## Hope99 Group Title From my studies of Electromagnetism I've discovered that magnetic fields are quite similar to electric fields(Hence the name?). They almost act the same, two heads of different coins I guess? What do you all think? 2 years ago 2 years ago

1. Hope99

@experimentX @Carl_Pham @Jemurray3

2. Vincent-Lyon.Fr

Although they act differently, they are actually part of the same electromagnetic tensor, and cannot be transformed independently. $$F_{\mu\nu} = \begin{bmatrix} 0 & E_x/c & E_y/c & E_z/c \\ -E_x/c & 0 & -B_z & B_y \\ -E_y/c & B_z & 0 & -B_x \\ -E_z/c & -B_y & B_x & 0 \end{bmatrix}$$ Even in classical mechanics, if you change frame of reference, the coordinates of the new magnetic field depend on the old coordinates of the magnetic field AND of the electric field.

3. Vincent-Lyon.Fr

Source of the tensor: http://en.wikipedia.org/wiki/Electromagnetic_tensor

4. ghazi

NO they are totally different

5. Hope99

I confused myself. Because the magnetic field's are generated from the electrical filed or vice versa I thought they were the same. But their not. True you guys have a point.

6. ghazi

electric field lines are also called as lines of force but magnetic field lines are not, because tangent to electric field lines at any point gives us the direction of force at that point but magnetic field lines don't ...it is a very fundamental difference

7. ghazi

also magnetic field lines arise from magnetic dipole but electric field lines come out from monopoles....

8. Hope99

But @ghazi M fields produce E fields and vice versa right? Based on Maxwell's-Farady law?

9. Hope99

But not as simple as I put it... Under certain conditions/circumstances they do?

10. ghazi

right , they are produced from each other but if you judge or classify them independently the are different and somewhere interrelated

11. Hope99

@ghazi True, and yet they relie on one another to be created what an interesting thing!

12. Hope99

hey @experimentX ! I was wondering if you can note the difference between magnetic field and electrical field? Btw guys both fields are considered a force? Because once a man said all fields are force I wasn't so sure about that point...

13. Hope99

@Vincent-Lyon.Fr Thank you for that "tensor point!".

14. experimentX

look for the last two equations of Maxwell's equations. They give the relation between magnetic and electric field http://en.wikipedia.org/wiki/Maxwell's_equations

15. experimentX

i wonder if it has got to do something with relativity.

16. Vincent-Lyon.Fr

Maxwell's equations were consistent with special relativity even before SR was theorised by Einstein.

17. Hope99

If SR is involved that makes it more complicated then ever... Since I know nothing about SR...

18. Carl_Pham

Electric and magnetic fields are not the same thing, of course. But it is true, as Vincent points out, that they are both manifestations of the same underlying force -- the electromagnetic force. The fact that whether a given force looks like it comes from an electric or a magnetic field, depending on your frame of reference, tells you that relativity plays an intimate role here. In fact, there would be no magnetic fields or forces at all, if the speed of light were not finite. (You will find that every equation for the force produced by a magnetic field has magnetic field strength divided by c -- if c goes to infinity, the force vanishes.)

19. Hope99

Guys what do you mean with "Electomagnetic force" I couldn't understand it... @Carl_Pham @Vincent-Lyon.Fr

20. Algebraic!

they're the same thing, just different manifestations of the same potential. "oh, well different manifestation! they must be different then!" The only difference is you. if you move along with a charge in a potential you see one manifestation and if you fall behind or speed ahead of the charge you see another. "oh well, then the physics in different frames doesn't agree!" it does actually, because it's formulated in terms of the potential and the only way there can be disagreement about what's happening in one frame with respect to another is if you think that E and B are independent, semi-dependent, related but not the same, etc etc. A famous example of this, from Einstein himself, if I'm not mistaken, is the current carrying wire and the free charge moving relative to each other. If you travel with the wire and the charge appears to you to be moving past, you see it experiencing a magnetic force. But if you travel along with the charge, so that the wire seems to be falling behind and the charge appears stationary to you, there can be no magnetic force, because the charge isn't moving. Yet, magically, it does experience the exact same force, only now it appears to you to be an electric force.

21. Algebraic!

It's not magic of course, it's the same potential doing the same thing it always does. The only thing that's changed is you.

22. Algebraic!

Feynman famously complained about the confusion around this. I don't remember the exact quote but it went along the lines of, I used to think kindly of EB White for the very cute stuff he wrote, but now I hate him solely for his initials. Oh how I wished I'd never learned the letters E and B. Why can't they just teach A right from the start.

23. Hope99

What is this talk about SR and Feyman @Algebraic! ?! I really am LOST with all of this... I know know about magnetic fields and electrical fields are the basics. However, I tried to relate them interms of "creation" magnetic fields are created from the electrical fields and the same happens when there is a change in a magnetic fields, an electrical field is created... That is what I know is a shared thing between each. Now I do agree that they are totally different but noticed there is a lot of deep info. about it! Now I ask again what does special relativity have to do with this?

24. ghazi

@Hope99 don't get in the bingle bog of feynman and SR i would suggest you to go through the basic manifestation involved in electric and magnetic field. Later it'll help you to go further in relativity. For now, you can go through the Chapter 28 volume-1 of lectures by Richard P. Feynman

25. ghazi

@hope99 also chapter -1 , volume 2 of lectures by feynman

26. Vincent-Lyon.Fr

Electromagnetic force aka Lorentz force is the action of fields $$\vec E$$ and $$\vec B$$ on a charge q with velocity $$\vec v$$ relative to the frame of reference where it is measured. $$\vec{F} = q\;(\vec{E} + \vec{v} \times \vec{B})$$

27. Hope99

@Vincent-Lyon.Fr Aha! Lorentz force! Is what I'm more familiar of... :P Now I know when B fields are produced from the E fields... But I don't know much of the E fields generated from the B fields?! I know only when B fields are changing induced E fields are generated... But what are some specific example you guys could give out to help?

28. Hope99

@ghazi Yea, I'm already struggling with the fundamental's and relativity and Feynman's work is to complex for me right now! So I'll stay with the basics :P But I have to ask what do I gain if I understood more about relativity and relating it with our subject?!

29. ghazi

you will definitely explore a dimension beyond space and time :) i am also fighting for the same :( ...and correlation is way too important than gaining the knowledge , it makes a mountain of wisdom, and broadens your knowledge and thinking ability

30. Hope99

I agree with you on that @ghazi its true these things tend to make you always wiser about life. Do you have any examples of changing B fields creating E fields by any chance? I think that one example is 2 bar magnets attracting/repelling? E fields are induced right? That causes them to attract/repel? "I'm not sure about that point its a "I think" for me"

31. Hope99

Another example of induced EF is a generator. That's the only thing I know :P

32. ghazi

you can have example of current passing through solenoid there is generation of both

33. Hope99

So its true when magnets attract/repel E fields are generated due to that attraction/repulsion?

34. ghazi

i guess no, it's just not that, it also involves current too

35. Hope99

@ghazi "i guess no, it's just not that, it also involves current too" could you explain more about that point? What does current have to do with it?

36. ghazi

changing magnitude of magnetic field produces electric field

37. Hope99

So in case of two bar magnet they have equal magnitudes so no electric field is induced? However, if there were changing magnitudes E fields are induced? Ok fair enough, whats the make cause that makes a magnet attract/repel? I thought it was the E fields :S

38. ghazi

the reason of attraction of magnets is the alignment of domain in magnets, in a particular direction

39. Hope99

true but what is the main force that causes that attraction and repulsion? That does all the "work" between them? Isn't it the electric?

40. ghazi

it is magnetic force that causes attraction , fundamentally there are four forces in nature weak, strong, electromagnetic and gravitational ..these are the causes of attraction and repulsion

41. Hope99

That's really strange although studying a lot about electrodynamics I found that magnetic fields can't really do much work they only do work "indirectly" by inducing electrical fields on another "magnet/dipole" and thus work is being done... And I would always say the main force responsible of that action is the magnetic force creating the electrical force to do work... Its more of that teacher student example where the teacher gives our homework his not doing it... But just gives out that homework... The student ends up doing all the hard work! More homework = more work for the student... Now the "Teacher" is the M field and the "Student" is the E field. That really made me connect the two and realized how sometimes the two work together or are sometimes depent on one another... Maybe? What do you all think @ghazi @experimentX @Carl_Pham @Vincent-Lyon.Fr ?

42. experimentX

I still need to work out quite on lot on electrodynaimcs. Probably I'll work out Mathematical physics first. consider me out of discussion ... anyway I'll follow the post.

43. Hope99

@experimentX fair enough. Electrodynamics is one of those fundamental things that once you understand it you can relate it with a lot of things in the world! I've always said that electric fields and magnetic fields are of the same coin but different sides... That means they both do the complete opposite! Yet the have certain traits that re really similar... Now I've invite @Jemurray3 because he probably knows my failed history of this matter :P and @TuringTest too! Its been a while but he knows I've asked about this previously multiple times! So help me out guys!

44. Hope99

Finally I'd like to point out the fact that a lot of people "physicists" agree that magnets do work but "indirectly" I don't know what they mean by that point... Ow well! Its a puzzle I guess :P

45. Hope99

There not much info. About magnet to magnet interaction or more exact magnetic fields interactions! Much less info about that matter really :S

46. ghazi

it is not that complex...you need to go through everything in a sequence and to an advanced level....i am sure you'll get everything

47. Hope99

@ghazi I "hope" so :P I just want to understand more about magnet to magnet interactions that all... I found less about that information for some reason :S Maybe its more of a mystery?

48. ghazi

it ain't a mystery ...you will get to know everything and you'll have a lucid explanation ..refer to every available material

49. ghazi

see, electric field is responsible for the drift of electrons in a wire (current) and changing flux of magnetic field causes production of emf and emf is the force that causes drift of electrons , and again drifting produces current and current produces magnetic field...this way cycle runs

50. Hope99

@ghazi When magnets attract or repel their is a change in flux... So that gives sense to the E field being produced?

51. Jemurray3

Magnetism is a essentially a relativistic manifestation of electricity. Didn't we have a month-long discussion about this? Can you formulate a question rather than just say "I want to understand better"?

52. Hope99

@Jemurray3 Yea we did but going through electromagnetism and studying more about it made me really REALLY confused... So my only hope to get back track is to realize where I failed in my understanding from you guys and continue on with the right mindset. Now for some proper questions @Jemurray3: 1) What is the common relation between E field and M fields? 2) When one M field interacts with other M field is there an induced E field(Bar magnet attracting/repelling another bar magnet what force is doing work and who's responsible for that force?). Thank you "again" for clearing that out.

53. Jemurray3

As a charged particle moves through a region in which there are electric and magnetic fields, it feels forces and therefore traces out a well-defined trajectory through that region. In electrodynamics, these fields are velocity dependent in the sense that two observers in different inertial frames of reference would calculate different fields based on the same configuration of charges and currents. However, it is found that even though the fields may change for different observers, the effect that they have on a charged particles turn out to be the same, which is to say that the fields transform into one another in such a way that the trajectory traced out by a charged particles is always unique and well-defined. That transformation is captured by the electromagnetic field tensor which is listed above. If you are not familiar with tensor calculus don't worry about it, but the point is that Electric and Magnetic fields are two different manifestations of the same phenomenon which mix with one another when one moves through different frames of reference.

54. Jemurray3

Secondly, it is important to realize that magnetic and electric fields do not interact with one another or themselves. They interact with charged particles only. The question of bar magnets traces itself back to the interactions of atoms and electrons with magnetic fields, an area which does not have a direct analog in classical electrodynamics because these magnetic interactions do not obey the law F = q(v x B). In the case of electrons, for example, we see that electrons have an intrinsic magnetic dipole moment that behaves like a current loop and creates the same magnetic field in the limiting case that the current goes to infinity and the area of the loop goes to zero in such a way that their product, the magnetic moment of the electron ,remains constant. Because this magnetic interaction does not obey the classical magnetic force laws, quantum mechanical dipoles can do work on each other. Real current loops cannot because they have physical dimension -> as they shrink to points, as in the case of an electron, the situation changes, but again this phenomenon can only be approximated in the limiting case by classical electrodynamics.

55. Hope99

Thank you @Jemurray3 ! Why the heck does David Griffiths author of the book "Introduction to electrodynamics" say that electric fields are created from changing magnetic fields based on our example magnet attracting/repelling -_-" Guess now I know classical dipole interaction are out of that field! Thanks @Jemurray3 !

56. Hope99

"The forces of attraction field of magnets are due to microscopic currents of electrically charged electrons orbiting nuclei and the intrinsic magnetism of fundamental particles (such as electrons) that make up the material." The same is applied in repulsion so in a sense the electric force is in charge/responsible for that "work". In some way Wikipedia describes this as a complex effect from the rules of electrodynamics... @Jemurray3

57. Hope99

Theses small currents are also "magnetic dipoles" right? @Jemurray3 ? I think the ONLY force that can act in this case is the electrical force don't you agree?

58. Hope99

I need to study more about magnetic dipole moment and their interactions because there lay's the key of my question!

59. Jemurray3

I said that electric and magnetic fields don't interact, which means that they don't exert forces on each other. Changing electric fields induce magnetic fields and changing magnetic fields induce electric fields, as Griffiths says.

60. Hope99

Thanks @Jemurray3

61. Hope99

@Jemurray3 So in case of a bar magnets being attracted/repelled what is the main cause of that effect and what kind of force is doing work? Finally when they are attracted/repelled is there a change in M field that induces an E field? If you can clear things out a bit in case of a bar magnet that would be great!

62. Jemurray3

The magnetic field inside a ferromagnetic material is due primarily to the magnetic dipole moments due to the electrons "orbiting" ( the word "orbiting" here being used very loosely ) the atomic nuclei and to the intrinsic magnetic dipole moments of the electrons, the latter contribution being most important. In Ferromagnetic materials, these intrinsic dipole moments tend to be aligned with one another in such a way that a powerful magnetic field is produced. That's a bar magnet. In the presence of an external, spatially-varying magnetic field, the dipoles in the bar magnet feel a magnetic force. This force is NOT explained by classical electrodynamics because the electrons are point particles and therefore can only be approximated in the limiting case I mentioned awhile ago. Unlike the previous case of an ACTUAL current loop in a spatially varying magnetic field, we cannot find a sneaky electric field doing the work, so in this and only this instance, the magnetic field can be said to do work.

The magnetic field moves perpendicularly with the electric field|dw:1348855212350:dw|

64. Hope99

@Jemurray3 you explanation is fair enough! I was really interested in that example and found many would argue that its the electrical force that's doing the work generated by M and that would confuse me... So when looking at a magnet to magnet interaction I should look more at magnetic dipole to dipole interaction? Because really this matter is not a popular one and not that easy to find and understand... A troublesome example really... @Jemurray3 Can't we say that a bar magnet would produce a E FIELD when its M field is being changed? And the other bar magnet does the same exact effect thus E fields are acting on one another? How can I be certain what force is the cause of work and all of this action. I dubt its the magnetic field because many are against that idea...

65. Hope99

Im confused...

66. Jemurray3

I don't know what you mean when you say it's M field is being changed. The forces between magnets, be they electrical or magnetic in nature, act on the charged particles in the magnet, not the fields themselves. If the fields interacted, we would not be able to superpose them.

67. Hope99

@Jemurray3 pretty much what you said is my final conclusion.

68. Hope99

+ Personally what ever force is in charge (Magnetic,electric) all I know that this force is generated when two magnet's or more are interacting with each other. You can't get it only with those interactions and I think that is all due to the atomic level of things really. The force is rated based of the feature's of the magnet hence there are types, and sizes that would have various effects of interactions some would have a weak interaction because the magnets are and some greater because of the magnet. So yea, at the end of the day I would say its all about the atomic size of thing were we start to understand :P @Jemurray3 Hope you got this point because its more philosophical then ever. Take care-