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posted by  adduhleenuh on 7/24/2008 10:40:19 AM  |  status: Live  

lenz's law

Course Textbook Chapter Problem
Classical Physics Principles of Physics (3rd) by Jewett, Serway 23 20
Question Details:
20. Use Lenz’s law to answer the following questions concerning the direction of induced currents. (a) What is the direction of the induced current in resistor R in Figure P23.20a when the bar magnet is moved to the left? (b) What is the direction of the current induced in the resistor R immediately after the switch S in Figure P23.20b is closed? (c) What is the direction of the induced current in R when the current I in Figure P23.20c decreases rapidly to zero? (d) A copper bar is moved to the right while its axis is maintained in a direction perpendicular to a magnetic field as shown in Figure P31.28d. If the top of the bar becomes positive relative to the bottom, what is the direction of the magnetic field?

Figure 23.20: http://i35.tinypic.com/2a4v7zb.jpg

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posted by Steve (retired) on 7/24/2008 11:49:03 AM  |  status: Live
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answer (a)

 since north pole is being taken away , magnetic flux in left to right direction is decreasing. so, induced current will be to left, counterclockwise direction from end opposite to motion of magnet(right).
 answer (b)
 
 clockwise from left side and out of the page in resistor, as on closing switch, magnetic field is produced in left direction.
 
answer (c)
 
 flux due to straight wire is into the page inside loop according to right hand thumb rule. so, current will be in anticlockwise direction or towards left in resistor, to oppose this flux.
 
answer (d)
 
 force on electrons is clearly towards bottom and on positive charges towards top.
considering the latter,  F = qv x B
 
 so, field is into the page according to converse of of vector product rule.
 
 
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