Quiz06_solutions

Quiz06_solutions - Physics 222 Name:_ Section:_ Quiz 6A...

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Physics 222 Fall 2011 Name:_______________ Section:______________ Quiz 6A A long, thin wire is wrapped a number of times around a non-conducting cylinder forming a coil (a solenoid.) It is being unraveled such that it loses windings at a constant rate. A magnetic field is directed along the axis of the cylinder such that when you look from one side, it points away from you. Let the positive direction be toward you (i.e. let the surface perpendicular to the axis have an area vector pointed toward you.) a) What is the sign (+ or −) of the magnetic flux (total flux through all loops of the coil)? Describe/show your reasoning. Since the magnetic field and the area vector are pointed in opposite directions, the magnetic flux must be negative. b) What is the sign (+ or −) of the rate at which the magnetic flux through the coil changes? Describe/show your reasoning (Hint: as the coil unravels, the area of the coil changes.) Since the coil is collectively losing area as the wire is unraveled, the magnitude of the flux is decreasing. Since the magnetic flux is negative, and its magnitude is decreasing, the rate is positive. c) What is the direction (clockwise or counterclockwise) of the induced current in the wire? Describe/show your reasoning. Since the magnitude of the flux is decreasing, the induced field generated by the current must act to reinforce the external field. Since the induced field is pointed away from you, by the right hand rule the current must run clockwise in the coil.
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Quiz06_solutions - Physics 222 Name:_ Section:_ Quiz 6A...

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