A straight wire segment of length $25 \mathrm{~cm}$ carries a current of $33.0 \mathrm{~A}$ and is immersed in a uniform external magnetic field. The magnetic force on the wire segment has magnitude $4.12 \mathrm{~N}$. (a) What is the minimum possible magnitude of the magnetic field? (b) Explain why the given information enables you to calculate only the minimum possible field strength.