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# Electron Enters Magnetic Field

APPHEM-XDSHNB

An electron enters a uniform magnetic field with a path perpendicular to the field lines and moves in a circular path of radius $R$.

A positron (same mass of an electron but opposite charge) enters the same magnetic field in a path perpendicular to the field with four times the kinetic energy.

Compared to the electron’s path, the radius of the proton’s circular path is:

A