Magnetic field changing speed of particle?

[sparkle123]

From my textbook:

The example seems to contradict what is said above. The electron is accelerated in the negative z direction, so the velocity now has both an x-component and a z-component so the speed DOES change (magnitude larger)?

 

[bp_psy]

Since the acceleration/Force is always perpendicular to the direction of motion of the electron only the velocity is changed not the speed. The electron does not gain any KE since W=Fdcos(theta) and theta in this case is pi/2.

 

[sparkle123]

got it thanks!