# Second Year Physics Electrmagnetic Induction

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$15.7 The Fig.Q.15.7 shows a coil of wire in the xy plane with a magnetic field directed along the y -axis.Around which of the three coordinate axes should the coil be rotated in order to generate an emf and a current in the coil?$

$15.17 a) Can a step-up transformer increase the power level?$

$15.16 A copper ring has a radius of 4.0 \mathrm{~cm} and resistance of 1.0 \mathrm{mS} . A magnetic field is applied over the ring perpendicular to its plane. If the magnetic field increases from 0.2 \mathrm{~T} to 0.4 \mathrm{~T} in a time interval of 5 \times 10^{-3} \mathrm{~s} what is the current in the ring during this interval?(Ans: 201A)$

$15.8 How would you position a flat loop of wire in a changing magnetic field so that there is no emf induced in the loop?$

$15.14 The back emf in a motor is 120 \mathrm{~V} when the motor is turning at 1680 \mathrm{rev} per min. What is the back emf when the motor turns 3360 rev per min?(Ans: 240 V)$

$Example 15.5: A solenoid coil 10.0 \mathrm{~cm} long has 40 turns per \mathrm{cm} . When the switch is closed the current rises from zero to its maximum value of 5.0 \mathrm{~A} in 0.01 \mathrm{~s} . Find the energy stored in the magnetic field if the area of cross-section of the solenoid be 28 \mathrm{~cm}^{2} .$