For the surface S+S2, we have d D d dt D·dS+ D·d at at =0+ S d2 According to Gauss's law: 手:4=q() (对本例,在数值上) we have d④ OD dq(t) d 2 d(1+2) ds dt at dt 传For the surface S1+S2 , we have   +  = =  S S D d D dS dt d t Φ I   d d According to Gauss’s law: D dS q(t ) S S  =   +    I 传 dt dq(t ) S t D t Φ I I D d d ( )  = = =   = = =  +    d d d (对本例，在数值上） q(t) L S1 S2 I     =  +  S S D dS dt d D dS dt d     =  + d  I we have