Electrons fly straight out of an electron gun with velocity $$v$$. However, since the electron is located in a homogeneous magnetic field $$B$$, which is directed towards the screen plane, the electron is deflected due to Lorentz force. A charged particle like the electron is subjected to a magnetic force $$F$$ (magnetic component of the Lorentz force) when it flies through a magnetic field. The magnetic force is always orthogonal to the velocity, which causes the electron to move circular.