2、正弦量的有效值 对于正弦电流,设(t)= I sing(ot+g,) I=1÷ Im sin(ot+,)dl [1-cos 2(at lat 2T 2T 0 2 0.707I, 2 同理U=U=0.707U 1212 2、正弦量的有效值 ( ) sin( ) m i 对于正弦电流，设 i t = I t + I I m m m m T T i m T T m i I I t T I t dt T I I t dt 0.707 2 2 2 [1 cos 2( )] 2 ( ) 2 0 2 0 2 0 2 2 1 sin = = = = = − + = +       同理 U Um 707Um 0. 2 1 = =