Final answer:
The electron beam in a focusing cup spreads out due to electrostatic repulsion among the negatively charged electrons.
Step-by-step explanation:
In the focusing cup, all the electrons (e-) are negative; the electron beam tends to spread out due to electrostatic repulsion. The repulsion occurs because objects with the same electric charge exert a force on each other, pushing them apart. This concept is supported by Coulomb's law, which states that the force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. J. J. Thomson first observed phenomena related to cathode rays and made significant discoveries about the electron, including determining its charge-to-mass ratio after deflecting it with both magnetic and electric fields.