JEE Main & Advanced Physics Current Electricity, Charging & Discharging Of Capacitors / वर्तमान बिजली, चार्ज और कैपेसिटर का निर Question Bank Electric Conduction Ohms and Resistance

  • question_answer
    A current I is passing through a wire having two sections P and Q of uniform diameters d and d/2 respectively.  If the mean drift velocity of electrons in sections P and Q is denoted by vP and vQ respectively, then    [Roorkee 1999]

    A)            \[{{v}_{\text{P}}}={{v}_{\text{Q}}}\]                            

    B)            \[{{v}_{\text{P}}}={{v}_{\text{Q}}}\]

    C)            \[{{v}_{\text{P}}}=\frac{1}{4}{{v}_{\text{Q}}}\]

    D)            \[{{v}_{\text{P}}}=\text{ 2}{{v}_{\text{Q}}}\]

    Correct Answer: C

    Solution :

                 Drift velocity \[{{v}_{d}}=\frac{i}{neA}\Rightarrow {{v}_{d}}\propto \frac{1}{A}\text{ or}\,{{v}_{d}}\propto \frac{1}{{{d}^{2}}}\]            \[\Rightarrow \,\frac{{{v}_{P}}}{{{v}_{Q}}}={{\left( \frac{{{d}_{Q}}}{{{d}_{P}}} \right)}^{2}}={{\left( \frac{d/2}{d} \right)}^{2}}=\frac{1}{4}\Rightarrow {{v}_{P}}=\frac{1}{4}{{v}_{Q}}\].     


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