Self-propagating wave question

[Lucretius]

I'm currently reading about EM waves in my physics course, but there is something that is not clear to me.

I know, from Maxwell's equations, that a changing electric field produces a magnetic field and a changing magnetic field likewise produces an electric field. However, it seemed from my reading that the strengths of these fields were constant.

Now, my book is talking about the self-propagation of EM waves, and from the sound of it; it would seem as if a changing electric field produces a changing magnetic field, which in turn produces a changing electric field, and so on; propagating forever.

So, there are two things I can think of: either I am misunderstanding Maxwell's equations; or there is some other factor I am missing here in understanding what is going on. Does a changing E-field produce a changing B-field or one with a constant strength?

 

[Chris Hillman]

The former, of course. There is an excellent discussion in Feynman's Lectures on Physics which should be perfect for you.

 

[sir-pinski]

Great question! It's important to understand that in the context of EM waves, the term "constant" refers to the amplitude or strength of the fields, not their values. In other words, the fields are constantly oscillating back and forth, but the amplitude remains the same. This is because EM waves are self-sustaining, meaning they do not require any external source to maintain their energy.

To answer your question, a changing electric field does indeed produce a changing magnetic field, and vice versa. This is known as the self-propagation of EM waves. As the electric field changes, it creates a magnetic field, which in turn creates a new electric field, and so on. This process continues indefinitely, allowing the EM wave to propagate through space.

It's also important to note that the strength of the fields is not constant throughout the entire wave. As the wave travels, the fields will decrease in strength due to factors such as distance and absorption. However, the overall pattern of changing electric and magnetic fields continues, allowing the wave to propagate.

I hope this helps clarify your understanding of EM waves and their self-propagation. Keep up the good work in your physics course!