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A long hairpin is formed by bending a very long wire as shown in Fig. 30-43. If

ID: 2005297 • Letter: A

Question

A long hairpin is formed by bending a very long wire as shown in Fig. 30-43. If the wire carries a 20 A current, what are the direction and magnitude of B at point a and midpoint b? Take R = 9.0 mm and the distance between a and b to be much larger than R (each straight section is "infinite").

 

 

 

 

 

 

(a) Point a              What is the direction?
1____________ T      _____________2

(b) Midpoint b         What is the direction?
3____________ T      _____________

 

 

 

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Explanation / Answer

current through the wire is   i = 20 A   here we find the field by superposing the results of two semi infinite wires and a semi circular arc    .     magnetic field due to semi infinite straight wire          B   = o i / 4 R        magnetic field at the center of circular arc is             B = o i / 4 R     where = rad   a) magnitude of B at point a is              Ba   = 2  o i / 4 R     + o i / 4R                        =    o i /2 R  ( 1/ + 1/2 )                     = [ ( 4*10 -7 )( 20 ) / 2 R ] ( 1/ + 1/2 )                     = 0.82   [ ( 4*10 -7 )( 20 ) / 2 (9*10-3 m ) ]                     = 1.144*10-3 T           The direction of field is out of page b)   b as a point mid way between two infinite wires       magnitude of B at point b is            Bb   = 2 ( o i / 2R   )                        = o i / R                         =  ( 4*10 -7 )( 20 ) /   R                     = 8.88*10-4 T                        = 1.144*10-3 T           The direction of field is out of page b)   b as a point mid way between two infinite wires       magnitude of B at point b is            Bb   = 2 ( o i / 2R   )                        = o i / R                         =  ( 4*10 -7 )( 20 ) /   R                     = 8.88*10-4 T   
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