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Unformatted text preview: Magnetism Problem 1 (medium): Two infinite wires are both parallel to the yaxis, and both lie in the xyplane. Wire 1 carries a current I 1 > 0 in the positive ydirection, and crosses the xaxis at x = a . Wire 2 carries a current I 2 > 0 in the negative ydirection, and crosses the xaxis at x = a . 1: Find the magnetic field at a point ( x,y ) where (i) x < a , (ii) a < x < a , and (iii) x > a . 2: Find the force per unit length exerted on wire 2 by wire 1. 3: A point charge q moves along the xaxis, in the positive direction, at a speed v . At the instant this charge crosses the origin, (0 , 0), what magnetic force does it experience? & & & & & & Solution: Because both wires lie in the xyplane, we know that at any point in the xy plane the magnetic field must point in the zdirection, i.e., either + k or k . In wire 1, located at x = a , the current I 1 flows in the positive ydirection. Therefore, from the righthand rule, the magnetic field due to wire 1 at a point ( x,y ) is in the + k direction if ( x,y ) is to the left of the wire ( x < a ), and is in the k direction if ( x,y ) is to the right of the wire ( x > a ). The perpendicular distance to wire 1 from ( x,y ) is  x + a  . The magnetic field at ( x,y ) contributed by wire 1 is therefore 1 B 1 ( x < a,y ) = I 1 2  x + a  k B 1 ( x > a,y ) = I 1 2  x + a  k In wire 2, located at x = a , the current I 2 flows in the negative ydirection. Therefore, from the righthand rule, the magnetic field due to wire 2 at a point ( x,y ) is in the k direction if ( x,y ) is to the left of the wire ( x < a ), and is in the + k direction if ( x,y ) is to the right of the wire ( x > a ). The perpendicular distance to wire 2 from ( x,y ) is  x a  . The magnetic field at ( x,y ) contributed by wire 2 is therefore B 2 ( x < a,y ) = I 2 2  x a  k B 2 ( x > a,y ) = I 2 2  x a  k The total magnetic field is the sum of the contributions from each wire. For x < a , the total magnetic field at ( x,y ) is B ( x < a,y ) = B 1 ( x < a,y ) + B 2 ( x < a,y ) = I 1 2  x + a  k I 2 2  x a  k = 2 I 1  x + a  I 2  x a  k For a < x < a , the total magnetic field at ( x,y ) is B ( a < x < a,y ) = B 1 ( x > a,y ) + B 2 ( x < a,y ) = I 1 2  x + a  k I 2 2  x a  k = 2 I 1  x + a  + I 2  x a  k For x > a , the total magnetic field at ( x,y ) is B ( x > a,y ) = B 1 ( x > a,y ) + B 2 ( x > a,y ) = I 1 2  x + a  k + I 2 2  x a  k = 2  I 1  x + a  + I 2  x a  k Notice that the magnetic field has no dependence on y , only on x ....
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 Spring '09
 WAUNG

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