Can an antenna with a positive charge emit a radio wave, and why or why not?

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Discussion Overview

The discussion centers on whether a positively charged antenna can emit a radio wave when shaken, exploring the conditions under which electric charges emit electromagnetic waves. Participants examine the relationship between charge acceleration, energy transformation, and the production of radio waves, touching on both theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that shaking a positively charged antenna at high frequencies could result in the emission of radio waves due to the acceleration of charges.
  • Others argue that without acceleration, only an electric field is produced, not electromagnetic waves.
  • One participant suggests that the energy used in shaking the antenna would primarily transform into heat energy, questioning its significance for EM wave production.
  • There are claims that the frequency of the emitted wave would depend on how the antenna is shaken, with a specific example of 500 MHz mentioned.
  • Some participants discuss the role of particle accelerators in producing EM waves, noting that while they do emit radiation, it is often a side effect of their design focused on speed rather than acceleration.
  • There is a mention of synchrotron radiation as a significant source of EM waves in certain accelerators, although some participants note that this is not always desired.
  • One participant simplifies the question by removing the term "antenna," suggesting that any charged object shaken at high frequencies would create a radio wave.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for radio wave emission, with some agreeing that acceleration is key while others emphasize the role of energy transformation into heat. The discussion remains unresolved regarding the significance of heat energy in the context of EM wave production.

Contextual Notes

Participants highlight the dependence on definitions of acceleration and the conditions under which electric charges emit waves. There are unresolved questions about the significance of heat energy compared to electromagnetic wave production.

Who May Find This Useful

This discussion may be of interest to those studying electromagnetism, radio wave propagation, and the physics of charged particles, as well as individuals curious about the practical applications of these concepts in technology and engineering.

JayKo
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1. If you took a positively charged antenna and shook it up and down 500 million times a second, would it emit a radio wave. If so, why? If not, why not?

I would say, the EM wave that it emit is heat due to friction with the air. anyone have any other suggestion.


2. Electric charges are what emit radio waves, so why can't the cellular telephone emit a radio wave by just putting a positive charge on its antenna permanently?

not sure on this answer though.
 
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JayKo said:
Electric charges are what emit radio waves...
Electric charges emit radio waves only at very special conditions, when they are accelerated.
If no acceleration, they may produce only EM field, not waves.
If no speed, only E field, no waves, no magnetic field.

JayKo said:
1. If you took a positively charged antenna and shook it up and down 500 million times a second, would it emit a radio wave. If so, why?
Yes, it would. Actually it would emit a radio wave at frequency 500 MHz
PLUS vulgar electric field, because the antenna is not electrically neutral.

P.S. If you counted UPs and DOWNs together, when you said "shook it up and down 500 million times a second", then frequency of wave would be only 250 MHz... :smile:
 
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jdg812 said:
Electric charges emit radio waves only at very special conditions, when they are accelerated.
If no acceleration, they may produce only EM field, not waves.
If no speed, only E field, no waves, no magnetic field.

cool, i see.thanks for the explanation. how about the 1st question,is my reasoning correct?
my reasoning is since energy is used in shooking the antenna, it must have transform to another energy which is heat energy, or is the heat energy significant in this case? anyone?
 
jdg812 said:
Electric charges emit radio waves only at very special conditions, when they are accelerated.
If no acceleration, they may produce only EM field, not waves.
If no speed, only E field, no waves, no magnetic field.


Yes, it would. Actually it would emit a radio wave at frequency 500 MHz
PLUS vulgar electric field, because the antenna is not electrically neutral.

P.S. If you counted UPs and DOWNs together, when you said "shook it up and down 500 million times a second", then frequency of wave would be only 250 MHz... :smile:

what is producing this EM wave? lambda=c/f = 3x10^8/250x10^6. =1.2m (radio wavelength)
 
JayKo said:
my reasoning is since energy is used in shooking the antenna, it must have transform to another energy which is heat energy, or is the heat energy significant in this case? anyone?
If you would really shake your antenna by hand, I guess 99% of mechanical energy would be transformed into heat energy. This energy is not significant for EM waves.
Then, 0.99% of energy would be transformed in the energy of acoustic waves.
This energy is not significant for EM waves as well.
 
JayKo said:
what is producing this EM wave?
When you shake your antena, charges in it are accelerated. This acceleration produces the EM wave.
 
jdg812 said:
If you would really shake your antenna by hand, I guess 99% of mechanical energy would be transformed into heat energy. This energy is not significant for EM waves.
Then, 0.99% of energy would be transformed in the energy of acoustic waves.
This energy is not significant for EM waves as well.

yeah, the antenna is shook by hand. so i can conclude that the dominant energy here is heat only. thanks.
 
jdg812 said:
When you shake your antena, charges in it are accelerated. This acceleration produces the EM wave.

but this acceleration is insignificant if we compare to the speed of electron. hence the EM wave would be extremely small.

if so, those particle accelerator when in operation would produce significant EM wave energy, right? thanks
 
JayKo said:
particle accelerator when in operation would produce significant EM wave energy, right? thanks
Yes, particle accelerators produce some EM waves, but not so significant, because they were designed to produce speed, not acceleration.
 
  • #10
JayKo said:
if so, those particle accelerator when in operation would produce significant EM wave energy, right? thanks

Some accelerators do produce significant EM radiation. Look up "synchrotron radiation," for example on Hyperphysics.
 
  • #11
In particle accelerators the "synchrotron radiation" is side effect. They try to avoid that. Anyway, cost of such source of EM waves is of the order $10 Billions. On the other hand, cost of a capacitor, consisting of a piece of wire and the Earth, is of the order $0.1. Thus an efficiency of such cheap device as a source of EM radiation is about 10^{12} times better... :smile:
 
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  • #12
  • #14
jtbell said:
Some accelerators do produce significant EM radiation. Look up "synchrotron radiation," for example on Hyperphysics.

interesting subject for further reading, thanks .;)
 
  • #15
JayKo said:
1. If you took a positively charged antenna and shook it up and down 500 million times a second, would it emit a radio wave?

i would simply the question and removed the word "antenna". if you took a positive charge (or negative) and shook it up and down (or left and right) 500 million times per second, you would be creating a 500 MHz radio wave.

this is what happens with a radio transmitting antenna. charge sloshes back and forth along the antenna element and that creates a radio wave.

see the "thought experiment #1" at:
https://www.physicsforums.com/showthread.php?p=1556816

I would say, the EM wave that it emit is heat due to friction with the air.

so you think it would work differently if you were shaking this charged object up and dowm 500,000,000 times per second in outer space where there is no air to have friction with?

anyone have any other suggestion.

how 'bout, the reason it works is that: unlike charges attract and like charges repel.
 
  • #16
rbj said:
i would simply the question and removed the word "antenna". if you took a positive charge (or negative) and shook it up and down (or left and right) 500 million times per second, you would be creating a 500 MHz radio wave.

this is what happens with a radio transmitting antenna. charge sloshes back and forth along the antenna element and that creates a radio wave.

see the "thought experiment #1" at:
https://www.physicsforums.com/showthread.php?p=1556816



so you think it would work differently if you were shaking this charged object up and dowm 500,000,000 times per second in outer space where there is no air to have friction with?



how 'bout, the reason it works is that: unlike charges attract and like charges repel.



what a interesting read, well i know understand how charges interact with each other to produce the EM wave. so in microscopic sense, we analyze the charges, in macroscopic sense, we talk about the antenna movement interact with the air friction, is that sound logical? but in vacuum, there is nothing to cause that friction.
 

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