21.25.Set Up:Use Lenz’s law to determine the direction of the induced current. The force requiredto maintain constant speed is equal and opposite to the force that the magnetic field exerts on the rod because ofthe current in the rod.Solve: (a)(b)is into the page. The flux increases as the bar moves to the right, so the magnetic field of the induced current isout of the page inside the circuit. To produce magnetic field in this direction the induced current must be counter-clockwise, so from bto ain the rod.(c)is to the left. To keep the bar moving to the right at constant speed an external force with magnitude and directedto the right must be applied to the bar.Reflect:The force on the rod due to the induced current is directed to oppose the motion of the rod. This agrees withLenz’s law.21.26.Set Up and Solve:(a)Each slab of flowing blood has maximum width dand is moving perpendicular to thefield with speed v.becomes (b)(c) The blood vessel has cross sectional area
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