大学物理双语版奥本汉姆课件Chapamp;MagFie.pptVIP

(1) A long straight wire (载流长直导线的磁场 P614) The magnitude of the differential magnetic field produced at P by the current-length element Ids located a distance r from P is given by: 2. The application of Biot-Savart law r =R /sin? , s = -R cot? x P R s L I ? ?1 ? ?2 I I Discussions: ①The magnetic field from all current element of straight wire are in the same direction and into the screen. ②For straight and infinite wire (无限长载流直导线), ?1=0,?2=?,then —— The magnitude of magnetic field of infinite straight wire at a point with distance R is in inverse proportion to the distance. The case given in P614 (2) In a circular arc of wire (载流圆线圈轴线上的磁场) As the figure, given I, R, x I ? ? p x x R 0 The direction of is perpendicular to the plane made of Ids and r and . I ? ? p x x R 0 Discussions: ①At center O, x=0, ②For a circular arc of wire (一段圆弧电流圆心处的磁感应强度): ③The direction of is along the axis direction (right-hand rule) as right figure and Fig.26-18 in P615. Solution: (direct into the screen) (into the screen) You may comparing with the calculation of electrostatic field. Example 26-11 of P616: One quarter of a circular loop of wire carries a current I, and two straight sections whose extensions intersect the center C of the arc. Find at point C? 1 3 2 For parts 1 and 2, The number of turns per unit length of the solenoid(螺线管,单位长匝数) n. As vertical cross section (纵剖面) shown, (I, n, R, L, ?1, ?2 ) (3) A stretched-out solenoid (载流直螺线管轴线上的磁场) I I Think about: how to choose a current-element dI ? . dx x ? . ? ? ? ? ? ? ? ? ? ? ? ? . . . . . . . . . . . . ? ? ? . . L ?1 ?2 P choose dI=nIdx x=Rcot? , dx=Rcsc2 ? d ?, R2+x2= R2 csc2 ? Same direction! Explanations: ②For a “infinitely long” solenoid, The at the axial line: –5R 5R 0.439 B x ①Direction inside the solenoid is along the axis and right-hand rule as Fig.26-12. ③B-x curve inside the solenoid (L=10R): 25-7. Ampere’s Loop Law Application Andre Marie Ampere (1775—1836) French mathematician