A magnetic field of 0.27 T exists in the region enclosed by a solenoid that has 599 turns and a diameter of 9.29 cm. Within what period of time must the field be reduced to zero if the average magnitude of the induced emf within the coil during this time interval is to be 12.8 kV


  1. Answer: 8.54\times 10^{-5}\ s



    The initial magnetic field is B=0.27\ T

    No of turns N=599\ \text{turns}

    Diameter of the solenoid d=9.29\ cm

    Induced EMF E=12.8\ kV

    Induced emf is the product of no of turns and rate of change of flux.

    \Rightarrow E=-N\cdot \dfrac{\Delta \phi }{\Delta t}\\\\\Rightarrow E=-N\cdot \dfrac{\Delta (B\cdot A)}{\Delta t}\\\\\Rightarrow E=-NA\cdot \dfrac{\Delta B}{\Delta t}\\\\\text{Insert the values}\\\\\Rightarrow 12.8=-599\times \pi r^2\cdot \dfrac{(0-B)}{\Delta t}\\\\\Rightarrow \Delta t=\dfrac{599\times \pi \times (4.64\times 10^{-2})\times 0.27}{12.8\times 10^3} \\\\\Rightarrow \Delta t=854.71\times 10^{-7}\ s\\\\\text{Taking absolute value}\\\Rightarrow \Delta t=8.54\times 10^{-5}\ s

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