# Can someone please explain magnetic vs electric fields?

I've looked through about 20 different explanations, from the most basic to the most complex, and yet I still don't understand this basic concept. Perhaps someone can help me.

I don't 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. Isn't 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 doesn't imply a net force, and thus it wouldn't 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, isn't that an example of a magnetic field?

6. I just generally don't understand why moving electrons create magnetism (unless i was correct in my net charge hypothesis) and I don't understand the exact difference between electrostatic and magnetic fields.

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:
$\hspace{50px}$ $\hspace{50px}$ .$$\begin{array}{c} \textit{electric dipole field lines} \\ \hspace{250px} \end{array} \hspace{50px} \begin{array}{c} \textit{magnetic dipole field lines} \\ \hspace{250px} \end{array}$$

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)

1. Isn't 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 observed 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.

1. 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.

1. 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 doesn't imply a net force, and thus it wouldn't 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.

1. If i could take any object with a net charge, (i.e. a magnet), even if it's sitting still and not moving, isn't 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.

1. I just generally don't understand why moving electrons create magnetism (unless i was correct in my net charge hypothesis) and I don't 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.

• Thank you very much for taking the time to correct my mistakes and explain it to me! – user1299028 Feb 14 '13 at 14:35
• 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 '14 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 '14 at 18:21
• @Val the above comment is also addressed to your comment which I just noticed today. – anna v May 1 '14 at 18:22
• @annav Thank you. I think your use of "isomorphic" is apposite. I hope I'll get the time to look in to the subject further and see how the two interrelate. I imagine considering both view points together will be instructive. – Benjohn May 1 '14 at 22:12

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.

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.

The electric and magnetic fields arise as Lorentz duals of each other, with them mixing and transforming between each other through Lorentz boosts. The full picture of the field comes from the electromagnetic field tensor

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

Which satisfies simple identities (see ) equivalent to Maxwell's equations. The electric and magnetic fields are different components of this tensor, placed in similar positions as e.g. the momemtnum and shear stress in the 4d stress tensor.

• Also, shout out to the whole "division by c" that makes the whole magnetic field less impressive in amplitude – Vendetta Oct 10 '17 at 13:16
• Can this matrix then be viewed as the "stress tensor" of light? – zwep May 20 '19 at 19:19
• @zwep. No. One can see e.g. that this tensor is antisymmetric while the stress energy tensor is symmetric. I can't tell why you think they are related at all. – Abhimanyu Pallavi Sudhir May 20 '19 at 23:07

Another difference is that an electric charge experiences a force in a magnetic field only if it is moving. This is different from an electric field in that a charge experiences a force in an electric field even when it is stationary. Also, the direction of the force in a magnetic field is perpendicular to both the direction of the velocity and the magnetic field lines. In an electric field, the force vector is in the same direction as the electric field lines.

• Why is the electric force connected to a movement? – Ben Sep 11 '19 at 5:33

One difference that i really like is that electric field lines are terminating lines which means that they originate from +ve charge and end at -ve charge whereas the magnetic field lines are non-terminating i.e, they form a loop.

• There are non-conservative closed electric fields. To know more, check induced electric fields. – Yashas Feb 18 '17 at 14:16