EM field and wave interactions of a point charge

Click For Summary

Discussion Overview

The discussion revolves around the interactions of electromagnetic (EM) fields and waves produced by point charges, particularly focusing on the scenario of two point charges in static equilibrium and the effects of moving one charge. Participants explore the implications of such movements on the EM field and the resulting wave propagation, as well as the relationship between energy, frequency, and the Planck constant in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes a scenario where moving a point charge alters the EM field experienced by another charge, suggesting that this change propagates at the speed of light and constitutes an EM wave.
  • The same participant proposes calculating the frequency of the emitted light based on the work done on the second charge, questioning if it could be equated to the energy of light divided by Planck's constant.
  • Another participant argues that the nature of the light emitted depends on the motion of the charges, stating that aperiodic motion would lead to a complex emission pattern that cannot be assigned a single frequency.
  • This participant notes that frequency is typically associated with periodic waves and suggests that periodic motion of charges is necessary to produce a coherent wave, such as in synchrotrons or oscillating antennas.
  • A later reply acknowledges the previous participant's insights and emphasizes the importance of understanding the classical theory of light before delving into quantum theory.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the light emitted from the charges, particularly regarding the conditions under which frequency can be assigned. There is no consensus on the implications of the proposed scenario or the calculations suggested.

Contextual Notes

The discussion highlights limitations in understanding the relationship between charge motion and EM wave characteristics, particularly regarding the assumptions about periodicity and the complexity of aperiodic emissions.

mishima
Messages
576
Reaction score
43
I've been thinking of 2 point charges separated by some distance in static equilibrium. When one charge is moved from rest, the EM field would change the way it looks at the location of the other point charge. This "changing in the looks" of the EM field as I understand propagates from the first charge at the speed of light and constitutes an EM wave. This is easy to see using a computer simulation that allows dragging charges around. When the change in the EM field reaches the other point charge it accelerates, trying to restore equilibrium.

So, I know about cases where an excited atom returns to its ground state and how to calculate the frequency and wavelength of the emitted photon. I'm wondering how I might do that with the somewhat unreal situation above. Could I just consider the work done on the second charge to be equal to the light's energy, then divide by Planck's constant to get frequency (if both charges had charge e)? Thanks.
 
Physics news on Phys.org
The nature of the light will depend on how the charges move. In such general situation as you described, the motion would be most probably aperiodic, so the light will move in all directions in a complicated way. One cannot assign frequency to it in such situation (one can resolve it into Fourier components, but the result would be complicated as well.)

Frequency is a quantity which refers to periodic wave, like sine wave. In order to get such wave, the charges have to move periodically - in circles in synchrotron, or around each other, like in excited state of hydrogen atom, or oscillate rectilinearly as in antenna.
 
Thanks, I never thought of it that way.
 
No problem. From your post, it seems you learned bit about quanta. The Planck constant and the connection between the energy and frequency is important, but rather subtle and not entirely clear, so I recommend to learn bit about the classical theory of light (wave optics/electrodynamics ) which is quite clear and natural, and only then move on to learn about the quantum theory.
 

Similar threads

High School Does the Electric Field Change When One Charge Moves Relative to Another?
  • · Replies 7 ·
Replies
7
Views
2K
Graduate EM wave generation using a single charge in a vaccum?
  • · Replies 4 ·
Replies
4
Views
4K
Graduate EM waves, longitudinal EM propagation?
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Mechanism of Energy Conservation in Zero-Amplitude Sum of EM Waveforms
  • · Replies 21 ·
Replies
21
Views
3K
Graduate Voltage and current waves in a transmission line
  • · Replies 4 ·
Replies
4
Views
3K
Graduate Charge at constant velocity emitting EM waves?
  • · Replies 12 ·
Replies
12
Views
3K
Undergrad Energy emitted by EM sources under constructive interference
  • · Replies 7 ·
Replies
7
Views
2K
Graduate From Maxwell's equations to EM waves
  • · Replies 9 ·
Replies
9
Views
9K
Graduate Correction to the field energy due to the existence of discrete charges
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Electromagnetic waves/radiation properties?
  • · Replies 8 ·
Replies
8
Views
2K