Electromagnetic waves and charge

1. Jul 22, 2014

arvindsharma

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

2. Jul 22, 2014

Drakkith

Staff Emeritus
What do you mean? Charges do experience a force from the light wave.

3. Jul 22, 2014

Matterwave

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. Jul 31, 2014

arvindsharma

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. Aug 1, 2014

davenn

because the force due to electric field is very very small

D

6. Aug 1, 2014

ehild

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.

ehild

7. Aug 1, 2014

e.bar.goum

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. Aug 1, 2014

ehild

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. Aug 1, 2014

e.bar.goum

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. Aug 1, 2014

sophiecentaur

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.