NEET AIPMT SOLVED PAPER SCREENING 2005

  • question_answer
                    A network of four capacitors of capacity equal to \[{{C}_{1}}=C,\,\,{{C}_{2}}=2C,\,\,{{C}_{3}}=3C\] and \[{{C}_{4}}=4C\] are connected to a battery as shown in the figure The ratio of the charges on \[{{C}_{2}}\] an \[{{C}_{4}}\] is:

    A)                                                            \[\frac{22}{3}\]                

    B)                 \[\frac{3}{22}\]

    C)                 \[\frac{7}{4}\]                   

    D)                 \[\frac{4}{7}\]

    Correct Answer: B

    Solution :

                              Key Idea: Charge on a capacitor is the product of capacitance and potential difference across it.                 The charge flowing through \[{{C}_{4}}\] is                 \[{{q}_{4}}={{C}_{v}}\times V=4CV\]                 The series combination of C1, C2 and C3 gives                 \[\frac{1}{C'}=\frac{1}{C}+\frac{1}{2C}+\frac{1}{3C}\]                 \[=\frac{6+3+2}{6C}=\frac{11}{6C}\]                         \[\Rightarrow C'=\frac{6C}{11}\]                 Now, C? and \[{{C}_{4}}\] form parallel combination giving                 \[C''=C'+{{C}_{4}}\]                 \[=\frac{6C}{11}+4C=\frac{50\,C}{11}\]                 Net charge \[q=C''V\]                 \[=\frac{50}{11}CV\]                 Total charge flowing through \[{{C}_{1}},\,{{C}_{2}},\,{{C}_{3}}\] will be                 \[q'=q-{{q}_{4}}\]                 \[=\frac{50}{11}CV-4C\,V=\frac{6CV}{11}\]                 Since, \[{{C}_{1}},\,{{C}_{2}}\] and \[{{C}_{3}}\] are in series combination hence, charge flowing through these will be same.                 Hence,                 \[{{q}_{2}}={{q}_{1}}={{q}_{3}}=q'=\frac{6\,C\,V}{11}\]                 Thus,     \[\frac{{{q}_{2}}}{{{q}_{4}}}=\frac{6CV/11}{4CV}=\frac{3}{22}\]


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