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The magnetic field for to a moving charge depend on its velocity (Biot Savart's law). My question is that is it then not frame dependent? If it is, it means if a man is walking and other is standing magnetic field at same point to both will be different. But magnetic field at that point can have only on "constant value". Can you please explain this.

PS I know nothing about special theory of relativity. But I read on Google it has got something to do with "Lorentz force"

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I'm afraid your statement that magnetic field at a point can have only on(e) "constant value" is not true, and you will have to learn a bit about special relativity to understand why.

There is a classic thought experiment, which is to imagine the electromagnetic fields due to a charged particle at rest in what we'll call the stationary frame. Clearly, the electric field is radial and diminishes as $r^{-2}$ as we move away from the particle. There is NO magnetic field.

But now consider what this looks like from the point of view of an observer moving past the charge at high speed. They will "see" a moving charge - or a current. This moving charge will generate an electric field (but one that is different at any point in space from that measured in the stationary frame), but more importantly, they would be able to detect a magnetic field due to the current.

So, electric field and magnetic field are not invariant when measured in different reference frames - there are a set of transformations that allow you to work out how they change.

http://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity#The_E_and_B_fields

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But magnetic field at that point can have only on "CONSTANT VALUE".

Not true. If a particle is acted on by some combination of electrical and magnetic forces in one frame of reference, then in another frame of reference, it will be a different combination of electrical and magnetic forces. It's possible to have a force that's purely electrical in one frame and purely magnetic in another.

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