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Q4. An infinite long metal rod of radius a has an off-center cylindrical hole (radius b) through its entire length, as shown in Figure 4. The rod axis and hole axis are parallel to each other with a distance of c. This hollow conductor carries a current / that is uniformly distributed throughout the cross section. (a) Find the magnitude of the magnetic field at the point O and P. Point O is at the axis of rod, and point P is at the axis of hole. (b) When a = 2.0 cm, b = 0.8 cm, c = 0.5 cm, and / = 40 A, calculate the B value at the point O and P. a b Figure 4