186 CHAPTER 20 20.27 Since the magnetic force, sin m Fq v B θ =, on a positive charge is directed toward the top of the bar when the velocity is to the right, the right hand rule says that the magnetic field is directed into the page . 20.28When the switch is closed, the current from the battery produces a magnetic field directed toward the right along the axis of both coils. (a) As the battery current is growing in magnitude, the induced current in the rightmost coil opposes the increasing rightward directed field by generating a field toward to the left along the axis. Thus, the induced current must be left to right through the resistor. (b) Once the battery current, and the field it produces, have stabilized, the flux through the rightmost coil is constant and there is no induced current . (c) As the switch is opened, the battery current and the field it produces rapidly decrease in magnitude. To oppose this decrease in the rightward directed field, the
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