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Unformatted text preview: PHYS2020, Fall 2006 CAPA Set 9 1. [1pt] A vertical straight wire carrying an upward 11.3 A current exerts an attractive force per unit length of 8 . 00 10- 4 N/m on a second parallel wire 7.70 cm away. What is the magnitude of the current that flows in the second wire? The current in the first wire creates a magnetic field which acts on the current in the second wire. The magnetic field created by a current along a long straight wire is given by B = I 2 r . Labeling the current in the first wire as I 1 the field is B = I 1 2 r . The force that this magnetic field exerts on the second wire is given by F = IlscriptB . Here the current is that which is carried in the second wire I 2 so the force on the second wire is F 2 = I 2 lscript 2 B where B is given from before so we have F 2 = I 2 lscript 2 I 1 2 r . We are asked to find the current flowing in the second wire so we solve for I 2 to get I 2 = F 2 2 r lscript 2 I 1 . We dont know F 2 or lscript 2 but we do know F 2 /lscript 2 which is all that is necessary to solve the problem. For the numbers given here, the final answer is I 2 = F 2 lscript 2 2 r I 1 = 8 . 00 10- 4 N/m 2 . 0770 m 4 10- 7 T m/A 11 . 3 A = 8 . 00 10- 4 N/m . 0770 m 2 10- 7 T m/A 11 . 3 A = 27 . 3 A . 2. [1pt] Determine the magnitude of the magnetic field midway between two long straight wires 1.80 cm apart when one wire carries 5.40 A and the other carries 15.5 A . Assume these currents are in the same direction....
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