Problem: A current-carrying wire is bent into a circular loop of radius R and lies in an xy plane. A uniform external magnetic field B in the +z direction exists throughout the plane of the loop. The current has the magnitude of I and it is directed counterclockwise when observing from positive z axis.
Part A: What is the magnetic force exerted by the external field on the loop?
I need help to solve the above problem and express your answer in terms of some or all of the variables I, R, and B.