(1)当开关S1闭合、S2断开,滑动变阻器的滑片P置于B端时,等效电路如图甲所示.当开关S1闭合、S2断开,滑动变阻器的滑片P置于A端时,等效电路如图乙所示.当开关S1、S2都闭合,滑动变阻器的滑片P置于B端时,等效电路如图丙所示.乙图中R1和R2串联:有P=UI得:UR1=