感抗：XL = ωL = 2πfL=2*3.14*50*0.5 = 157欧姆，电流I=U/XL=220/157=1.4A
电流有效值I=U/R=U/(WL)=220/(2*pi*f*L)=220/(2*3.14*50*0.5)=1.4A.
