Relativistic electromagnetism

[Criticalsandy]

I have read:
F=q(v*B)

where q=charge of the particle
v=velocity of the charge particle
B=magnetic flux density
F= force experienced by the charged particle

Now,I could not understand what is the velocity v with respect to?

Please help me.

[I_am_learning]

V is with respect to that frame of reference from which you are observing that B.
Remember B field also needs frame of reference.
It was a nice question though. And, BTW, Welcome to P.F.

[jtbell]

Remember, the magnetic field is only part of the electromagnetic field. In relativity, you cannot escape dealing with the other part (the electric field) also, when switching between different reference frames (points of view):

\vec F = q (\vec E + \vec v \times \vec B)

When you switch from one reference frame to another, not only does v change, but so do E and B. If you start with only a B field in one frame (zero E field), in some other frame (different v) you also have an E field, in general. Likewise if you start out with only an E field in one frame, in another frame you have both an E and a B field.

[jfy4]

Purcell's book on E&M is fantastic for this sort of thing, I highly recommend it.