Sơ đồ mạch: `(R_Đ //// R_1)nt R_2`
`a)R_Đ=[U_[ĐM] ^2]/[\mathcal P_[ĐM]]=[6^2]/3=12(\Omega)`
`=>R_N=R_2+[R_Đ .R_1]/[R_Đ + R_1]=15+[12.24]/[12+24]=23(\Omega)`
Có: `I=E/[R_N +r]=24/[23+1]=1(A)=I_2=I_[Đ1]`
`U_[Đ1]=I_[Đ1].R_[Đ1]=1.[12.24]/[12+24]=8(V)=U_Đ =U_1`
`@I_Đ =[U_Đ]/[R_Đ]=8/12~~0,67(A)`
`@I_1=[U_1]/[R_1]=8/24~~0,33(A)`
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`b)U_Đ > U_[ĐM] (8 > 6)=>` Đèn sáng mạnh hơn bình thường.
`A=UIt=8.8/12 .10.60=3200(J)`
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`c)` Bỏ `R_1=>`Sơ đồ mạch lúc này: `R_2 nt R_Đ`
`=>R_N =R_2+R_Đ=15+12=27(\Omega)`
Có: `I=E/[R_N+r]=24/[1+27]=6/7(A)=I_Đ`
`=>U_Đ=I_Đ .R_Đ=6/7 .12~~10,29(V) > U_[ĐM] (6 V)`
`=>` Đèn sáng mạnh hơn bình thường.
Có: `H=[R_N]/[R_N +r].100=27/[27+1].100~~96,43%`
