chapterxv磁場magneticfield(已修改)
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【正文】 202006(2) Lecture Notes WY 1 ChapterXV 磁場 Magic Field Main Content： 畢奧－沙伐爾定律 BiotSavart Law 安培環(huán)路定理 Ampere’s cycle law 安培環(huán)路定律的應(yīng)用 Applications 與變化電場聯(lián)系的磁場 Magic field induced by a change EF 平行電流間相互作用力 202006(2) Lecture Notes WY 2 BiotSavart Law 202006(2) Lecture Notes WY 3 Introduction ? We have discussed how an existing magic field influences moving charges (and thus currents) ? We have not yet discussed the origin of magic fields ? We will now see that currents (moving charges) produce magic fields ? This can be thought of as the basic mechanism by which all magic fields are produced 202006(2) Lecture Notes WY 4 History ? 1819 Hans Christian Oersted discovered that a pass needle was deflected by a current carrying wire ? Then in 1820s JeanBaptiste Biot and Felix Savart performed experiements to determine the force exerted on a pass by a current carrying wire ? There results were as follows … 202006(2) Lecture Notes WY 5 JeanBaptiste Biot amp。 Felix Savart’s Results ? dB the magic field produced by a small section of wire ? ds a vector the length of the small section of wire in the direction of the current ? r the positional vector from the section of wire to where the magic field is measured ? I the current in the wire ? ? angle between ds amp。 r r ds dB ? 202006(2) Lecture Notes WY 6 Biot amp。 Savart’s Results ? dB perpendicular to ds ? dB perpendicular to r ? |dB| inversely proportional to |r|2 ? |dB| proportional to current I ? |dB| proportional to |ds| ? |dB| proportional to sin ? ? dB t h e m a g n e t i c f i e l d p r o d u c e d b y a sm a l l se c t i o n o f wi re? ds a v e c t o r t h e l e n g t h o f t h e sm a l l se c t i o n o f w i re i n t h e d i rec t i o n o f t h e c u r rent? r t h e p o si t i o n a l v e c t o r fr o m t h e se c t i o n o f wi re t o wh e re t h e m a g n e t i c f i e l d i s m e a su red? I t h e cu rr e n t i n t h e wi re? ? a n g l e b e t wee n ds amp。 rrdsdB?202006(2) Lecture Notes WY 7 Biot – Savart Law ? All these results could be summarised by one “Law” 2?rrdsB ?? Id20 ?4 rrdsB ???????? Id??Putting in the constant Where ?0 is the permeablity of free space ATm104 70??
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