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Magnetic Field vs Electric Field

by Aspchizo
Tags: electric, field, magnetic
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Aspchizo
#1
Feb20-13, 10:46 PM
P: 26
Can someone explain the difference? thanks.
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vanhees71
#2
Feb21-13, 02:52 AM
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There is no "difference". There is one and only one electromagnetic field. What you call electric and magnetic field depends on your reference frame, and it is rather a splitting in electric and magnetic components of this field (like you split your space-time vector wrt. to a reference frame in one temporal and three spatial components).
Aspchizo
#3
Mar1-13, 02:00 PM
P: 26
From videos I've watched it seems that they do break it down into two components (electric and magnetic) and each component has specific influence on a charged particle passing through the field.

This is what I don't understand. Like for example in this video he says...

"First lets determine the direction of the forces that each field will apply to the charged particle"

http://www.youtube.com/watch?v=oduicUCLPs8

He says the magnetic and electric field are working in opposition to eachother. I don't understand this. If someone could give me a explanation for this that would be great.

Naty1
#4
Mar1-13, 03:49 PM
P: 5,632
Magnetic Field vs Electric Field

He says the magnetic and electric field are working in opposition to eachother. I don't understand this. If someone could give me a explanation for this that would be great.
not really 'opposite', but at right angles to each other....

If I observe a stationary electric field in my room, and you race by, you'll observe a magnetic field...perpendicular to the one I observe. [very roughly analogous to me looking at 'heads' on a coin while you see 'tails' on your side....]

See the illustration of actual current flow, and fields here:

Magnetic field due to moving charges and electric currents

http://en.wikipedia.org/wiki/Magneti...ctric_currents

The direction of the magnetic field is shown as RED circles. The corresponding electric field is in the direction of the THUMB....shown by the white arrow.
Menaus
#5
Mar1-13, 06:51 PM
P: 54
Quote Quote by vanhees71 View Post
There is no "difference". There is one and only one electromagnetic field. What you call electric and magnetic field depends on your reference frame, and it is rather a splitting in electric and magnetic components of this field (like you split your space-time vector wrt. to a reference frame in one temporal and three spatial components).
Ugh... NO!

Electromagnetism is the conjugate forces of the electrostatic (or dielectric) and magnetic fields of a conductor, or what have you. If what you say is true than we could switch the visualizations of magnetic and dielectric fields freely according to reference point, we cannot. Dielectric field lines terminate on charged objects/particles and never connect, and magnetic field lines are concentric circles that always connect.

Dielectricity and Magnetism, however, are more like two separate and distinct parts to a whole, like male and female. We can have dielectricity without magnetism, just like we can have magnetism without dielectricity. But they are most often seen together.

While the magnetic and dielectric fields are not in opposition to each other, they do have some contradictory characteristics, they are also perpendicular to each other. Even so, they also complement each other very much and one can define the other if both are in conjugation.

Magnetic fields are made of concentric circles, they always connect to each other like the latitude lines on a map or the Earth. They look like this:



Magnetic fields are created by the movement of charged particles in a circuit, the magnetism created by these charged particles (usually electrons) is the same magnetism created by your normal magnet at your home. The magnetism in your home magnet comes from the alignment of the atoms. When they are aligned correctly, the tiny magnetic fields of the atoms (created by the electrons outside the nucleus) add up and make a large one outside.

The magnetism in an electric circuit is created by the movement of electrons within the circuit. Since electrons can effect magnetic fields, magnetic fields can also effect electrons, and this is part of what the video is talking about.

Electrostatic fields are different, they are straight lines which only terminate at other charged bodies, they are like the lines of longitude you see on a map or on the Earth.



Another name (older name, but I like it better, because electrostatic fields aren't necessarily 'static' at all times) for electrostatic, is dielectric. The dielectric field arises from charged particles. When you rub two objects together (usually an insulating object and a conducting one), the electrons from the insulator move to the conductor, causing the two objects to be charged. Because electrons are negatively charged, and there are more electrons than protons (positively charged particles) on the conductor, a negative electric charge exists on the conductor. The opposite is true for the insulator--a positive charge exists because the electrons lost from the insulator mean that the protons have more room to manifest their energy in the form of a field.

There is then, the electromagnetic field, or the field that arises when both dielectric and magnetic fields are in conjugation (most of the time). Steinmetz simply called it the electric field of a circuit.



Here the dielectric field directly effects the magnetic, and vise versa. As you can see there is a clear distinction between the two, while they are together in one.
WannabeNewton
#6
Mar1-13, 07:26 PM
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Quote Quote by Menaus View Post
Ugh... NO!
Maybe you shouldn't be so quick to attack vanhees like that don't you think? The electromagnetic field is described by the 2 - form [itex]F_{ab}[/itex] which goes by many names but I like electromagnetic field strength tensor. The components of [itex]F_{ab}[/itex] are split up into components of the magnetic and electric field but the 2 - form itself encapsulates the electromagnetic field. Other than the main diagonal, which is always zero, which components of [itex]F_{ab}[/itex] are zero or non zero depends on the frame of reference and as noted the components correspond to components of the E and B field. Maxwell's equations can be written down entirely in terms of [itex]F_{ab}[/itex] and the 4 - current density, as you can see in my signature. So really you just have the electromagnetic field as the one, all encompassing object and the components of the E and B field are frame dependent components of [itex]F_{ab}[/itex]. What vanhees said is probably one of the most beautiful things about electromagnetism.
ModusPwnd
#7
Mar1-13, 07:28 PM
P: 1,100
Cant transform an electric field completely away though. I think his remark was snarky and missed the point of the question. There is a difference. Yes, you can transform some of one into the other. Regardless, there is a difference. I think its just needless showing off... If somebody asked what was the difference between energy and momentum you wouldn't reply "no difference" and appeal to relativity. No, you would explain the difference and then add in some about their connection.
WannabeNewton
#8
Mar1-13, 07:58 PM
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Quote Quote by ModusPwnd View Post
I think its just needless showing off... If somebody asked what was the difference between energy and momentum you wouldn't reply "no difference" and appeal to relativity. No, you would explain the difference and then add in some about their connection.
Well the OP's question was rather ambiguous and vague to begin with so it is hard to gauge (no pun intended ) what to respond with. If the OP was more specific then more detailed responses could be constructed but as it is stands, the OP should have researched his very broad question first, otherwise it is only natural to get a large spectrum of responses. I don't see how it is showing off and honestly I think it motivates people to go learn about the deep connections between the E field, B field, and special relativity because this is one of the most amazing things about electromagnetism imo.
BruceW
#9
Mar1-13, 08:57 PM
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Quote Quote by Aspchizo View Post
He says the magnetic and electric field are working in opposition to eachother. I don't understand this. If someone could give me a explanation for this that would be great.
Yes, there is the force due to the electric field and the force due to the magnetic field. In region 1, he wants these two forces to be equal and opposite so that the total force on the charge is zero. Therefore, he does some calculations to find out what the electric field and magnetic field values must be, so that the total force on the charge would be zero. Note that in general, the two forces are not usually equal and opposite, it is just in the particular example he gives, he wants them to be equal and opposite, so that the charge will go through the little 'gap' he drew.

Then in region 2, there is only a magnetic field, so there is no electric field in that region, so there is zero force on the charge due to the electric field. The only force on the charge in this region is due to the magnetic field.

Sort of related to the OP's question, but at a higher level than is necessary: The only real difference between the E and B fields is that there are no magnetic monopoles. So if we communicated with an alien civilization, to try to tell them which we define to be E field and which we define to be B field, the only way to do this is to say that the B field is the one which has no monopoles associated with it.

Edit: or you could describe one of the consequences of there being no magnetic monopoles. e.g. the magnetic field does not diverge. Then the alien would know which field you are talking about. Essentially, any difference in the theoretical definitions of the E and B fields can be attributed to the statement "there are no magnetic monopoles".
micromass
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Mar2-13, 11:07 AM
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Quote Quote by ModusPwnd View Post
Cant transform an electric field completely away though. I think his remark was snarky and missed the point of the question. There is a difference. Yes, you can transform some of one into the other. Regardless, there is a difference. I think its just needless showing off... If somebody asked what was the difference between energy and momentum you wouldn't reply "no difference" and appeal to relativity. No, you would explain the difference and then add in some about their connection.
Quote Quote by WannabeNewton View Post
Well the OP's question was rather ambiguous and vague to begin with so it is hard to gauge (no pun intended ) what to respond with. If the OP was more specific then more detailed responses could be constructed but as it is stands, the OP should have researched his very broad question first, otherwise it is only natural to get a large spectrum of responses. I don't see how it is showing off and honestly I think it motivates people to go learn about the deep connections between the E field, B field, and special relativity because this is one of the most amazing things about electromagnetism imo.
You can't blame vanhees here for giving a response that is too advanced. The OP really should have put more effort in his question. Some more information would have given us some indication about his current level.

Please report original posts like this, thanks.
Aspchizo
#11
Mar3-13, 09:03 PM
P: 26
Thanks for the replies, reading over them now.

I got a warning "Asking questions cold "

Is this because the question is so short and vague? If so, my bad, won't happen again.
BruceW
#12
Mar4-13, 07:25 AM
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Maybe because your first post is unspecific. Usually in the first post, you are supposed to also say more detail of what you want to know / some kind of context to the question / some specific examples of things you are not sure about.
jtbell
#13
Mar4-13, 08:03 AM
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Quote Quote by Aspchizo View Post
Is this because the question is so short and vague?
Yes, that's what it was about. When you ask a question, you should give enough context to indicate the level of answer that you're expecting or are prepared for. Also, even at a given level, a very short "simple" question like yours can be interpreted in different ways, giving different answers which may or may not be what you are really looking for.
Naty1
#14
Mar4-13, 08:39 AM
P: 5,632
Because there was no context to your question, several people posted illustrations...while wannabe posted tensor based answer.....quite an extreme range!!
DaleSpam
#15
Mar4-13, 09:54 AM
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Quote Quote by ModusPwnd View Post
Cant transform an electric field completely away though.
In some cases you can. If you have an E and a B field in a given frame and if B-E/c is greater than 0 then there is a reference frame where E=0.
BruceW
#16
Mar4-13, 03:35 PM
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true that. Also, to be able to completely transform away an electric field, the original B field must be perpendicular to the original E field at all points (due to the pseudoscalar invariant). I guess moduspwnd meant that you can't transform away a general electric field.


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