A proton moving at a constant velocity 10 times 105 ms enter

A proton moving at a constant velocity 10 times 10^5? m/s enters a relatively unexplored region of space. It continues to move at its initial velocity. Can we assume there are no electric fields and/or magnetic fields present in the vicinity? Explain your reasoning.

Solution

It is given that the The velocity of the proton is not changing when it is passing through the unexplored space.

It means that the net force on the proton is zero.

But this doesnt in anyway imply that there are no electric and magnetic fields . There could be a situation in which both Electric and magnetic forces could exist in such a way that both cancel each other and therefore the net force is zero.

e.g

Let E = E0 in x direction.

Let velocity (v) be in z direction.

Let B be in y direction

Therefore,

F_el =qE in x direction

F_mag= q(vB) in -x direction

So if E/B = v , in that case F_el = F_mag but in opposite directions

So they cancel each other! Net force is zero . So the particle travels as if there is no force