It does not. Currents are necessary to create a magnetic field, but it is notgoing to be only where the currents are. Again, this is described by Maxwell's equations.

If that were true, then EM waves would be projected in all directions in a sphere from the source.
What makes you think this cannot be the case? Do you think the Sun only shines towards Earth?

However, it is not necessarily the case depending on how the light is created and focused.

If a photon is equivalent to a standing wave probability function, and photons move in a single direction, how does this make sense (as these two things are equivalent, but seem different)?
You really should not think in terms of photons here. They are among the absolutely most difficult particles to quantise. To put it short: no, photons are not waves of probability. No, photons do not move in a single direction. No, it does not make sense because you are using a mixture of popularised physics and trying to put pieces of it together. You will not learn physics in this way. You will learn physics by studying physics for real.

• Surya97 and vanhees71
vanhees71
Science Advisor
Gold Member
I know that magnetic fields create moving charges (an electric current) normal to the plane of the magnetic force lines. I also have heard that magnetic and electric fields create each other in a perpendicular direction to the other (badly worded). Electric currents are moving charges (usually electrons), but electric fields are propagated through lines of force, not in one direction. How are these equivalent? Also, if there are no charges in the area of the magnetic field (it is emitted from a ferromagnetic substance), then how does the magnetic field create moving charges?

I know that oscillating magnetic and electric fields is basically what EM radiation is. However, I cannot visualize light (a transverse wave with a finite amplitude, frequency and wavelength which travels in a single direction) as two fields that turn into each other rapidly. If that were true, then EM waves would be projected in all directions in a sphere from the source. If a photon is equivalent to a standing wave probability function, and photons move in a single direction, how does this make sense (as these two things are equivalent, but seem different)?
First of all there are not two fields but only one, the electromagnetic field. The split in electric and magnetic components is dependent on the frame of reference and chosen just for convenience of the calculation.

Then, do not even try to think about photons before you haven't understood the classical theory. Photons cannot be visualized as massless little billard balls but only described using relativistic quantum field theory, which tells you that there is not even a position operator in the literal sense for a single-photon state.