Question

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?

Answer 1

The force on a charged particle moving in a magnetic field is F = q(\(\bar { v }\) x \(\bar { B }\)) = qvB sin θ. If θ = 0° or 180°, force is zero and the particle goes undeflected. So its initial velocity is parallel to \(\bar { B }\).