Energy in AC Circuits: No Frequency Involved

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Discussion Overview

The discussion centers on the relationship between energy, frequency, and alternating current (AC) in electrical circuits. Participants explore the formulas used to describe energy in AC circuits and whether frequency plays a role in these calculations, contrasting this with the behavior of electromagnetic waves.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why frequency is not included in the energy formulas for AC circuits, citing specific equations for average power and energy in inductors.
  • Another participant asserts that electromagnetic waves do not propagate in AC circuits, suggesting that AC current and electromagnetic waves are fundamentally different phenomena.
  • A different participant references an external source, suggesting that the vibration of electrons in AC may lead to the generation of electromagnetic waves, indicating a potential connection between the two concepts.
  • One participant clarifies that energy can be stored in capacitors and inductors, while power in electrical circuits is defined differently, mentioning the Poynting vector for electromagnetic waves.
  • Another participant introduces the idea that AC flowing through wires generates electromagnetic radiation, which can lead to energy loss and interference, implying a continuous energy supply is necessary for oscillation.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between AC circuits and electromagnetic waves, with no consensus reached on whether frequency should be considered in energy calculations for AC circuits.

Contextual Notes

Some participants reference specific equations and concepts without fully resolving the implications of those equations in the context of AC circuits versus electromagnetic radiation. The discussion includes assumptions about the behavior of electrons and energy transfer that remain unexamined.

Who May Find This Useful

This discussion may be of interest to individuals studying electrical engineering, physics, or anyone exploring the principles of alternating current and electromagnetic theory.

p75213
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Given that:

E = hν where

E = energy of a photon
h = Planck's constant = 6.626 x 10-34 J·s
ν = frequency

Why is it that the energy (electromagnetic waves) in AC electrical circuits does not include frequency as part of the formula? eg.
\begin{array}{l}<br /> P = \frac{1}{2}{V_m}{I_m}\varphi \to \omega = \int_0^t {P\,dt} \\ <br /> {\rm{Where: }} \\ <br /> {\rm{P = average power, }} \\ <br /> {V_m} = {\rm{voltage magnitude, }} \\ <br /> {I_m}{\rm{ = current magnitude, }} \\ <br /> \omega = {\rm{energy, }} \\ <br /> \varphi {\rm{ = power factor(}}\cos ({\theta _v} - {\theta _i})) \\ <br /> \end{array}

The same thing applies to the formula for the energy contained in an inductor:
\begin{array}{l}<br /> \omega = \frac{1}{2}L{i^2} \\ <br /> {\rm{Where:}} \\ <br /> L{\rm{ = inductance}} \\ <br /> i{\rm{ = current}} \\ <br /> \end{array}
 
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Electromagnetic waves do not propagate in an AC electrical circuit. Alternating Current flows in an electrical circuit. They are not the same, so they are not described by the same formulas.
 
That's what I thought until I read this: http://amasci.com/miscon/whatis2.html#2
The guy says he is an electrical engineer so I figured he must know what he is talking about.
Seems to make sense as the electrons in AC electricity are simply vibrating inplace. As I understand it a vibrating electron will set up an electromagnetic wave.
 
Last edited:
Energy (Joules) can be stored in electrical circuits only in capacitors (½CV2) and inductors (½LI2). Power (watts) in electrical circuits is given by V·I (real and reactive). Power (watts per m2) in an electromagnetic wave (including waveguides) is given by the Poynting vector \overrightarrow{S} = \overrightarrow{E}\times \overrightarrow{H} (vector cross product).
 
p75213...I think your reference is referring to electromagnetic radiation (radio waves usually) from the wires of the circuit.
Whenever AC flows through a wire there is electromagnetic radiation from the wire. It is this radiation that can cause 'interference' in radio reception etc.
This is an energy loss from the circuit and shows that energy must be continuously supplied to maintain oscillation.
 

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