物理专业英语洛伦兹力原文和翻译.doc

物理专业英语组别：19组专业：物理学姓名：9.5 THE LORENTZ FORCEA charge moving in a magnetic field experiences a force which we shall call magnetic . The force is determined by the chang q,its velocity v ,and the magnetic induction B at the point where the charge is at the moment of time being considered .The simplest assumption is that the magnitude of the force F is proportional to each of the three quantities q,v,and B .In addition ,F can be expected to depend on the mutual orientation of the vectors v and B .The direction of the vector F should be determined by those of vectors v and B.To”construct”che vector F form the scalar q and the vectors v and B ,let us find the vector of v and B and then multiply then multiply the result obtained by the scalar q.The result is the expression qvB (9.31）It has been established experimentally that the force F acting on a charge moving in a magnetic field is determined by the formula Fa=kqvB （9.32）Where k is a proportionality constant depending on the choice of the units for the quantities in the formula .It must be borne in mind that the reasoning which led us to expression(9.31)must by no means be considered as the derivation of Eq.(9.32)This reasoning does not have conclusive force .Its aim is to help us memorize Eq(9.32).The correctness of this equation can be established only experimentally .We must note that Eq.(9.32)can be considered as a definition of The magnetic induction B.The unit of magnetic induction B -the tesla-is determined so that the proportionality constant k in Eq.(9.32)equals unity .Hence,In SI units ,this equation becomes F=qvB (9.33）The magnitude of the magnetic force is F=qvBsin (9.34) Where is the angle between the vectors v and B .It can be seen from Eq.(9.34) that a charge moving along the lines of a magnetic field does not experience the action of a magnetic force .The magnetic force is directed at right angles to the plane containing the vectors v and B.If the charge q is positive ,then direction of the force coincides with that of the vector vB.Where q is negative ,the directions of the vectors F and vB are opposite (Fig.9.6).Since the magnetic force is always directed at right angles to the velocity of a charged particle ,it does no work on the particle .Hence ,we cannot change the energy of a charged particle by acting on it with a constant magnetic field .The force exerted on a charged particle that is simultaneously in an electric and a magnetic field is F=qE+qvB (9.35) This expression was obtained from the results of experiments by the Dutch physicist Hendrik Lorentz (18531928)and is called the Lorentz force.Assume that the charge q is moving with the velocity v parallel to a straight infinite wire along which the current I flows(Fig.9.7).According to Eqs .(9.30)and(9.34),the charge in this case experiences a magnetic force whose magnitude is F=qvB=qv (9.36)Where b is the distance from the charge to the wire .The force is directed toward the wire when charge is positive if the directions of the current and motion of the charge are the same ,and away from the wire if these directions are opposite (see Fig.9.7).When the charge is negative ,the direction of the force is reversed ,the other conditions being equal. Let us consider two like point charges qand q2 moving along parallel straight lines with the same velocity v that is much smaller than c (Fig.9.8).When vc,the electric field does not virtually differ form the field of stationary charges (see Sec。9.3）.Therefore the magnitude of the electric force F exerted on the charges can be considered equal to F,1=F,2=F= (9.37) Equations （9.21）and (9.33)give us the following expression for the magnetic force exerted on the charges : (9.38)(the position vector r is perpendicular to v). Let us find the ratio between the magnetic and electric forces It follows from Eqs.(9.37)and(9.38)that (9.39) see Eq(9.15).We have obtained Eq.(9.39) on the assumption that vc.This ratio holds ,however,with any vs. The forces Fand are directed oppositely .Figure 9.8 has been drawn for like and positive charges.For like negative charges, the directions of the forces will remain the same ,while the directions of the electric and magnetic forces will be the reverse of those shown in the figure . Inspection of Eq.（9.39）shows that the magnetic force is weaker than the Coulomb one by a factor equal to the square of ratio of the speed of the charge to that of light.The explanation is that the magnetic interaction between moving charges is a relativistic effect. Magnetism would disappear if the speed of light were infinitely great. 9.5 洛伦兹力移动的电荷在磁场中收到磁场力的作用，这种力是由电荷的电量q，速度v和在磁场中所在的位置和该时刻所对应的磁感应强度B决定。假设作用力F与q，v，b成正比，另外，F的方向主要是靠v和B决定的。F是由标量q和矢量v B作用得到的，v与B的矢量积再乘以标量q，表达式为qvB (9.31）移动电荷在磁场中所受力的作用F由实验确定。公式F=kqvB （9.32）k是由磁场决定的等比例常量。必须记住9.31式的原因是为了能够帮助引出9.32式。当然这个推理有些牵强。其目的是为了记住9.32式。该式。的正确性可通过实验检验。必须注意到9.32式中考虑到了磁感应强度B。磁感应强度B的单位特斯拉已经被定义。所以与在9.32式中等比例常量k是一样。因此，在国际单位制中这个等式变为F=qvB (9.33) 磁场力的大小F=qvBsin （9.34）是v和B的矢量夹角。从9.34式中可以看出电荷在磁场中沿着磁场线运动。则不会受到磁场的力作用。磁场力的方向在v和B矢量构成的矢量平面内。如果所带电量为正电荷。F的矢量方向与vB所指的方向相同，若所带电量为负电荷，则与vB所指方向相反（见图9.6）由于磁场力的方向是由带电粒子的速度方向决定的。而对粒子本身不起作用。因此，粒子在磁场中运动但并没有产生能量。电荷在既有电场又有磁场的空间中运动。受到的力为F=qE+qvB （9.35）这个表达式里有荷兰物理学家亨德.里克.洛伦兹力（1853-1928）通过实验得出，并命名该力为洛伦兹力。假设带电粒子q以速度v沿平行电流I方向（见图9.7）根据9.30式和9.34式，电荷在实验中受到力的大小为F=vqB=qv (9.36) b是指电荷到电流I的距离。当带正电时，电流方向和带电粒子运动方向相同时，磁场力的方向垂直。指向电流的方向。如果它们的方向相反时，磁场力的方向将反向。当带负电时，电场力的方向反向，在其他条件下一样。假设俩个相同点电荷，沿着磁感应线方向以相同的速度v运动。其中v远远小于c当vc时，在电场中与电点电荷没有本质的区别（见9.3部分）。因此作用在点电荷上的电场力的大小为可表示为 F,1=F,2=F= (9.37) 由9.21式和9.33式可得出磁场中电荷所受到的作用力大小为 (9.38)（矢量r的方向垂直于v）由9.37式和9.38式得电场力与磁场力之比 (9.39)当vc时，此式对任意v都成立。图9.8已经画出，当俩电荷同时带正电时， F和方向相反；当俩电荷带同时带负电时，它们力方向不会变，只是方向发生转变。当俩电荷带不同电荷时，电场力和磁场力的方向将会发生转变（如图）由9.39式得出通过速度与光速的平方之比可看出磁场力比库仑力小，这是因为磁场力会对俩移动电荷产生作用力。磁场力将会随着速度的无限增大而消失。