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Unformatted text preview: Physics 202 Thursday, April 1, 1999 Announcements: Lecture notes: Example of Biot and Savart Law Find the electric field due to current in a straight wire of infinite length What is B at point P? o Take point O to be the orgin. The positive z axis is out of the screen. o i ds and r are on the x -y plane. dB is in the +z direction. o dB = ( o /4 )( i ds(cross)r / r 2 ) o Absolute value of dB = ( o /4 )( i ds / r 2 )sin We now need to express the "sum" of the absolute value of dB in terms of a and 1 and 2 . o absolute value of ds = dx o x = a cot (180 - ) = - a cot . o dx = a csc 2 d . o r 2 = a 2 + x 2 = a 2 + a 2 cot 2 = a 2 ( 1+ cot 2 ) = a 2 csc 2 . o Therefore, Absolute value of dB = ( o /4 )( i a csc 2 d sin / a 2 csc 2 ) o Simplified, Absolute value of dB = ( o /4 )( i sin / a) d o B = ( o i/4 a) I (integral from 1 to 2 ) sin d = -( o i/4 a) cos (Evaluated from 1 to 2 ) o...
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