A cosmic$-$ray proton in interstellar space has an energy of $23\mathrm{.}5$ MeV and executes a circular orbit having a radius equal to that of Mars' orbit around the Sun $(2\mathrm{.}281011$ m$).$ What is the magnetic field $($in T$)$ in that region of space?

T

$($b$)$

What If$?$ The cosmic ray proton enters our solar system where the interplanetary magnetic field has a magnitude of $5\mathrm{.}00$ nT and is perpendicular to the velocity of the proton. What is the radius $($in m$)$ of the proton's circular orbit in this field?

m