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I require only a simple answer. One sentence is enough... (It's for high school physics)

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9 Answers 9

Electric forces are created by and act on, both moving and stationary charges; while magnetic forces are created by and act on only moving charges.

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This would have been my answer as well (aiming for high-school level). As an aside, it also gives an intuitive sense of why, in light of relativity, they're really the same force -- since what's moving or stationary depends on your frame of reference. –  Tim Goodman Feb 21 '11 at 5:50
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To be slightly more specific, I might say "The electric force on a particle is proportionate to its charge but not its velocity. The magnetic force is proportionate to charge and velocity." In other words $\mathbf{F} = q \mathbf{E}$ vs. $\mathbf{F} = q \mathbf{v} \times \mathbf{B}$ –  Tim Goodman Feb 21 '11 at 6:00
    
Very nicely stated. –  Carl Brannen Feb 22 '11 at 4:53
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Electric monopoles exist. Magnetic monopoles don't* exist.

This is the reason that Maxwell's laws governing electricity and magnetism aren't symmetric. Maxwell's laws say that $\nabla \cdot B = 0$ and $\nabla \cdot E = 4\pi\rho_{e}$, but if magnetic monopoles existed this would be $\nabla \cdot B = 4\pi\rho_{m}$ and $\nabla \cdot E = 4\pi\rho_{e}$. A similar asymmetry in induction would be corrected.

The experimental lack of evidence for magnetic monopoles can then be taken as the main reason we distinguish between the two forces.

*(Probably.)

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My question is about the forces specifically. I think it has something to do with static charges vs moving charges (electricity)... something like that... –  Šime Vidas Feb 21 '11 at 3:49
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@Šime Vidas Same statement; Maxwell's laws are the rules for how the force fields behave and are generated. Here's how to think of it: Sources of electric force are (1) electric charges and (2) moving magnetic fields. Sources of magnetic force are (1) moving electric fields. This asymmetry - that there are no "magnetic charges" that can be sources - is the reason that we draw a distinction between electricity and magnetism. –  kharybdis Feb 21 '11 at 3:54
    
Sources of electric force are electric charges, and sources of magnetic force are moving electric charges (conductors, for instance). - this answer should do just fine :) –  Šime Vidas Feb 21 '11 at 4:02
    
@Šime Vidas Fair enough. It's worth thinking about the deep reasons for a bit. Symmetry of this sort turns out to be very, very important in physics; if you can start to think in terms of looking for symmetry, you'll find your intuition for physics problems will grow immensely. Just general advice. This is a particularly elegant example of that. –  kharybdis Feb 21 '11 at 4:05
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The down vote is totally undeserved. This is the correct answer. –  Mark Eichenlaub Feb 21 '11 at 7:18
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For the purposes of high school physics it is reasonable to say that electric fields start and stop on charges but magnetic fields always run in loops.


This fails to be rigorous in at least two ways:

  • Relative velocity mixes the two types of fields in a manner governed by the Lorentz transform, so the truth is like wsc says.
  • It is an open question if there exist any magnetic monopoles, but theoretical prejudice favors their existence for reasons of beauty.
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Is this true?: The electric force is caused by a static charge, and the magnetic force is caused by a moving charge. –  Šime Vidas Feb 21 '11 at 3:45
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@Šime Vidas: Yes it's true. Electric fields are caused by charges (either starting or ending on the charges) and by time varying magnetic fields (but only in loops). Magnetic fields are caused by the motion of charges (in loops around the current) and by time varying electric fields (again, in loops). If there exist magnetic monopoles, then magnetic fields can start and stop on those, but we've never seen them. –  dmckee Feb 21 '11 at 3:50
    
Yea, I think i got it now :) I just need a basic answer, so I'll leave out the scenarios with varying fields. –  Šime Vidas Feb 21 '11 at 3:56
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Unlike electric fields, magnetic fields do no work on charges.

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An electric field doesn't do work on charge moving normal to it. –  Physiks lover Sep 20 '12 at 20:59
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Not to be slick, but starting with Hans Christian Ørsted, then on through James Clerk Maxwell and Albert Einstein we time and again reinforce that there is none -- they are heads and tails of the same coin.

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The question has been set by the teacher and will appear in an exam. I'm pretty sure that "there is none" results in zero points :) –  Šime Vidas Feb 21 '11 at 3:37
    
-1 "there is none" is very misleading. They are components of the electromagnetic field and their effects as defined in the Lorentz force equation are very different. But this is too advanced for what was asked for anyway. –  John McVirgo Feb 21 '11 at 4:11
    
@John: I agree that this is slightly misleading but nevertheless it is absolutely correct. And saying there is a difference without mentioning relativity is at least as misleading ;) Your down-vote is very much undeserved (you should down-vote your answer too, in the same spirit...). –  Marek Feb 21 '11 at 7:51
    
@Marek if Wsc removes "there is none" I will be happy to remove the down vote. –  John McVirgo Feb 21 '11 at 17:34
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Magnetic forces are always normal to the direction of the velocity of the charge it acts upon, whereas electric force are independent of the direction the charge moves in.

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Consult this document by Hans de Vries:

The simplest, and the full derivation of Magnetism as a Relativistic side efect of ElectroStatics

He uses only Electrostactic field and the non-simultaneity to derive the Magnetic Field.

It appears that the Magnetic field is a side effect of movement of charges in the electric field.

If we pretend that the magnetic field exists beyond a mere artifact then the following question must be answered: How can the particles communicate information about its position and velocity so that, in the past, they can adjust the source of the electromagnetic energy?

(*) Hans de Vries has a very interesting online book (not yet finished) in his site.

added:

Magnetic forces are induced by the motion and, much like Coriolis force, is not a real force. ...

wikipedia ..the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating reference frame. (a moving frame induces forces, there is no need to be a rotating one)

ps: it is frustating to be down voted without a comment :( it´s an injustice......yes it is - Calimero.

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Electric fields are divergent, magnetic fields are convergent.

Why the downvote? Anyone?

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Uhm...no. Apply a minus sign and "divergent" and "convergent" switch places. The difference is that electric fields have plentiful sources and sinks and magnetic fields have so few that we've never detected any (i.e. possibly none, though many theorist don't like that option much). –  dmckee Feb 21 '11 at 3:46
    
I'm just gonna walk away from this one and nod my head in agreement. –  Alex C Feb 21 '11 at 3:50
    
What do you think "divergent" and "convergent" mean? The divergence of the magnetic field is zero. dmckee is totally right on this, and to be honest your answer is gibberish. -1 –  Mark Eichenlaub Feb 21 '11 at 4:46
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Electric forces act between charged bodies (charges), magnetic forces act between magnetized bodies (magnets). The latter can be said differently: magnetic forces act between currents of neutralized charges.

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protected by Qmechanic Feb 20 '13 at 15:58

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