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=1 00.A out of the page (the current in theinner conductor), soBa=×⋅×=−−4101 0021 001020073ππµTmAAmT toward top of pageejafej...Similarly at point b: BIrbbb=µπ02, where Ibis the net current through the area of the circle havingradius rb.Taking out of the page as positive, Ib=−= −1 003 002 00...AAA , or Ib=2 00.A into the page.Therefore,Bb=×⋅×=−−4102 0023 001013373ππµTmAAmT toward bottom of pageejafej...P30.24(a)In BIr=µπ02, the field will be one-tenth as large at a ten-times larger distance: 400 cm(b)Bkk=+−µπµπ010222IrIr±±e jso B=×⋅−FHGIKJ=−410210 398 510 401 57 507ππTm 2.00 AAmmnTaf..
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Magnetic Field, single hundredth wire, ten-times larger distance, coaxial-cable power cords, weak magnetic fields