15.1 An emf of 0.45V is induced between the ends of a metal bar moving through a magnetic field of 0.22 T. What field strength would be needed on emf of 1.5 V between the ends of the bar, assuming that all other factors remain the same? (Ans; 0.73 T)



15.6 A pair of adjacent coils has a mutual inductance of 0.75 H. If the current in the primary changes from 0 to 10 A in 0.25 s, what is the average induced emf in the secondary? What is the change in flux in it if the secondary has 500 turns? (Answer: 300 V, 1.5 𝐱 10⁻¹ Wb). 



15.7 A solenoid has 250 turns and its self inductance is 2.4 mH. What is the flux through each turn when the current is 2 A? What is the induced emf when the current changes at 20 A s⁻¹? (Answer: 1.5 𝐱 10⁻⁵ Wb, 48 mV). 




15.10 Like any field, the Earth's magnetic field stores energy. Find the magnetic energy stored in a space where strength of earth's field is 7 ｘ10⁻⁵ T, if the space occupies an area of 10 ｘ10⁸ m² and has a height of 750 m. (Answer: 1.46 ｘ10⁹ J). 



15.11 A square coil of side 16 cm has 200 turns and rotates in uniform magnetic field of magnitude 0.05 T. If the peak emf is 12 V, what is the angular velocity of the cell? (Ans: 47 rad s⁻¹). 



15.16 A copper ring has radius of 4.0 cm and resistance of 1 mΩ. A magnetic field applied over the ring, perpendicular to its plane. If the magnetic field increases from 0.2 T to 0.4 T in a time interval of 5 ｘ10⁻³ s. What is the current in the ring during? (Ans: 201 A).