Do magnetic Feilds Exist?

1. Oct 16, 2009

SpartanG345

Hello sorry if this is noob question... Its my 1st year!

Well we have learn that magnetic fields are created by moving charges, current loop or straight conductor. I realised that in some frames of reference, the magnetic field will not exist, rather it is replace entirely by an electric field.

I also know magnetic fields cannot be isolated and therefore as far as i know there are no fundamental "magnetic" particles. but I have heard of magnetic monopoles.

I also learned that magnetic fields in all materials are actually created by small "current loops" or moving charge interactions between atoms.

Does this mean magnetic fields do not exist, or are a simplification of complex dynamic electric interactions, which can be simplified by changing your frame of reference?

2. Oct 16, 2009

Bob_for_short

It is sufficient to consider one reference frame in which the magnetic field exists. Field mutual transformations do not invalidate their physical sense and existence. For example, the electron is a magnetic dipole in all reference frames.

3. Oct 16, 2009

Stingray

Magnetic fields cannot always be transformed away. It turns out that $E^2-B^2$ is the same in all frames of reference. So is $\vec{E} \cdot \vec{B}$. This means that you cannot change frames in order to make a purely electric field look purely magnetic (or vice versa).

Magnetic fields are produced by moving charges, although many elementary particles also produce them even when at rest. The resultant fields look the same as they would if produced by an ordinary loop of current.

4. Oct 18, 2009

amannikus26

yes
electric fields are funn!

5. Oct 18, 2009

Andrew Mason

"Fields" are one way to model how electric, magnetic and gravitational forces work at a distance. But it is just a model that allows us to give predictable results. Whether there is something "actually there" is probably not answerable. It depends on what is meant by "something" and what is meant by "there".

Richard Feynman put it this way:

"Many different physical ideas can describe the same physical reality. Thus, classical electrodynamics can be described by a field view, or an action at a distance view, etc. Originally, Maxwell filled space with idler wheels, and Faraday with fields lines, but somehow the Maxwell equations themselves are pristine and independent of the elaboration of words attempting a physical description. The only true physical description is that describing the experimental meaning of the quantities in the equation - or better, the way the equations are to be used in describing experimental observations. This being the case perhaps the best way to proceed is to try to guess equations, and disregard physical models or descriptions."

"[URL [Broken] Development of the Space-Time View of Quantum Electrodynamics
Richard Feynman, Nobel Lecture, December 11, 1965
[/URL]

AM

Last edited by a moderator: May 4, 2017
6. Oct 19, 2009

mathfeel

In classical physics, all these electric/magnetic fields business is but a good way to describe effects of matter have on each other's motion when they possess a property call "charge". At the end of the day, no body actually measures electric or magnetic field but velocities and accelerations of the particle whose motion they affect. Nevertheless fields are useful because they make it easy for us to visualize certain effects.

I am still quite unsure about the above statements when we are talking about quantum mechanics though. Not only are electromagnetic field (or Gauge field) treated identically (in principle) to other "matter field" in QFT as just another term in the Lagrangian; there are phenomena like the Aharonov-Bohm effect that tells us not only magnetic field, but the potential also have physical reality.

7. Oct 19, 2009

Bob_for_short

It is not so. Static electric and magnetic forces are measurable directly, so it is not a problem to figure out what the field values are.
In a wave mechanics (QM) the wave is present everywhere. The topology of experimental set is important.

8. Oct 19, 2009

Andrew Mason

And how do you measure forces if not by measuring masses and accelerations in some direct or indirect fashion?

AM

9. Oct 19, 2009

Bob_for_short

By dynamometers, for example. No accelerations, just an elastic force to counter-balance.

10. Oct 19, 2009

Andrew Mason

This is just a clever indirect way of measuring force using mass and acceleration. You have to determine the spring constant in order to measure the electro-magnetic force. You calibrate the spring by measuring masses and accelerations.

AM

11. Oct 19, 2009

Bob_for_short

Let it be!

12. Oct 24, 2011

Dickfore

An electromagnetic field has a magnetic field component in every reference frame. So, yes, magnetic fields exist.