Manipal Engineering Manipal Engineering Solved Paper-2008

  • question_answer
    In a coil when current changes from 10A to 2A in time 0.1 s, induced emf is 3.28V. What is self-inductance of coil?

    A)  4H                         

    B)         0.4H

    C)  0.04H                   

    D)         5H

    Correct Answer: C

    Solution :

    If a current \[i\] is passed in the circuit and it is changed with a rate\[\frac{di}{dt}\], the induced emf\[e\]produced in the circuit is directly proportional to the rate of change of current. Thus,                                 \[e\propto \frac{di}{dt}\] When the proportionality constant is removed, the constant L comes here. \[\therefore \]                  \[e=-L\frac{di}{dt}\]                       ... (i) The minus sign here is a reflection of Lenzs law. Here,\[di=(2-10)A=-8A\],             \[dt=0.1\,\,s\] \[e=3.28\,\,V\]. \[\therefore \]  \[3.28=\frac{L(-8)}{0.1}\] \[\therefore \]  \[L=\frac{3.28\times 0.1}{8}=0.04\,\,H\]


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