# Does protons moving parallel to each other exert magnetics force

If two protons are moving parallel to each other in same direction with equal velocities then do they exert magnetic force on each other.? As protons are moving they should produce magnetic field and there should be some magnetic force + electric force on each of them. But if look from their refrence frame then both the protons are at rest and hence there will be no magnetic force (as magnetic field is created by a moving charge), only electric force on each of them. How can this be possible that force(magnetic force) on a particle becomes different on changing the reference frame even though the reference frames are non accelerating with respect to each other?

Whether they exert both electric and magnetic forces on each other, or only electric forces, depends on which reference frame you use. Electric and magnetic fields transform into each other under changes of reference frame. This is why we talk about the electromagnetic field as if it were one unified tensor field $$\mathbf{F}$$ rather than two different vector fields $$\vec{E}$$ and $$\vec{B}$$.