Electromagnetic waves and charge

In summary: Is the vacuum that good that a good turn is seen?In summary, light interacts with charged particles, causing them to vibrate and produce an electric field. This can result in reflection and absorption of the light, leading to a force on the particles. This can be seen in the Crookes radiometer, where the black side absorbs more energy and creates a temperature difference, leading to a force and causing the vane to rotate. However, the vacuum in the radiometer can affect the direction of the rotation, showing the importance of a good vacuum for accurate observations.
  • #1
arvindsharma
21
0
Dear All,

Can anyone explain me if light is an electromagnetic wave then why a charge does not experiences an electric force(or magnetic force) due to electric field component(or magnetic field component)of electromagnetic wave? this is a very challenging question asked by my friend to me.


Regards

Arvind
 
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  • #2
What do you mean? Charges do experience a force from the light wave.
 
  • #3
arvindsharma said:
Dear All,

Can anyone explain me if light is an electromagnetic wave then why a charge does not experiences an electric force(or magnetic force) due to electric field component(or magnetic field component)of electromagnetic wave? this is a very challenging question asked by my friend to me.


Regards

Arvind

Classically, see Thompson scattering to understand how a charge responds to the oscillating electric and magnetic fields of a light wave. A relativistically correct treatment of the charge's motion yield Compton scattering.
 
  • #4
it means if i keep a disc of charge in sunlight then it will experience a force due to electric field component and hence it should move but practically this does not happen.why?
 
  • #5
because the force due to electric field is very very smallD
 
  • #6
The electric field is perpendicular to the ray, the direction of propagation of the light wave. When the light shines perpendicularly to the disk, it will move the electrons and ions in it parallel to the disk, but in opposite directions. The disk will not move.

If you have a very light disk, able to rotate around a fixed axis, and make half the disk like a mirror, and black the other half, it will rotate when exposed to light. The photons colliding with the mirror part are reflected, and their momentum change from p to -p, they transfer 2p momentum to that half of the disk. The photons falling to the black part are absorbed, and they give momentum p to the black part. Change of momentum means force acting for some short time, and it provides torque. The disk will rotate.
See also http://en.wikipedia.org/wiki/Crookes_radiometer
Radiometer_9965_Nevit.gif


ehild
 
  • #7
ehild said:
The electric field is perpendicular to the ray, the direction of propagation of the light wave. When the light shines perpendicularly to the disk, it will move the electrons and ions in it parallel to the disk, but in opposite directions. The disk will not move.

If you have a very light disk, able to rotate around a fixed axis, and make half the disk like a mirror, and black the other half, it will rotate when exposed to light. The photons colliding with the mirror part are reflected, and their momentum change from p to -p, they transfer 2p momentum to that half of the disk. The photons falling to the black part are absorbed, and they give momentum p to the black part. Change of momentum means force acting for some short time, and it provides torque. The disk will rotate.
See also http://en.wikipedia.org/wiki/Crookes_radiometer
Radiometer_9965_Nevit.gif


ehild

If you read the link you provided, you will notice that your explanation is not the correct one. The modern interpretation is that the Crookes radiometer is simply a heat engine.

ETA: But of course, to be relevant to the OP, charges can feel a force from light.
 
  • #8
Anyway, light interacts with sheets of the mill, and reflected from the silver part, absorbed by the other. Both reflection and absorption occurs as the electric field of light interacts with the charged particles, force them to vibrate, and producing electric field which radiates back or heats up the black surface. What is the real reason of rotation is an other question.

ehild
 
  • #9
ehild said:
Anyway, light interacts with sheets of the mill, and reflected from the silver part, absorbed by the other. Both reflection and absorption occurs as the electric field of light interacts with the charged particles, force them to vibrate, and producing electric field which radiates back or heats up the black surface. What is the real reason of rotation is an other question.

ehild

Totally. The fact that you and I can see to read this post tells us all we need to know about light interacting with charge.
 
  • #10
e.bar.goum said:
If you read the link you provided, you will notice that your explanation is not the correct one. The modern interpretation is that the Crookes radiometer is simply a heat engine.

ETA: But of course, to be relevant to the OP, charges can feel a force from light.

To expand . . . . The Crooke's Radiometer you can buy works because there is a temperature difference between the black and white sides. The black has absorbed more energy. The vacuum is not very good so there are a significant number of collisions with air molecules. The ones hitting the hot side will leave with more momentum than the ones hitting the cold side. So the vane is pushed the 'wrong' way round (away from the black side) - different from one in a good vacuum, where the momentum change is maximum on the white side, causing it to turn away from the white side. I have never seen a 'proper' radiometer.
 

1. What are electromagnetic waves?

Electromagnetic waves are a type of energy that is created by vibrating electric and magnetic fields. They are invisible to the naked eye but can be detected by specialized equipment.

2. How do electromagnetic waves travel?

Electromagnetic waves travel through space at the speed of light. They can also travel through some materials, such as air and water, but may be absorbed or scattered by other materials.

3. What is the relationship between electromagnetic waves and charge?

Electromagnetic waves are created by the movement of charged particles, such as electrons. These waves also have an electric and magnetic field, which can interact with other charged particles and cause them to move.

4. How are electromagnetic waves classified?

Electromagnetic waves are classified based on their wavelength and frequency. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

5. How are electromagnetic waves used in everyday life?

Electromagnetic waves are used in a variety of ways, including communication (radio waves), cooking (microwaves), thermal imaging (infrared), and medical imaging (X-rays). They are also used in electronics, such as TVs, computers, and cell phones.

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