Answer
Choice C.
Work Step by Step
The magnetic energy stored is $U=\frac{1}{2}Li^2$.
The heat Q needed to evaporate a mass m of liquid is $Q=mL_v$.
The magnetic energy stored in the magnet is converted into thermal energy. This evaporates the liquid helium. U = Q.
$$ \frac{1}{2}Li^2=mL_v$$
$$ m=\frac{ Li^2}{2L_v} =\frac{ (4.4H)(750A)^2}{2(20900J/kg)}=59kg$$
That’s about 60 kg.
