College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 81

Figure P19.35a is a diagram of a device called a velocity selector, in which particles of a specific velocity pass through undeflected while those with greater or lesser velocities are deflected either upwards or downwards. An electric field is directed perpendicular to a magnetic field, producing an electric force and a magnetic force on the charged particle that can be equal in magnitude and opposite in direction (Fig. P19.35b) and hence cancel. Show that particles with a speed of v = E / B will pass through the velocity selector undeflected.