Question

如圖所示，電流表讀數(shù)為0.75A，電壓表讀數(shù)為2V，R₃=4Ω，經(jīng)過一段時(shí)間后某一電阻燒斷，電流表讀數(shù)變?yōu)?.8A，電壓表讀數(shù)變?yōu)?.2V，設(shè)電表對電路的影響不計(jì)，試問：                                                   

（1）燒斷的是哪一個(gè)電阻？                                                                                        

（2）電源的電動(dòng)勢和內(nèi)阻分別是多少？                                                                      

                                                                                           

                                                                                                                                    

Answer 1

（1）因某電阻燒斷，電流表、電壓表示數(shù)均增大，若R₁、R₃斷路，電流表或電壓表無讀數(shù)，可斷定發(fā)生故障的電阻是R₂．

（2）由R₂燒斷后：電壓表的示數(shù)等于路端電壓，則R₁==Ω=4Ω

根據(jù)閉合電路歐姆定律得 U=E﹣Ir即有  3.2=E﹣0.8r…①

R₂未燒斷時(shí)：路端電壓U′=R₁I′=4×0.75V=3V

通過R₃的電流I₃==A=0.25A

總電流：I₀=I₃+I′=0.25A+0.75A=1.0A

則有U′=E﹣I₀r

即 3=E﹣r…②

聯(lián)立①②解得 E=4V r=1Ω

答：

（1）發(fā)生故障的電阻是R₂．

（2）電源的電動(dòng)勢和內(nèi)阻各為4V和1Ω．