Clarification on the Poynting theorem

  • Context: Graduate 
  • Thread starter Thread starter fisico30
  • Start date Start date
  • Tags Tags
    Theorem
Click For Summary

Discussion Overview

The discussion centers on the Poynting theorem and its implications for energy balance in electromagnetic fields. Participants explore the conditions under which electromagnetic energy changes within a specified volume, including the roles of currents, charges, and energy transformations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant outlines that the energy in a volume can change due to net flux through the bounding surface, energy dissipation by currents and charges, and the introduction of new energy from impressed sources.
  • Another participant questions whether lost energy is related to the charge density interacting with other field conditions.
  • A participant seeks clarification on the type of work done by the electromagnetic field on charges and currents, suggesting that it provides kinetic energy to charges and inquiring about the behavior of currents.
  • There is a mention that moving charges and currents emit electromagnetic radiation, which may not return all absorbed energy back to the field, indicating a potential energy loss.
  • It is noted that the electric field in the Poynting vector does work on currents rather than charges directly, with the nature of this work varying based on the context, such as in resistive elements where it results in heat.

Areas of Agreement / Disagreement

Participants express various interpretations of the Poynting theorem and its implications, indicating that multiple competing views remain without a consensus on specific aspects of energy transformation and the roles of currents and charges.

Contextual Notes

Participants have not fully resolved the implications of charge density in the context of energy loss or the specifics of how currents behave under the influence of the electromagnetic field. There are also unresolved questions regarding the complete energy dynamics involved in the processes discussed.

fisico30
Messages
362
Reaction score
0
hello forum,

the famous Poynting theorem is an expression for the energy balance. Please, could anyone make sure I have the right understanding ?

The EM energy in a volume V with bounding surface S can change with time if:

1) energy leaves or enter that volume through S (net flux).

2) if the energy in the volume is used up by the free and induced currents and charges and then transformed into heat. Those currents and charges then behave as sinks.

3)If there are impressed sources (currents and charges) that generate new energy.

Regarding 2): the free and induced currents and charges could not only dissipate EM energy but also re-emit some of the absorbed energy once set into motion... so they would act as both sources and sinks.

In free space, there can only be free sources (if any) in the volume.
The dot product of E and J represents the lost power. Where is the charge density gone?

thanks
fisico30
 
Physics news on Phys.org
Fisico. Are you asking if there is lost energy due to the charge density acted upon some some other field conditions?
 
Hi Pharak:

my questions are:

1) The EM field does work on the charges and currents that are present in the volume and loses some of its energy. What type of work are talking about? I guess the EM field energy causes motion of the charges (it gives them Kinetic energy). What happens to the currents? Do they move, speed up?...

2) The same currents and charges, when moved, emit EM radiation too, that gets returned to the field. Probably some of the energy that they absorb is turn into heat, so not all the energy they receive is return...

Is this correct?
thanks
 
fisico30 said:
Hi Pharak:

my questions are:

1) The EM field does work on the charges and currents that are present in the volume and loses some of its energy. What type of work are talking about? I guess the EM field energy causes motion of the charges (it gives them Kinetic energy). What happens to the currents? Do they move, speed up?...

The electric field expressed in the Poynting vector does work on current, not charge per se. What sort of work depends. The typical, first example is a resistor, where the work done results in heat.
 

Similar threads

Undergrad Does Poynting's Theorem only involve external fields?
  • · Replies 1 ·
Replies
1
Views
1K
Graduate Understanding Poynting's Theorem intuitively
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Jackson: justification of the Poynting vector by GR
  • · Replies 38 ·
2
Replies
38
Views
3K
Poynting theorem and electromagnetic density
  • · Replies 7 ·
Replies
7
Views
2K
Graduate Understanding Poynting Theorem
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Poynting energy flow in Eath's atmosphere?
  • · Replies 5 ·
Replies
5
Views
3K
Graduate Interpretation and application of Poynting's theorem?
  • · Replies 59 ·
2
Replies
59
Views
17K
Graduate Poynting Theorem: Why is 1st Term Ignored?
  • · Replies 2 ·
Replies
2
Views
3K
Undergrad Boundary conditions and discontinuity of EM fields
  • · Replies 22 ·
Replies
22
Views
3K
Graduate Understanding the Coaxial Inductor: A Study of its Electric and Magnetic Fields
  • · Replies 1 ·
Replies
1
Views
2K