196 Sources of the Magnetic FieldP30.23From Ampere’s law, the magnetic field at point ais given by BIraaa=µπ02, where Iais the net currentthrough the area of the circle of radius ra. In this case, Ia=100.A out of the page (the current in theinner conductor), soBa=×⋅×=−−41010021 00 1020073TmAAmT toward top of pageejaf...Similarly at point b: BIrbbb=02, where Ibis the net current through the area of the circle havingradius rb.Taking out of the page as positive, Ib=−=−300200...AAA, or Ib=.A into the page.Therefore,Bb=×=−−020023 00 1013373AmT toward bottom of page...P30.24(a)In BIr=02, the field will be one-tenth as large at a ten-times larger distance: 400 cm(b)Bkk=+−010222IrIr±±so B=−FHGIKJ=−021039851040157507T m 2.00 AA mmnTaf.(c)Call rthe distance from cord center to field point and 23 00d=.mm the distance betweenconductors.BIrd rdIdrd=−−+FHGIKJ=−00211227 50 102 00 102 00300 10225 10107326.
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