184CHAPTER 20 20.22During each revolution, one of the rotor blades sweeps out a horizontal circular area of radius A, 2Aπ=AverticaBA B=. The number of magnetic field lines cut per revolution is . The induced emf is then BlA⊥∆Φ=()( )25235.010T3.0 m2.810V2.8 mV10.50serticaltf−−×==× =AvBBε∆Φ==∆20.23(a) To oppose the motion of the magnet, the magnetic field generated by the induced current should be directed to the right along the axis of the coil. The current must then be left to right through the resistor. (b) The magnetic field produced by the current should be directed to the left along the axis of the coil, so the current must be right to left through the resistor. 20.24(a) As the bottom conductor of the loop falls, it cuts across the magnetic field lines coming out of the page. This induces an emf of magnitude Bwv=Ein this conductor, with the left end at the higher potential. As a result, an induced current of magnitude BwvIRREflows clockwise around the loop. The field then exerts
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