物理学 7-1恒定电流 第五版 电流强度电流密度 (Current intensity Current density) 1电流强度( Current intensity) 大量电荷的定向有规则移动形成电流。 The directed regular motion of the large quantities f electric charges forms the electric current. 形成电流的条件:( Conditions for forming current) (1),自由电荷( Free charges) (2),恒定电势差( Constant potential difference) 第七章恒定磁场 1/8
第七章 恒定磁场 物理学 第五版 7-1 恒定电流 1/8 一 电流强度电流密度 (Current intensity Current density) 大量电荷的定向有规则移动形成电流。 1 电流强度(Current intensity) 形成电流的条件： (Conditions for forming current) (1),自由电荷 (Free charges) (2),恒定电势差(Constant potential difference ) The directed regular motion of the large quantities of electric charges forms the electric current
物理学 7-1恒定电流 第五版 电流强度为通过截面S的电荷随时间的变化率 we define the intensity of current through the cross section area s to be the net charge rate flowing through the area with the time dt lg=env, Sdt l=enO,s 7为电子的漂移速度 ( drift velocity)大小 单位(Unin:安培 (Ampere) 1A=103mA=100uA 第七章恒定磁场 2/8
第七章 恒定磁场 物理学 第五版 7-1 恒定电流 2/8 I = envd S vd为电子的漂移速度 (drift velocity)大小 单位(Unit): 安培 (Ampere) A 3 6 1A 10 mA=10 uA = t q I d d = d d q en S t = vd 电流强度为通过截面S 的电荷随时间的变化率 I S + + + + + + We define the intensity of current through the cross section area S to be the net charge rate flowing through the area with the time
物理学 7-1恒定电流 第五版 2电流密度( Current dens)j 方向规定( Direction provision)该点正电荷运动方向 The motion direction of positive charge at that point d 大小规定( Vaule provision C 等于在单位时间内通过该 点附近垂直于正电荷运动 方向的单位面积的电荷 It is equal to the charges flowing per unit cross-sectinn area perpendicular to the direction ofpositive charge near that point per unit time dI=jds=jdS cosa do eno dtds cos a ds cos a rj. dS 第七章恒定磁场 3/8
第七章 恒定磁场 物理学 第五版 7-1 恒定电流 3/8 该点正电荷运动方向 2 电流密度(Current density) j 方向规定 (Direction provision) 大小规定(Vaule provision ) The motion direction of positive charge at that point 等于在单位时间内通过该 点附近垂直于正电荷运动 方向的单位面积的电荷 It is equal to the charges flowing per unit cross-sectinn area perpendicular to the direction of positive charge near that point per unit time S d j I d d cos d d d cos d env S I t S Q j = = = = s I j S d dI = j dS = jdS cos
物理学 7-1恒定电流 第五版 电流的连续性方程恒定电流条件 ( Current continuity equation Constant current conditions 单位时间内通过闭合曲面向外流出的电荷, 于此时间内闭合曲面里电荷的减少量 e charges outward through the closed surface per unit time is equal to the charges decrease at same time in that closed surface do dQ ds ds dt /S=0 第七章恒定磁场 4/8
第七章 恒定磁场 物理学 第五版 7-1 恒定电流 4/8 单位时间内通过闭合曲面向外流出的电荷， 等于此时间内闭合曲面里电荷的减少量 . 二 电流的连续性方程 恒定电流条件 (Current continuity equation & Constant current conditions ) The charges outward through the closed surface per unit time is equal to the charges decrease at same time in that closed surface S S d j t Q t Q j S i s d d d d d = = − d d 0 d i s Q j S t + =
物理学 7-1恒定电流 第五版 若闭合曲面S内的 电荷不随时间变化 dQ1=0 If the charge does not change with time in the closed surface s 恒定电流 Constant current 7ds=0 Ⅰ+1+Ln=0 1=1+ 2 第七章恒定磁场 5/8
第七章 恒定磁场 物理学 第五版 7-1 恒定电流 5/8 d = 0 s j S 恒定电流 Constant current 0 1 2 − I + I + I = 0 d d = t 若闭合曲面 S 内的 Qi 电荷不随时间变化 S I 1 I 2 I If the charge does not change with time in the closed surface S 1 2 I I I = +