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I would tentatively say that it depends on the phenomenon, but I'm not quite sure what you mean. Could you elaborate?
i mean can an electric field exist without a magnetic field and vice versa ?
For any given observer, yes -- however, somebody moving with respect to you might see a different mix.
how is that possible ?
I'll give you an example. Imagine I have a metallic loop and a bar magnet. My bar magnet will be fixed on a platform, and I will be rolling toward it with my loop on a cart.|dw:1360721069350:dw|
For me, sitting on my cart, I see a magnet approaching. This causes a changing magnetic flux through my loop, which creates and electric field WITHIN my loop, forcing the electrons to move around and create a current.
My friend, however, sits on the magnet. All he sees is the electrons in my loop moving through a magnetic field. He knows that charged objects moving through a magnetic field feel a force due to the magnetic field, and he calculates that there should be a current flowing.
We do our separate calculations and find out that the current we predict is exactly the same, even though mine was due to an induced electric field and his was due to a purely magnetic force on my charges. Special relativity resolves this strange paradox by revealing that electric fields and magnetic fields transform into one another when you change reference frames. Therefore, one observer may see an effect he claims to be magnetic, while another might attribute it to some electric effect, but they would both calculate the same effect on the system at the end of the day, so they're equally right.
Thank you so much ! But what would happen in a single reference frame ? Can they be independent ?
They can appear to be, yes. For instance, if you charge the plates of a capacitor and then stand next to it, you observe a static electric field and no magnetic field.
Thank you very much !