Motion of a charged particle in a uniform magnetic field. The Lorentz force
A charged particle moving in a magnetic field is acted on by the Lorentz force F = |q|·υ·B·sinα, where α is the angle between the velocity and B. Its direction for a positive charge is found with the left-hand rule (it is reversed for a negative charge). If the particle moves along the field lines (α = 0° or 180°) the force is zero; if it enters perpendicular to them (α = 90°) the force is greatest. Because the Lorentz force is perpendicular to the velocity it does no work: the speed and kinetic energy do not change, only the direction does. A particle entering perpendicularly moves uniformly along a circle of radius R = mυ/(|q|B) (the Lorentz force plays the role of the centripetal force), and the period T = 2πm/(|q|B) does not depend on the speed. This principle is used in the mass spectrometer to separate particles by mass: among particles of the same speed and charge the heavier one traces the larger circle.
In your notebook draw the paths of a positive and a negative particle in a field directed into the page (which way does each circle turn?). Explain the same rule using the electron beam in an old cathode-ray tube. Keep strong magnets away from electronic devices and cards.