This question already has an answer here:
When the initial velocity is perpendicular to the magnetic field, the force acting on the charged particle
F = qvBsin(theta)
F = qvBsin(90)
F = qvB
In my textbook, it is given that the force will continually deflect the particle making it move in a circle. It also says that as the magnetic field acts on a particle perpendicular to its velocity, it does not do any work on the particle, it doesn't change the kinetic energy of the speed or the particle.
However, how come that it doesn't change the speed or the kinetic energy? We do have force acting on the particle which means that there will certainly be some accerleration.