Poynting Theorem: Why is 1st Term Ignored?

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In summary, when using the Poynting theorem to calculate power of current flowing down a wire, only the 2nd term is used and not the 1st term. The 1st term gives the total energy stored in the electromagnetic fields, while the 2nd term gives the energy flowing out of the fields. The 3rd term represents the work done per unit time on the charges. If the fields are time independent, the 1st term is not included. However, if there are changing fields, the 1st term must be included.
  • #1
quietrain
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why is it that when we use the poynting theorem to calculate power of

e.g, current flowing down a wire , which gives joule heating ,

we don't make use of the 1st term, but only the 2nd term is used?

910f60a9378cc62bee1e0bd603e62733.png


doesn't the first term give the total energy stored in the EM fields?

the 2nd term gives the energy flowing out of the EM fields

the 3rd term is the workdone per unit time on the charges right?
 
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  • #2
If you look at stationary states only, i.e., if the fields are time independent, the first term vanishes. Otherwise it has to be included.
 
  • #3
oh! so as long as i have E or B induction(changing) fields, i have to include the first term because its d/dt is not 0?

thanks!
 

FAQ: Poynting Theorem: Why is 1st Term Ignored?

What is Poynting Theorem?

Poynting Theorem is a fundamental concept in electromagnetism that describes the relationship between the electric and magnetic fields in a system and the flow of energy. It was first introduced by physicist John Henry Poynting in 1884.

What does the Poynting Theorem state?

The Poynting Theorem states that the energy transfer rate (or power) in an electromagnetic field is equal to the cross product of the electric and magnetic fields at a given point, multiplied by the speed of light. This theorem is essential in understanding the behavior of electromagnetic waves and their interactions with matter.

Why is the first term ignored in the Poynting Theorem?

The first term in the Poynting Theorem, which represents the energy associated with the electric field, is often ignored because it is typically much smaller than the second term, which represents the energy associated with the magnetic field. This is because the speed of light is a very large number, making the first term negligible in most cases.

Is the Poynting Theorem applicable for all types of electromagnetic waves?

Yes, the Poynting Theorem is applicable for all types of electromagnetic waves, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. This is because all of these waves are composed of electric and magnetic fields that propagate through space.

What are some real-world applications of the Poynting Theorem?

The Poynting Theorem has many real-world applications, including in the design of antennas, understanding the behavior of electromagnetic fields in electronic circuits, and in the development of technologies such as wireless power transfer, solar panels, and satellite communications. It is also used in the study of electromagnetic interactions in the Earth's atmosphere and in space.

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