Poynting vector - uniform vs accelerated charge

In summary, the Poynting vector is a mathematical representation of the energy flow of an electromagnetic field. In the case of a uniform charge, where the charge is not moving or accelerating, the Poynting vector is constant and points in the direction of the electric field. However, for an accelerated charge, the Poynting vector varies in both magnitude and direction, reflecting the changing energy flow of the electromagnetic field. This difference is due to the emission of radiation by the accelerating charge, which carries energy away from the system. Therefore, the Poynting vector provides a valuable tool for understanding the behavior of electromagnetic fields and the energy they carry.
  • #1
arlesterc
43
3
I am trying to understand why an accelerating charge emits radiation/electromagnetic waves but a uniformly moving one does not. I saw one video on Youtube where it seemed that it was explained by the fact that with a uniformly moving charge the Poynting vector was pointing 'in to the volume' - no energy was flowing out - but that with an accelerating charge the Poynting vector was pointing 'out of the volume' hence there was energy out that would be detected as electromagnetic waves. Is that the gist of it?
 
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  • #2
For a uniformly moving charge there is an inertial frame of reference in which the charge is not moving and does not radiate. For an accelerated charge there is no inertial frame of reference in which the charge is stationary.. However there is more to the story; for an accelerated charge, radiation is only detected if the detector is not accelerating with the charge. You might want to look at the thread at https://www.physicsforums.com/threa...ravitational-field.950608/page-2#post-6020694

The mentors and forum staff were most patient with my questions in that thread.
 

FAQ: Poynting vector - uniform vs accelerated charge

What is the Poynting vector?

The Poynting vector is a mathematical concept in electromagnetism that describes the direction and magnitude of energy flow in an electromagnetic field. It is named after John Henry Poynting, who first introduced the concept in 1884.

How is the Poynting vector calculated?

The Poynting vector is calculated by taking the cross product of the electric field vector and the magnetic field vector at a given point in space. This results in a vector quantity with units of watts per square meter.

What is the difference between uniform and accelerated charge in relation to the Poynting vector?

A uniform charge refers to a charge that remains stationary in a particular location, while an accelerated charge refers to a charge that is in motion. In terms of the Poynting vector, the direction and magnitude of energy flow will be different for a uniform charge compared to an accelerated charge.

How does the Poynting vector change in a uniform electric field?

In a uniform electric field, the Poynting vector will point in the same direction as the electric field and will have a magnitude proportional to the square of the electric field strength. This means that the energy flow will be constant and in a single direction.

What happens to the Poynting vector in the presence of an accelerated charge?

In the presence of an accelerated charge, the Poynting vector will not be constant. It will change in both direction and magnitude as the charge accelerates. This is because the electric and magnetic fields are constantly changing around an accelerating charge, resulting in a constantly changing Poynting vector.

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