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I've looked through about 20 different explanations, from the most basic to the most complex, and yet I still dont understand this basic concept. Perhaps someone can help me.

I dont understand the difference between the electric and magnetic force components in electromagnetism.

I understand that an electric field is created by electrons and protons. This force is attractive to particles carrying opposite charge and repulsive to like-charge particles.

So then you get moving electrons and all of a sudden you have a "magnetic" field. I understand that the concept of a magnetic field is only relative to your frame of reference,

  1. but there's no ACTUAL inherent magnetic force created, is there?

  2. Isnt magnetism just a term we use to refer to the outcomes we observe when you take a regular electric field and move it relative to some object?

  3. Electrons tend to be in states where their net charge is offset by an equivalent number of protons, thus there is no observable net charge on nearby bodies. If an electron current is moving through a wire, would this create fluctuating degrees of local net charge? If that's the case, is magnetism just what happens when electron movement creates a net charge that has an impact on other objects? If this is correct, does magnetism always involve a net charge created by electron movement?

  4. If my statement in #2 is true, then what exactly are the observable differences between an electric field and a magnetic field? Assuming #3 is correct, then the net positive or negative force created would be attractive or repulsive to magnets because they have localized net charges in their poles, correct? Whereas a standard electric field doesnt imply a net force, and thus it wouldnt be attractive or repulsive? A magnetic field would also be attractive or repulsive to some metals because of the special freedom of movement that their electrons have?

  5. If i could take any object with a net charge, (i.e. a magnet), even if it's sitting still and not moving, isnt that an example of a magnetic field?

  6. I just generally dont understand why moving electrons create magnetism (unless i was correct in my net charge hypothesis) and i dont understand the exact difference between electrostatic and magnetic fields.

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4 Answers 4

up vote 9 down vote accepted

So then you get moving electrons and all of a sudden you have a "magnetic" field.

But at the same time if you take a magnetic dipole ( a magnet as we know it) and move it around you will all of sudden get an electric field.

It was a great step forward in the history of physics when these two observations were combined in one electromagnetic theory in Maxwell's equations..

Changing electric fields generate magnetic fields and changing magnetic fields generate electric fields.

The only difference between these two exists in the elementary quantum of the field. The electric field is a pole, the magnetic field is a dipole in nature, magnetic monopoles though acceptable by the theories, have not been found.

Electric dipoles exist in symmetry with the magnetic dipoles

electric dipole magnetic  dipole

electric dipole field lines magnetic dipole field lines

1 but there's no ACTUAL inherent magnetic force created, is there?

There is symmetry in electric and magnetic forces

(the next is number 2 in the question)

2 Isnt magnetism just a term we use to refer to the outcomes we observe when you take a regular electric field and move it relative to some object?

Historically magnetism was obsereved in ancient times in minerals coming from Magnesia, a region in Asia Minor. Hence the name. Nothing to do with obvious moving electric fields.

After Maxwell's equation and the discovery of the atomic nature of matter the small magnetic dipoles within the magnetic materials building up the permanent magnets were discovered.

3 Electrons tend to be in states where their net charge is offset by an equivalent number of protons, thus there is no observable net charge on nearby bodies. If an electron current is moving through a wire, would this create fluctuating degrees of local net charge? If that's the case, is magnetism just what happens when electron movement creates a net charge that has an impact on other objects? If this is correct, does magnetism always involve a net charge created by electron movement?

No . See answer to 2. Changing magnetic fields create electric fields and vice versa. No net charges involved.

4 If my statement in #2 is true, then what exactly are the observable differences between an electric field and a magnetic field? Assuming #3 is correct, then the net positive or negative force created would be attractive or repulsive to magnets because they have localized net charges in their poles, correct? Whereas a standard electric field doesnt imply a net force, and thus it wouldnt be attractive or repulsive? A magnetic field would also be attractive or repulsive to some metals because of the special freedom of movement that their electrons have?

No. A magnetic field interacts to firs order with the magnetic dipole field of atoms. Some have strong ones some have none. A moving magnetic field will interact with the electric field it generates with the electrons in a current.

5 If i could take any object with a net charge, (i.e. a magnet), even if it's sitting still and not moving, isnt that an example of a magnetic field?

A magnet has zero electric charge usually, unless particularly charged by a battery or whatnot. It has a magnetic dipole which will interact with magnetic fields directly. See link above.

6 I just generally dont understand why moving electrons create magnetism (unless i was correct in my net charge hypothesis) and i dont understand the exact difference between electrostatic and magnetic fields.

It is an observational fact, an experimental fact, on which classical electromagnetic theory is based, and the quantum one. Facts are to be accepted and the mathematics of the theories fitting the facts allow predictions and manipulations which in the case of electromagnetism are very accurate and successful, including this web page we are communicating with.

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Thank you very much for taking the time to correct my mistakes and explain it to me! –  user1299028 Feb 14 '13 at 14:35
    
Have you heard of en.wikipedia.org/wiki/Relativistic_electromagnetism, -- the basis of Einstein's special relativity. When you say that magnetism comes from "magnetic dipoles" but not from charges, produced by motion, do you say that Einstein's electrodynamics is a bullshit? Is it popular to disdain the Einstein's theory these days? –  Val May 7 '13 at 7:28
    
I'm also trying to understand this, and Wikipedia's article on Relativistic Electromagnetism seems very helpful. It seems to be a conceptualisation in which there is "just" charge, and magnetism is the relativistic squashing together of charge, but I've also asked a question about this, so don't trust me! –  Benjohn May 1 at 18:01
    
@Benjohn As an experientalist it seems simpler to me to anchor my understanding on the data, which is how I have answered this question. Now once one has a theory that fits the known data and predicts accurately new experimental situations, one can use any type of isomorphic assumptions, use the mathematics and get the same results. This is the case of just using charges and the Lorenz transformation to express the predictions for measurements. This does not eliminate electric and magnetic fields, just describes them in a different basis. –  anna v May 1 at 18:21
    
@Val the above comment is also addressed to your comment which I just noticed today. –  anna v May 1 at 18:22

As you pointed out correctly, a magnetic field is created by a moving charge (current) with respect to the observer. If the charge is at rest with respect to the observer, only an electric field is seen. The fact that a magnetic field looks the way it does can be derived by making use of a Lorentz transformation.

Along those lines, the answer to your question regarding net charge is that the relevant quantity is net current. The magnetic field follows the superposition principle: if you have an equal (in magnitude) and opposite (in sign) current, the magnetic fields cancel.

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I will give an intuitive explanation (not too rigorous).

The electromagnetic force is given by:

$$\vec F=q\left(\vec E+\vec v\times\vec B\right)$$

Therefore, a stationary object only experiences an electric field while a moving object experiences an electric field and magnetic field.

Furthermore, Gauss's law for electricity (one of Maxwell's equations) is: $$\oint\oint\vec E\cdot\mbox{d}\vec S=\frac{q}{\epsilon_0}$$

Nore: The 2 integrals should be a single one with a large circle around but I don't know how to do that.

Gauss's law for magnetism, on the other hand is: $$\oint\oint\vec B\cdot\mbox{d}\vec S=0$$

Furthermore, a complex electromagnetic tensor is defined to put the 4 Maxwell's equations into a single one. In differential form, using abstract index notation,

$$\nabla^aW_{ab}=\frac{4\pi J}{\epsilon_0}$$

Then, the real part of this tensor in Minkowski spacetime is:

$$F_{\mu\nu} = \begin{bmatrix} 0 & E_x/c & E_y/c & E_z/c \\ -E_x/c & 0 & -B_z & B_y \\ -E_y/c & B_z & 0 & -B_x \\ -E_z/c & -B_y & B_x & 0 \end{bmatrix}$$

In other words, the Electricic Field and the Magnetic Field are in different places of the electromagnetic field tensor, like how momentum density and shear stress are in different places of the stress-energy-momentum tensor.

So, I would say they are different components of another field (electromagnetic (2,0) tensor field).

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Consider that there are two fields at the point:

  • electric field $\textbf E$, and
  • magnetic field $\textbf B$

Both fields act on charge, but in different manner.

Electric field is acting on charge as follows:

  • it accelerates the momentum vector of the charged object in the direction of the vector $\textbf E$, and the magnitude of acceleration is proportional to the magnitude of the vector $|\textbf E|$

Magnetic field is acting on charge as follows:

  • it rotates the momentum vector of the charged object around the direction of the vector $\textbf B$, and the magnitude of rotation is proportional to the magnitude of the vector $|\textbf B|$

The combination of acceleration and rotation is known as Lorentz transformation. If you are familiar with Special Relativity, you should know that parameters of the Lorentz transformation are dependent on the reference frame. That is why electric and magnetic fields transform into each other with the change of reference frame.

Something that looks like acceleration in one frame, might look as combination of acceleration and rotation in other frame.

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