4.18. (a) A magnetic field that varies in magnitude from point to point but has a constant direction (east to west) is set up in a chamber. A charged particle enters the chamber and travels undeflected along a straight path with constant speed. What can you say about the initial velocity of the particle?
The charged particle is not deflected by the magnetic field even while having a non-zero velocity; therefore, its initial velocity must be either parallel or anti-parallel to the magnetic field, i.e., its velocity is either in the east or west direction.