题目

如图所示，R1=R2=R3=R，电源内阻不计，电流表、电压表均理想．两表的示数分别为0.3A、2.0V，则电源电动势为                                                                                                V，R=                  Ω．若将两表的位置互换，则电压表示数为                                                                                              V，电流表示数为       A．                                                                                                                                                                                                               答案：4，20，4，0．

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