It is to kind of a funny argument you guys are having here: which component of EM wave is more important. As always it is the question of what are you measuring - or, conversely which component are you affecting. I am very much not sure that you can say that in polarization devices you affect E rather than H, look at the derivation of Fresnel equations. both of them are there :). Comaprison of amplitudes is to a sense a bad way to state the problem, since fields are forces acting on something. Since electric and magnetic fields are acting on different things (charges and currents) it is indeed the energy contained within those that should be compared. Now, think on regular spectroscopy vs say an NMR machine. Regular spectroscopy "works" on interaction of ELECTRIC dipole, naturally with E - field, while in NMR (or EPR) it is the MAGNETIC dipole interacting with H field. So, it is very much depended on what particular experimental situation that significe one component vs the other.

BTW, the factor of c is very much a question of units you are using.

Hello Kebbab Zoubir,

Very good question! The answer lies in the relative magnitudes of the electric and magnetic fields in the em wave. If we consider a plane wave in free space, in order to satisfy Maxwell's equations, the amplitudes of electric and magnetic field components of the wave are related by E_0 = c * B_0, where c is the speed of light. Since c is a very large number, the electric field component in an em wave is much larger than its magnetic component. In other words, an em wave is predominantly an electric wave. That is why if we have to change the polarisation (or any other characteristics) of an em wave, we need to affect the electric field.

Hello Kanad,

Clear explanation, thank you. I would like to extend the question asked by Mr. Kebbab. My question is, according to Maxwell's equations, plane wave in free space carries larger electric component than magnetic, if there is material (instead of free space), which can experience magnetic field higher than electric field, using such material can we consider only with magnetic field?. Are we lack of such material indeed?

Dear Kebbab,

Your answer lies in your question only. Since these two fields are correlated, any theoretical or experimental action applied on one automatically followed by other. About why everything is applied to electric field vectors only is that working with electric field is far more easier than working with magnetic fields (theoretically) -like applying boundary conditions. Therefore, it has been adapted in experiments also that everything is done according electric field vector direction. For example if we consider a linearly polarized wave in y-direction, it will have electric field vector in y-direction and magnetic field vector in x-z plane. So in experiments we can consider all the things according to the magnetic fields.

@ Shankar, you can always work with magnetic field.

Hello Shankar,

Excellent question! If you extend the arguments of the em wave propagation to a material medium, you need to replace the velocity of light in free space by that in the medium, ie by v = c/n, where n is the refractive index at the frequency of your interest. This implies that if you have to make B_0 > E_0, (which is your proposition) you need to make v < 1. In other words, you have to slow light down in the medium very very much. I do not know if there is any natural material where this happens, but researchers have made artificial metamaterials to achieve this at GHz frequencies. I'm not an expert of the area, but here is a reference that you can look at:

Applied Physics Letters vol 98, 17171907 (2011) . You will find a lot of other articles if you search internet with the keyword "slow light".

Enjoy!

Dear Prof. Kanad,

Let me make some comments on your statements concerning the electromagnetic waves. You said in your first comment: " the amplitudes of electric and magnetic field components of the wave are related by E_0 = c * B_0 where c is the speed of light. Since c is a very large number, the electric field component in an em wave is much larger than its magnetic component. In other words, an em wave is predominantly an electric wave".

The relation E_0 = c * B_0 is formally correct but it is between the electric field E and the magnetic induction B. Conceptually B is not comparable to E but it is comparable to the electric displacement D. The magnetic field H is comparable to E. Therefore, the relation is normally written in terms of E an H. Doing that we get E= Zw*H, Where Zw is the wave impedance . In order to see the wight of the two components we can write the above equation after manipulation in the form .5 epsilon E^2= .5 mu H^2 which means that the energy in the electric field is equal to the energy in the magnetic field. This is a property of electromagnetic waves, the two components are equally important. When one is reduced the other will be also reduced to satisfy the equal stored energy rule in electromagnetic wave.

Thank you.

Dear Dr Zekry,

What do you imply by "The relation E_0 = c * B_0 is formally correct" ?

You have ignored the fundamental difference between the field vectors - B: magnetic field (unit tesla) and H: magnetising field (unit A/m); and E: electric field (unit V/m) and D: electric displacement (unit C/m^2). This is a widespread confusion. H and D are required only in material media to take account of free (explicit) charges and currents in materials whereas E and B take into account all kinds of charges and currents. That is why E and B are also designated as microscopic field vectors while D and H are designated as macroscopic field vectors.

In free space there is no current, no charge, and Maxwell’s equations take the form:

Curl E = - dB/dt, Div E =0, Div B = 0, and Curl B = (1/c^2)* dE/dt

(all derivatives are partial).

You can clearly see that the relation of magnetic field B and electric field E does not involve anything but the velocity of light.

You can still argue that in free space, magnetising field, H = B/mu_0 (where the permeability of free space mu_0 = 1.257e-6 H/m) is a large quantity comparable to electric field E. Yes, that would be right, which means that an enormous current per unit length of free space (A/m) is required to support a plane em wave. However, this does not mean that the magnitude of the magnetic field of the wave is comparable to that of the electric field.

This is not a way to discuss the things. I am not confused at all and i mean every word written by me.What i said is perfectly correct and can be found in electrical engineering text books. The real scientist returns to the right.

@Dr Zekry

I do not doubt that you mean every word you had written. I meant that your comments to my earlier post were confusing, and similar statements regarding the electromagnetic field vectors are not uncommon. I feel that I need to explicitly spell them out to remove any misunderstanding. The following statements in your post were confusing:

1. "The relation E_0 = c * B_0 is formally correct but it is between the electric field E and the magnetic induction B." What do you imply by formally correct?

2. "Conceptually B is not comparable to E but it is comparable to the electric displacement D. The magnetic field H is comparable to E."

What does the first statement mean? Are you comparing magnitudes of B and E, or something else? Their magnitudes are related through B = E/c, and so, clearly they are not comparable. Then do you mean that is B comparable to D? But, we were discussing em waves in free space where electric displacement does not arise.

In the second sentence, you said that H is the magnetic field, which is incorrect. H is magnetising field, and again there is no H in free space.

3. "Therefore, the relation is normally written in terms of E an H." Which relation are you referring to? To be objective, you cannot write a relation between E and H in free space because H is related to materials.

Finally, no one questioned the fact that both E and B fields are important for an em wave. There was no question about the fact that energy densities of the two fields in an em wave in free space are equal. It's simply that magnitudewise, B = E/c.

So, overall I found your comments confusing, and since you addressed your comments to my answer, I had to clarify my earlier answer in detail by clearly stating the definitions of the field vectors and the Maxwell's equations. Why do you find that "This is not a way to discuss the things"? How else would you discuss?

Finally, I agree with you that "The real scientist returns to the right."

Dear Sirs Kanad and Zekry,

I would like to add something in your conversation.

@ Kanad about you sentence, "In the second sentence, you said that H is the magnetic field, which is incorrect. H is magnetising field, and again there is no H in free space." As far as I read in books and other articles on electromagnetic waves propagating in free space and different media, I know that H exists in free space also, only you can have a direct correlation between B and H and they will be in the same direction. B=mu0H. As far as magnetizing field is concerned, I never heard about it. According to my knowledge B is called magnetic induction, H magnetic field and M in case of magnetic materials is known as internal magnetization.

At your sentence, "To be objective, you cannot write a relation between E and H in free space because H is related to materials. " You can have a look on the Maxwell equation written in free space and in any other dielectric medium always H vector is calculate from E vector. In the literature of electromagnetics, B is always referred as magnetic induction and H is magnetic field. Electromagnetic wave equations are also given in terms of H only. The definition of Poynting vector and several other parameters related to electromagnetic waves are always in terms of H not B. H is not related to materials, M is basically a material property.

Thank you Dr Kumar. Well said. I agree with you completely. No more No less. NO extra word after your comment.

REAL SCIENTIST RETURNS TO THE RIGHT AND I INTRODUCE TO HIM MORE RESPECT AND LOVE. What do i benefit when i write a comment that may not in agreement with a colleague comment?

My respect to you and to Kanad.

For me the subject is closed

Thank you Kumar!

It is to kind of a funny argument you guys are having here: which component of EM wave is more important. As always it is the question of what are you measuring - or, conversely which component are you affecting. I am very much not sure that you can say that in polarization devices you affect E rather than H, look at the derivation of Fresnel equations. both of them are there :). Comaprison of amplitudes is to a sense a bad way to state the problem, since fields are forces acting on something. Since electric and magnetic fields are acting on different things (charges and currents) it is indeed the energy contained within those that should be compared. Now, think on regular spectroscopy vs say an NMR machine. Regular spectroscopy "works" on interaction of ELECTRIC dipole, naturally with E - field, while in NMR (or EPR) it is the MAGNETIC dipole interacting with H field. So, it is very much depended on what particular experimental situation that significe one component vs the other.

BTW, the factor of c is very much a question of units you are using.

Dr Zekry,

My love and respect to you too.

@ Arunandan

I hope, you will agree that there is good reason why both B and H, (and E and D) field vectors exist in electromagnetism. If only H were sufficient to describe magnetism, B would not have been there.

I already mentioned that D and H are macroscopic fields, which are used to evaluate average fields in material media due to free (explicit) charges and currents. E and B are the microscopic fields which take into account all kinds of charges and currents and have meaning everywhere. You can write Maxwell's equations in terms of any of B, H, E, D, just not in H as you mentioned. In free space, however, it will be meaningful only in B and E as there is no free charge or current there.

A similar situation is valid for the definition of Poynting vector. In general,

S = (1/mu_0) E x B. In non-magnetic media this is identical to writing it in terms of H, but has a different implication in magnetic media.

A thorough treatment of the microscopic and macroscopic field quantities is there in J.D. Jackson's Classical Electrodynamics, which is a well-known and widely available text. A compact, but comprehensive account is also available in the Wikipedia for quick reference:

http://en.wikipedia.org/wiki/Maxwell%27s_equations

All the best

My conclusions are:

- Maxwell equations are the basis of the electromagnetics and all of us agree om them.

-We have different interpretations regarding the importance of the two field components of the electromagnetic waves. According to my interpretation;

In order to see the wight of the two components we can rewrite the equation of MANAD E_0 = c * B_0, in the form: .5 epsilon E^2= .5 mu H^2 which means that the energy in the electric field is equal to the energy in the magnetic field. This is a property of electromagnetic waves, the two components are equally important. When one is reduced the other will be also reduced to satisfy the equal stored energy rule in electromagnetic wave.

According to Manad interpretation:

" the amplitudes of electric and magnetic field components of the wave are related by E_0 = c * B_0, where c is the speed of light. Since c is a very large number, the electric field component in an em wave is much larger than its magnetic component. In other words, an em wave is predominantly an electric wave.

We saw the same thing from two view angles.

I am sorry that i intervened again. I apology if i cause any pain for any colleague.

Thank you all.

@Dr Zekry,

It's certainly painful to see my name spelt like you've done in your previous post! Hope you'll at least correct it. :-)

Hi Yuri,

I agree with you that the discussion went a bit funny in more than one ways.

The original question I addressed was: although light is an em wave why the electric field component is discussed more in explaining its properties compared to its magnetic field component? It was not to imply that one of them was more important than the other.

The plane wave solution of the em wave in free space gives a standard relation,

E = c*B, which justifies the preference for the electric field component. It is much larger in magnitude and is detected more easily than the magnetic field. It's not clear why this is so upsetting.

You have raised a good point regarding interaction of the fields with charges and currents. If we look at the expression for the Lorentz force, F = q*(E + v x B), and replace B by E/c, the magnitude of the force becomes q*E*(1 + v/c). This immediately shows that the force due to the magnetic field component of an em wave on a moving charge (current) is a tiny (=v/c) fraction of the force due to the electric field unless the charge is moving at a relativistic speed. In non-magnetic materials this results in very small net magnetic force.

The magnetic field of light in lab conditions is a relativistic correction to the em field (B/E~10^-6). Polarizing light goes though light-mater interaction and therefore via the electric field interaction as the magnetic field interaction is negligeable. Only the future most intense lasers (>10^20 W/cm^2) might exhibit magnetic effects.

Prof Hirlimann,

Thanks for your post stating experimental facts that demonstrate the B/E ratio in em waves, and mentioning that the magnetic field is a relativistic manifestation of the electric field. Hopefully, this will help to dispel some confusions in fundamentals of electromagnetism.

Kanad, Hi.

I do not argue that in most experimental situations the electric interaction of the EM wave with material does dominate the measurements. Nevertheless, there are different situations, particularly in the cases where free currents (or their quantum analogs) do present. Particularly I think of NMR spectroscopy (MRI imaging), where the nuclear spin serves de-facto as a current (magnetic dipole can classically be seen as a current running in a single loop). In such cases, the measured interaction of the EM wave (RF in this case) comes solely from magnetic component of the field. Curiously, the detection of the interaction, namely the RF detectors use again the electric component to detect the field. Probably that is the most wonderful demonstration of the interplay between two components.

More to the daily use:

In nowadays there are so-called inductive stoves: basically it is a stove that does not heat by itself, but rather uses the metal of the pot to induce currents in it, and heat your food. There is no direct electrical contact between the stove and your pot, but rather stove generates alternating magnetic field, that generates electrical currents in the pot's metal, which in turn, generate heat and cook your food. This type of interaction is purely magnetic, while still relativistic in the sense that your food gets relatively hotter than it has been before :).

The last example is, although less scientific, but serves a good demonstration to the fact, that not only lasers can be sources of EM field :).

I am a second year master student,I learned much things from the previous comments with very good reasoning. Anyway, I have been thinking that the electric field has been considering much more than magnetic field in electromagnetism, because I think that the very very fundamental EM field source is the electric charge and the single magnetic charge has not been discovered. I mean that the electric charge is the very beginning point and EM field is the evolutionary consequence of the electric charge. I hope I will study much more than these he he he ...

E>B for em field.

The researchgate is a bad market of ideas and opinions. I am sorry for taking part in this forum. I relied on facts to introduce my opinions. To the second time i formulated my words as precise i could and i am not sorry for them. Execute me colleagues of this forum.For me It it was not the matter of winning. What did i win? Perhaps the only benefit is that i saw how the people see the same thing differently in a very demonstrative way .

Thank you

Yuri, Hi

I totally agree to what you say regarding the usefulness of magnetic field and its interactions with matter. In fact, we have been using magnetic field from a much earlier time than we were even aware of any other fields - I'm referring to the use of the loadstone and the compass. We all know how important they were in human history.

Yes, the magnetic induction cooker is a nice innovation for the kitchen. I use it regularly. It has brought the industrial induction heating furnace home. I cannot but make one interesting point here. In this gadget, the magnetic field of the ac mains supply is not used directly as it is too weak. The electric current flows in a coil that produces a sufficiently larger magnetic field, which drives eddy currents in ferromagnetic utensils to produce Joule heating etc etc.

In this thread the aim was mostly to talk about relative proportions of the electric and the magnetic components in an em wave. The em wave and electrodynamics are fascinating subjects. In fact, if you delve deeper, as already mentioned, the magnetic field is a relativistic effect of the electric field. This means that using the Lorentz transformations of the special theory of relativity, we can find a moving frame of reference in which only the electric field remains while the magnetic field vanishes.

@Abdurahman Amat,

I'm pleased to know that you enjoyed the discussion. Probably, now you can see that your thoughts about the fundamental nature of the charge and the electric field are not far from the truth. J D Jackson's Classical Electrodynamics is a very good text, which you may want to read at some stage. I would also recommend Feynman's Lectures vol2 (in fact, all the three volumes are fascinating reads).

@ Kanad Mallik

Thank you very much !!!! I am trying to study well he he he ...