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what is the difference between an electromagnetic field and a magnetic field?

Posted on 2012-03-11
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Last Modified: 2012-06-21
Hi Everyone,

            I am wondering if someone would mine sharing some insights into the differences between an electromagnetic field and a magnetic field.  Any information given to this question will be greatly appreciated.

             I will look forward to reading any scientific explanations given to this post.

             Thank you

             George
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Question by:GMartin
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edster9999 earned 150 total points
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If you have a metal bar magnet with no power attached and you move it over a pin, the pin moves.  That is a magnetic field.

If you have a coil and you pass electricity through it and when the power is turned on the pin moves, turn it off and nothing happens - that is elctromagnetic field.
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by:GMartin
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Hi There,

           Thank you so much for giving very easy to understand examples of the difference between a magnetic field and an electromagnetic field.  With respect to the earth, is it surrounded by a magnetic field?

            George
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by:ozo
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A magnetic field is an electromagnetic field.
An electromagnetic field with only a magnetic component and no electric component must be a static field.
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by:ozo
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If you have a metal bar magnet with no power attached and you move it over a pin, the pin moves.  That is a magnetic field.
True enough.
And when you move it, you also create an electric field.
(if you move it slowly enough, the electric field may be weak enough that you only notice the magnetic component)

It may occur to you that whether or not something is moving relative to you can depend on ones frame of reference.
That is actually a profound observation.
A magnetic field is essentially the way we describe the effects of special relativity on moving electric fields.
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by:GMartin
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Hi

          With regards to the most recent post given by ozo, is it a fair or accurate conclusion to say that magnetic fields and electric fields are interwined and can not be fully separated?  And, could you elaborate more upon your statement that magnetic fields are used to describe the special relativity on moving electric fields?  It sounds like you are making reference to Albert Einstein's Theory of Relativity; perhaps using that to explain magnetic fields.

            Thank you

            George
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by:ozo
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Magnetic fields and electric fields are components of the electromagnetic field.
Changing magnetic fields create electric fields and changing electric fields create magnetic fields.
They are intertwined by Maxwell's equations.
(in a static situation, they could be analyzed separately)

If you understand static electric fields from unmoving charges, and you understand relativistic transforms for motion in Albert Einstein's Theory of Relativity, you will also understand magnetic fields.
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by:aburr
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Ozo's last comment is by far the best. Many of the previous comments are oversimplifications  at best.
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The subject of course is as complicated as you want to make it or as required by the desired use of the answer.
From a general physics non relativistic view point you can consider three fields.
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1.  Magnetic  - set up by a permanent magnet (or coil through which an electric current is flowing), like a compass needle or the earth or a loudspeaker.
2. Electric - set up by charges like an electron or proton or static electricity
3. Electromagnet - which is a field combining the first two in a special way that permits the effects to propagate through space like radio waves or light rays.
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by:gismo51
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The only real difference involve relative motion between the source of the field and the observer.  A magnetic field without motion in any frame of reference implies no related  electrical force or field.  Similarly an electric field (e.g. the presence of a static electrical charge) with no relative motion implies no related magnetic field.  But if you have motion relative to an otherwise static magnetic field there is always an accompanying electric field and an electric charge in motion always involves a related magnetic field.  A marginally famous nuclear physicist (my uncle) once described a magnetic field to me as the "relativistic correction" for the delayed (by the speed of light) response to the motion of an electric charge.   In simplistic terms when an electrical charge is moved the effect of that motion will be propagated at the speed of light and while it's propagating a magnetic field will be present.
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by:ozo
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"Electromagnet" is a noun describing a magnet made with a coil of wire.
"Electromagnetic" is a an adjective describing the fundamental force which is mediated by photons.  Electromagnetic fields can have electric and magnetic components.

Magnetic fields are felt only by moving charges (or magnetic monopoles).
In a frame that is unmoving (relative to the magnetic field),
flowing charges may be electrically neutral and thus produce no net electric field.
But from a moving frame, relativistic transforms may foreshorten charges of one polarity differently than the charges of the opposite polarity, and thus an electric field would be seen.

And it may be more accurate to say that when an electrical charge is accelerated, the effect of that acceleration will be propagated at the speed of light.
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by:GMartin
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Hi Everyone,

            Thanks so much for the insightful feedback given to this post.  To be perfectly honest, I did not fully realize how complicated my question really was until I began reading the feedback given.  While I certainly appreciate the more scientific and technical explanations given,  the most simplistic difference between the two given by edster9999 did make the most sense.  Coming from a limited amount of science exposure in college, I guess it is more difficult for me to comprehend the more technical explanations despite of their truth and accuracy.  At any rate, I went ahead and split up points to help in being fair to those explanations given which also answered this question.  

             Many thanks once again everyone for the help.

             George
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by:ozo
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I wouldn't say that http:#37707218 describes the difference between an electromagnetic field and a magnetic field, because if you have a metal bar magnet with no power attached and you move it over a pin, the pin moves, that is also an electromagnetic field, and if you have a coil and you pass electricity through it, and when the power is turned on the pin moves, that is also a magnetic field.

http:#37707218 might have been an answer if the question was about the difference between a "magnet" and an "electromagnet".
But "magnetic field" and "electromagnetic field" refer to the force, rather than to the devices that produce the force.

"Electromagnetism" refers to all the intertwined phenomenon of electricity and magnetism.
And magnetic fields and electric fields are both manifestations of the electromagnetic field.
The main difference may be more in the usual usage of the words, than in the phenomenon to which they refer.
One might more more commonly speak of electromagnetic fields when one is talking about the propagating waves of mutually generated changing electric and magnetic fields, and one might more commonly speak of electric or magnetic fields, when one is primarily concerned with the static components of the electromagnetic field.
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