EM Waves: How Do They Get Generated?

In summary: So I'll have to continue reading to understand more about this.In summary, according to classical electrodynamics, an accelerating charge has to radiate. The radiation has both energy and momentum, which comes at the expense of the KE and momentum of the charged particle. So, the charged particle will actually accelerate less than a neutral particle of the same mass under the same force. It’s as if the radiation is having a recoil effect on the charge. That solves your energy conservation problem, I hope.But if you consider the system to be just the 2 charges, there is energy violation. Because there is no external force acting on the system. Unless the system now changes to the 2 charges plus the radiation. But I don't
  • #1
torquerotates
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I read that em waves get generated by oscillating and/or accelerating charges. I am confused on how this occurs. Doesn't this violate energy conservation? Say 2 charges( a +& a -) are separated by a certain distance. So they have potential energy. Then you let them go. The potential energy is converted to kinetic energy. But if the potential energy is converted to kinetic, where does this leave room for em waves?
 
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  • #2
According to classical electrodynamics, an accelerating charge has to radiate. The radiation has both energy and momentum, which comes at the expense of the KE and momentum of the charged particle. So, the charged particle will actually accelerate less than a neutral particle of the same mass under the same force. It’s as if the radiation is having a recoil effect on the charge. That solves your energy conservation problem, I hope.
 
  • #3
But if you consider the system to be just the 2 charges, there is energy violation. Because there is no external force acting on the system. Unless the system now changes to the 2 charges plus the radiation. But I don't think that is allowed because the system defined in the first place is just the 2 charges(+a &-a).
 
  • #4
(Remember, according to classical electrodynamics, an accelerating charge has to radiate.)

Isolated charges only occur in the first chapter of elementary electrostatics textbooks. In reality, you have the two charges, and their fields filling the Universe (somewhat loosely speaking, of course.) The charges and the fields interact mutually, giving rise to EM radiation. I am trying to answer as simply as possible.

Suppose you put a body in vacuum, and define it to be an isolated body. But then it cools off, giving off infrared radiation. What will you say now? Is it still an isolated body, by your “definition”?
 
  • #5
Suppose you put a body in vacuum said:
Yeah, that is what I'm confused about. So the initially defined system is just the isolated body. And the system after is still the same because the isolated body(previous)=the body+em energy. Therefore no external work is done on the system. And energy and momentum is conserved. Am I correct with this line of thinking ?
 
  • #6
torquerotates said:
But if you consider the system to be just the 2 charges, there is energy violation. Because there is no external force acting on the system. Unless the system now changes to the 2 charges plus the radiation. But I don't think that is allowed because the system defined in the first place is just the 2 charges(+a &-a).

The energy was already there, in a different form. Suppose an “isolated” bomb explodes, then the chemical energy just got converted into mechanical energy.

Isolated systems are mental constructs invented by scientists to study certain aspects of Physics without getting cluttered by non-essentials. For example, in thermodynamics, an isolated system may mean an insulated container which cannot exchange energy or matter with outside. In mechanics, isolated systems generally mean one on which no external force or torque is acting. But internal forces can always be there.

In the example I’d given, suppose we are only studying the angular momentum of the body (classically). Then it hardly matters whether the body cools off or not. But in reality, there is no system which is isolated.

Coming back to your original problem, your isolated system consisted of two charges. If the two charges interacted and produced EM radiation, it’s somewhat like the bomb exploding. The energy of EM radiation was already there as PE.

And the system after is still the same because the isolated body(previous)=the body+em energy. Therefore no external work is done on the system. And energy and momentum is conserved. Am I correct with this line of thinking ?

Yes.
 
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  • #7
Oh... I see. Thanks your last post was very insightful. I've been confused on how energy and systems work until you brought up your point about the virtual non-existence of isolated systems. Well, I've only taken mechanics and E&M. But not thermo yet.
 

1. How are EM waves generated?

EM waves are generated when an electric charge or current changes direction or magnitude. This produces a changing electric field, which in turn produces a changing magnetic field. These two fields oscillate and create an electromagnetic wave that travels through space.

2. What are the sources of EM waves?

The main sources of EM waves are accelerating electric charges, such as those found in radio antennas, light bulbs, and electronic devices. Other sources include nuclear reactions, stars and other celestial bodies, and even the human body.

3. How do EM waves differ from other types of waves?

EM waves are unique because they do not require a medium to travel through. They can travel through empty space, unlike sound waves which require a medium such as air or water. EM waves also have a wide range of frequencies and wavelengths, giving rise to different types such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

4. How do EM waves interact with matter?

EM waves can interact with matter in several ways. They can be reflected, absorbed, or transmitted. The type of interaction depends on the frequency and properties of the material. For example, visible light is mostly transmitted through glass but mostly absorbed by black surfaces.

5. Can EM waves be harmful?

EM waves can be harmful at high levels of exposure, such as those found in X-rays and gamma rays. However, most EM waves at lower frequencies are not harmful and are essential for various technologies and everyday life. It is important to follow safety guidelines and regulations to minimize potential harm from high-frequency EM waves.

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