Electromagnetiic wave power transfer

[Vanmathi Ravikumar]

Is the power in an electromagnetic wave equally transferred along both the electric and magnetic fields?

 

[tech99]

Is the power in an electromagnetic wave equally transferred along both the electric and magnetic fields?

Yes

 

[Vanmathi Ravikumar]

In electromagnetic waves

a) Power is equally transferred along both electric and magnetic fields.

b) Power is transmitted in a direction perpendicular to both the fields.

c) Power is transmitted along electric field.

d) Power is transmitted along magnetic field

 

[Vanmathi Ravikumar]

which one of these is most appropriate?

 

[BvU]

Well, tech already improved your odds from 25 to 50%. What's your best guess at this moment ?

 

[Vanmathi Ravikumar]

My selected option was (a). I just wanted to make sure if (a) was more appropriate than (b)

 

[BvU]

I don't think there's anything wrong with (b) when I look at the Poynting vector in e.g. this link

What are the arguments for (a) ? Or maybe (a) and (b) are both right ?

 

[Vanmathi Ravikumar]

option (b) does not specify what exactly are the fields. It just mentions it as both the fields. so i think that (a) is more accurate.

 

[tech99]

option (b) does not specify what exactly are the fields. It just mentions it as both the fields. so i think that (a) is more accurate.

I now think (a) is not accurate because of the word "along", as it should say "by".

 

[Vanmathi Ravikumar]

does'nt option (a) suggest that energy is equally divided between the electric and magnetic fields?

 

[tech99]

does'nt option (a) suggest that energy is equally divided between the electric and magnetic fields?

The wording suggests that power flows along the field lines, which is not correct.

 

[Vanmathi Ravikumar]

This is what i think that happens: When Photon carrying energy is moving in a straight line, power is transmitted in a direction perpendicular to both electric and magnetic fields. The transmitted power in that way gets transferred into creation of magnetic field and electric field in perpendicular directions perpendicular to the direction of movement of photon. Power is Energy flow per second.

 

[BvU]

Don't mix up photons and electromagnetic waves 'prematurely', so to say. They are one and the same, but considered from different points of 'view'.

 

[Vanmathi Ravikumar]

So what exactly happens?

 

[Vanmathi Ravikumar]

Thank you very much for improving my understanding on EM waves. But I would like to put my thoughts as below. Kindly clarify.

When EM wave propogates in a direction, Electric field and magnetic field get created perpendicular to each other and the energy available in each of the fields is equal. The energy possessed by the electromagnetic wave manifests itself by creation of electric and magnetic fields. Does the creation of changing magnetic field and electrical field not need energy to flow in the direction of field lines? Whether the creation of magnetic field and electrical field be assumed to happen perpetually without energy ?

 

[BvU]

Dear Vanmathi,
Electromagnetic waves are the propagation of disturbances (changes) in the electric and magnetic fields. They obey the Maxwell equations which can be combined to yield coupled wave equations for E and B fields. The energy does not flow in the direction of the field lines, but perpendicular to both. Same as with water waves: the water moves up and down but the wave transports energy outwards.

You can enjoy nice animations of EM waves; I liked the video here and the picture here then there is this video and the next and a near infinity of more of them. All this counts as classical physics. Photons don't.

 

[Vanmathi Ravikumar]

thank you for the links. Let us discuss em waves a little later. In case of the ripples formed in a pond, there is movement of fluid particles up and down. At any instant of time, work is also done in making the fluid particles go up and down which transfers energy in transverse direction , transverse to the direction of movement of wave. How do you account for energy transfer happening in transverse direction?

 

[BvU]

There is no net energy transfer in transverse direction. As a consequence of a neighboors up and down movement, it just bobs up and down, vibrates, oscillates until the energy is passed on sideways.

 

[Vanmathi Ravikumar]

I also accept that Power is transmitted in the direction of wave travel. But I do not understand why you do not want to accept that there is work done on the fluid particle in upward direction as well as in downward direction. If you take work done in upward movement is taken as positive and in downward movement is negative, net work done is zero. But that way of looking at the issue appears to me wrong because for raising a fluid particle from rest to upwards, work is to be done against gravity and also work is to be done for pushing down the fluid particle from rest to down against the fluid mass. So there is energy transfer to the fluid particle. This rate of transfer of energy is the Power of the fluid particle. This power is transmitted from one fluid particle to the adjoining fluid particle due to the cohesion and viscous forces between the fluid particle and the adjacent fluid particle.

 

[sophiecentaur]

viscous forces

When there are viscous forces, work is done on regions of the medium and is not retrieved. In that case, there will be a net transverse work done during each half cycle.

 

[BvU]

I agree with Sophie (perhaps I never should have brought up the water analogy ). Nevertheless, momentum is transferred sideways.

 

[sophiecentaur]

I agree with Sophie (perhaps I never should have brought up the water analogy ). Nevertheless, momentum is transferred sideways.

If you are dealing with EM, I have to ask you what transverse momentum? The momentum is in the longitudinal direction, surely.

 

[BvU]

For the water waves (I used the term bobbing up and down to keep it simple -- sigh) sideways wrt up and down is in the direction the wave propagates, longitudinal.

I wish we could direct Vanmathi's attention back to EM waves and the Maxwell equations again .

My estimate was that pointing to the Poynting vector for the energy transport and the momentum of an EM field would be a bit too ambitious -- I now hope I was wrong with that too !