P30_068 - R above C and the other a horizontal distance R...

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68. We refer to the center of the circle (where we are evaluating ~ B )as C . Recalling the straight sections discussion in Sample Problem 30-1, we see that the current in the straight segments which are colinear with C do not contribute to the Feld there. Eq. 30-11 (with φ = π/ 2 rad) and the right-hand rule indicates that the currents in the two arcs contribute µ 0 i ( π/ 2) 4 πR µ 0 i ( π/ 2) 4 πR =0 to the Feld at C . Thus, the non-zero contributions come from those straight-segments which are not colinear with C . There are two of these “semi-inFnite” segments, one a vertical distance
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Unformatted text preview: R above C and the other a horizontal distance R to the left of C . Both contribute Felds pointing out of the page (see ig. 30-6(c)). Since the magnitudes of the two contributions (governed by Eq. 30-9) add, then the result is B = 2 i 4 R = i 2 R exactly what one would expect from a single inFnite straight wire (see Eq. 30-6). or such a wire to produce such a Feld (out of the page) with a leftward current requires that the point of evaluating the Feld be below the wire (again, see ig. 30-6(c))....
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