# Do electromagnetic waves have potential and kinetic energy

arlesterc
Do electromagnetic waves have potential and kinetic energy like springs, strings, etc. If so how are they calculated, inter-related? What is the total energy? Are the energies fluctuating over time?

I am a physics hobbyist so generally the first answers should come with the least mathematics as possible - high level - and then I delve further/dig into the math if I need to.

• Dale

## Answers and Replies

Gold Member
Do electromagnetic waves have potential and kinetic energy like springs, strings, etc. If so how are they calculated, inter-related? What is the total energy? Are the energies fluctuating over time?

I am a physics hobbyist so generally the first answers should come with the least mathematics as possible - high level - and then I delve further/dig into the math if I need to.

There is a sort of analogy between the E field which we associate with Potential Energy and the H field, which is associated with motion. But the Kinetic Energy of electrons is not relevant to current in a wire so the analogy is a bit dodgy. The E and H fields are in phase in an EM wave but the Potential Energy (pressure) fluctuations in a sound wave and the Kinetic Energy (air flow) are out of phase. So use any analogy with care and 'in private'.

arlesterc
sophiecentaur:
Thanks for quick reply. However I do not know what E and H are that you refer to?

CWatters: Thanks for the link but I see nothing in there about potential and kinetic energy of an electromagnetic wave. I saw a video about a string wave on Youtube and with the string I understood the relationship between kinetic and potential as the string was 'waved' - each point of the string went up and down as the wave waved and where the point of the string was represented some combination of potential and kinetic energy. I don't have that understanding with electromagnetic waves. Is there a calculation for what the potential energy of an electromagnetic wave is? The kinetic energy? I believe the electromagnetic wave can also be viewed as a particle - photon. So as a charged particle moves photons are created that move away from the charged particle. So do these photons have potential and kinetic energy? It would seem to me that they have maximum potential energy at the start but the potential would change to kinetic as time went on. Or is it vice versa? Also as a photon always travels at the speed of light what does it mean for kinetic energy of the photon?

Or is there no real potential and kinetic when it comes to electromagnetism as there is with uniform fields like gravity or electricity - like between two charged plates? It's a concept that is not used, has no meaning?

I appreciate any further attention that either of you can provide.

Gold Member
Thanks for quick reply. However I do not know what E and H are that you refer to?
Sorry, I was sort of expecting you to be familiar with that, in the light of your OP. E is Electric Field and H is Magnetic Field. The two fields are oscillating about a mean value as the wave progresses. Google is your friend in these matters.

Mentor
I believe the electromagnetic wave can also be viewed as a particle - photon. So as a charged particle moves photons are created that move away from the charged particle.
It's best not to introduce photons into this discussion at all. First, they're only needed to explain the behavior of light in situations where quantum mechanical effects matter, and this isn't one. Second, they aren't anything like what you're thinking they are, because quantum mechanics uses the word "particle" to mean something completely different than the ordinary English meaning of the word suggests.

Without photons, light is classical electromagnetic waves. Electromagnetic fields store energy. This energy can move around as the fields change over time, so electromagnetic waves (which are a particular way for the electrical and magnetic fields to be changing) can carry electromagnetic energy from one place to another.

• nasu and sophiecentaur