# Electromagnetic Waves

## Homework Statement

Hi there! I am currently taking physics and we are in the process of learning about electromagnetic radiation. I am a bit confused as to how exactly an electromagnetic wave is produced. I understand that a periodically oscillating charge produces and oscillating magnetic field which in turn produces an oscillating electric field (so on and so forth) but what I don't understand is why a changing electric field produces a changing magnetic field or vice-versa. If, for example, a charge was moving up and down at some frequency, would the electric field be changing because the charge is changing position or because the change in position of the charge results in a changing magnetic field which in turn causes a changing electric field? Thank you in advance! Please note that we are using algebra and not calculus in our course so if your explanation requires mathematics, please use algebra. Thanks again!

## The Attempt at a Solution

marcusl
Gold Member
An oscillating charge produces both electric and magnetic fields. The phenomenological "reason" is that a charge possesses an electric field, and also generates a magnetic field when it moves. (Magnetic fields are generated by currents, which are moving charges.)

A deeper explanation of the relation between electric and magnetic fields is that they are different manifestations of the same thing, as related to each other through special relativity. That is, relativity predicts what an electric field looks like in one reference frame (the laboratory) from a charge in another moving frame. It looks odd, so we call it a magnetic field.

An oscillating charge produces both electric and magnetic fields. The phenomenological "reason" is that a charge possesses an electric field, and also generates a magnetic field when it moves. (Magnetic fields are generated by currents, which are moving charges.)

A deeper explanation of the relation between electric and magnetic fields is that they are different manifestations of the same thing, as related to each other through special relativity. That is, relativity predicts what an electric field looks like in one reference frame (the laboratory) from a charge in another moving frame. It looks odd, so we call it a magnetic field.

So, as a light waves moves through a vacuum, the changing magnetic field creates a changing electric field and the process continues, correct? Also(a bit off topic), what is it about a magnet (a bar magnet for example) that causes a continuously produced magnetic field? Are charges moving within the material the bar magnet is composed of? Thank you very much for your answer!

Sorry, one more thing. Since electromagnetic radiation is composed of changing electric and magnetic fields, does that mean that EMR (light) causes an electric and magnetic force on charged particles/objects? Thank you!

Last edited:
marcusl
Gold Member
Well, whether one "creates" the other or not, changing electric and magnetic fields go together. (In the example I gave, both are generated by the moving charge.) Propagating waves (light, radio waves, etc.) are electromagnetic and possess both fields.

You are correct, a permanent magnet possesses microscopic currents. It arises from the "spin" of the electrons in the iron and other metals that make up the magnet.

Correct again, EM waves produce forces on charges. The force is described by the Lorentz equation (it involves calculus but this site has a reasonable description
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html#c2). A simple example is a mirror. Incident waves accelerate the charges (electrons) in the metal, which in turn radiate waves back. You see your reflection as a result. In this case it is the electric field that interacts with the electrons.

Well, whether one "creates" the other or not, changing electric and magnetic fields go together. (In the example I gave, both are generated by the moving charge.) Propagating waves (light, radio waves, etc.) are electromagnetic and possess both fields.

You are correct, a permanent magnet possesses microscopic currents. It arises from the "spin" of the electrons in the iron and other metals that make up the magnet.

Correct again, EM waves produce forces on charges. The force is described by the Lorentz equation (it involves calculus but this site has a reasonable description
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html#c2). A simple example is a mirror. Incident waves accelerate the charges (electrons) in the metal, which in turn radiate waves back. You see your reflection as a result. In this case it is the electric field that interacts with the electrons.

Thank you so much for your response. Yes, after writing my previous question(s), I started researching a little more on what electric and magnetic fields really were and I came across the Lorentz transformation which dealt with time dilation and length compression. Basically, from what I understand, magnetic fields are manifestations of our relativistic view of electric fields. Apparently, as a charge (an electron for example) moves through space and gets closer to the speed of light, the electric field lines at the "front" (relative to the direction of motion) are compressed due to length compression and become perpendicular to the direction of motion. This, from what I understand, is what a magnetic field is. Am I correct in saying any of this or am I missing some concepts? Thanks again! Your help is greatly appreciated! By the way, did you go to school for physics? You seem to have a lot of knowledge on the subject and I was just curious since I am deciding between studying for a degree in engineering or physics.

marcusl
Gold Member
No, that's not a magnetic field. The magnetic field arises from the Lorentz contraction of the electron and crystal ion charge distributions in a current carrying wire, as seen by a moving charge. The compressed field lines you refer to are the electric field of a moving charge as seen by a stationary observer.

Treatments of the relativistic origins of magnetism are found in undergrad college texts:

Purcell, Electricity and Magnetism, vol2 of the Berkeley Physics Series
Schwartz, Principles of Electrodynamics
I'm not familiar with Griffiths' E&M book but I hear that it also covers this

Finally, yes, I did study physics in school.

No, that's not a magnetic field. The magnetic field arises from the Lorentz contraction of the electron and crystal ion charge distributions in a current carrying wire, as seen by a moving charge. The compressed field lines you refer to are the electric field of a moving charge as seen by a stationary observer.

Treatments of the relativistic origins of magnetism are found in undergrad college texts:

Purcell, Electricity and Magnetism, vol2 of the Berkeley Physics Series
Schwartz, Principles of Electrodynamics
I'm not familiar with Griffiths' E&M book but I hear that it also covers this

Finally, yes, I did study physics in school.

Thanks for the reply! Unfortunately, I am a bit more confused now. I understand what you mean by charge distribution in a current carrying wire but what is a crystal ion and what if we had a charged particle flowing through space? There would be no charge distribution since an electron with charge 1.6*10^-19C is the lowest charge a body can possess. So what causes the magnetic field when it is just one particle moving through space. According to the below thread, it is the fact that magnetic fields are just manifestations of electric fields that causes us to even define the concept of a magnetic field. Also, what exactly does charge distribution have to do with magnetic fields? Thanks again!

Note* Thanks for the references to the physics texts. I checked them out and they seem great (even though they are Calculus based, Calculus is just another field of mathematics waiting for me to absorb the information it has to offer!).

Last edited:
marcusl
Gold Member
Thanks for the reply! Unfortunately, I am a bit more confused now. I understand what you mean by charge distribution in a current carrying wire but what is a crystal ion
I will refer you to the books mentioned earlier since the demonstration is too long to reproduce here.

The reason for considering a moving test charge in the vicinity of a current-carrying wire is that it is a "pure" demonstration of the magnetic force. The wire has zero net electrical charge in the lab frame, so in the absence of relativistic effects there would be no force on the test charge whatsoever. The counter-intuitive existence of the magnetic force is most clearly and dramatically seen in this example.

The case of two moving point charges involves instead a mixture of electric and magnetic forces. However the magnetic force is miniscule compared to the electric force, so it's a less obvious demonstration.
and what if we had a charged particle flowing through space? There would be no charge distribution since an electron with charge 1.6*10^-19C is the lowest charge a body can possess. So what causes the magnetic field when it is just one particle moving through space.
There is indeed only the electric force. In a subtle distinction, what we call the magnetic field can (classically) only be detected by its effect on a moving test charge, so one must use a second moving charge to characterize the "magnetic" force from a first moving charge.

I will refer you to the books mentioned earlier since the demonstration is too long to reproduce here.

The reason for considering a moving test charge in the vicinity of a current-carrying wire is that it is a "pure" demonstration of the magnetic force. The wire has zero net electrical charge in the lab frame, so in the absence of relativistic effects there would be no force on the test charge whatsoever. The counter-intuitive existence of the magnetic force is most clearly and dramatically seen in this example.

The case of two moving point charges involves instead a mixture of electric and magnetic forces. However the magnetic force is miniscule compared to the electric force, so it's a less obvious demonstration.

There is indeed only the electric force. In a subtle distinction, what we call the magnetic field can (classically) only be detected by its effect on a moving test charge, so one must use a second moving charge to characterize the "magnetic" force from a first moving charge.