阿摩線上測驗
42. The current density in a particle beam with circular cross section of radius a points along the beam axis with a magnitude that decreases linearly from J0 at the center (r = 0) to half that value at the edge (r = a). What is the total current in the beam?
(A) 
(B) 
(C) 
(D)
(E) 
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統計： A(0), B(1), C(2), D(0), E(0) #3840152
統計： A(0), B(1), C(2), D(0), E(0) #3840152
