VIT Engineering VIT Engineering Solved Paper-2015

  • question_answer
    A condenser of capacitance C is fully charged by a 200V supply. It is then discharged through a small coil of resistance wire embedded in' thermally insulated block of specific heat 250 J/kg-K and of mass 100 g. If the temperature of the block rises by 0.4 K, then the value of C is

    A) 300 \[\mu F\]                    

    B) 200 \[\mu F\]

    C) 400 \[\mu F\]                    

    D) 500 \[\mu F\]

    Correct Answer: D

    Solution :

    Given that                       V = 200 V                       m = 100g = 0.1 kg                       s  = \[\text{ }\!\!\alpha\!\!\text{ =}\frac{\text{ }\!\!\tau\!\!\text{ }}{\text{I}}\text{=}\frac{\text{qEL }\!\!\theta\!\!\text{ }}{\frac{\text{m}{{\text{L}}^{\text{2}}}}{\text{2}}}\]                      \[\Rightarrow \]           C = ? The energy stored in the capacitor is given by                        \[{{\text{ }\!\!\omega\!\!\text{ }}^{\text{2}}}\text{ }\!\!\theta\!\!\text{ =}\frac{\text{2qEL }\!\!\theta\!\!\text{ }}{\text{m}{{\text{L}}^{\text{2}}}}\]                        \[\because \]                        \[\alpha ={{\omega }^{2}}\theta \] This energy is used to heat up the block. Let \[\Rightarrow \] be the rise in temperature, then heat energy is given by         \[{{\omega }^{2}}=\frac{2qE}{mL}\]                     = 10 J Now,              \[T=\frac{2\pi }{\omega }=2\pi \sqrt{\frac{mL}{2qE}}\] \[\frac{T}{4}.\]        \[\text{t=}\frac{\text{T}}{\text{4}}\text{=}\frac{\text{ }\!\!\pi\!\!\text{ }}{\text{2}}\sqrt{\frac{\text{mL}}{\text{2qE}}}\] \[\text{250Jk}{{\text{g}}^{\text{-1}}}{{\text{K}}^{\text{-1}}}\]           \[\Delta \theta =0.4K\]       


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