高二物理选修电势能和电势

第四节电势能和电势Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

1、功的计算：2、重力做功的特点：(1)与路径无关,由初末位置的高度差来决定.(2)重力做功WG与重力势能变化的关系(3)重力势能是相对的,须选定零势能参考面功是能量转化的量度！重力做正功，重力势能减少重力做负功，重力势能增加复习：Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

一个试探电荷在电场中某点由静止释放,将如何运动?E在电场力作用下电荷做加速运动,一段时间后获得一定的速度,试探电荷的动能增加.+F什么能转换为动能的呢?Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.一、电场力做功的特点

在匀强电场中移动电荷时，电场力做的功与电荷经过的路径无关,只与电荷的起始位置和终止位置有关。对于非匀强电场也适用B+MAFөq+

WAB=F·|AB|cosθ=qE·|AM|

把正电荷沿不同路径从A点移到B电场力做功？Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

将+q由A移到B的过程电场力对电荷做功为：将-q由A移到B的过程，电场力对电荷做功为：dAB+qEL做功的过程是一个能量转化的过程，那么电场力做功的过程是什么能转化为什么能呢？Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.二、电势能

电场中的电荷由于受到电场力作用而具有的势能。Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

正电荷从A点移到B点时,电场力做正功,电荷的电势能减少.

正电荷从B点移到A点时,电场力做负功,即电荷克服静电力做功,电荷的电势能增加.ABE+vABE+v如果是负电荷情况相同Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.二、电势能1、电荷在电场中具有与其位置有关的能量叫电势能。2、电场力对电荷做正功，电荷的电势能减小3、电场力对电荷做负功，电荷的电势能增加电场力做的功等于电势能变化量的负值Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例1.如图,在场强的匀强电场中,点电荷q=+1c从A移动到B,AB相距L=1m,电场力做功为多少?电势能如何变化?电荷在B点具有的电势能多大？电势能减少EqFAB设A点的电势能WAB=EPA－EPB=103J电势能为负表示的物理意义是什么？+Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

通常把电荷在离场源电荷无限远处的电势能规定为零，或把电荷在大地表面上电势能规定为零。

电荷在某点的电势能，等于电场力把它从该处移动到零势能位置所做的功Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.电势能为负表示电荷在该处的电势能比零还要小.电势能是标量电势能是相对的，与零势能面的选择有关。电势能是电荷和电场所共有的，具有系统性Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.电场力做正功，电势能减少；电势能与重力势能类比电场力做负功，电势能增加。与重力做功跟重力势能变化关系相类比Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.重力势能和电势能类比

重力场中，重力：地球和物体之间存在的吸引力电场中，电场力：电荷之间的作用力有重力就有重力势能Ep=mgh有电场力就有相应的能，叫电势能EPEp由物体和地面间的相对位置决定EP由电荷间的相对位置决定电场力做正功，电势能就减少电场力做负功，电势能就增加电场力作功，电势能改变重力做功，重力势能改变重力做正功，重力势能就减少重力做负功，重力势能就增加W重=EP1-EP2W电=EPA-EPBEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.1、下列说法中，正确的是：A、把两个同种点电荷间的距离增大一些，电荷的电势能一定增加B、把两个同种点电荷间的距离减小一些，电荷的电势能一定增加C、把两个异种点电荷间的距离增大一些，电荷的电势能一定增加D、把两个异种点电荷间的距离减小一些，电荷的电势能一定增加课堂训练：Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.2、把电量q1=5×10-5C的电荷从电场中的A点移到B点，电场力做功W1=10-3J，则电荷q1在A、B两点的电势能EA1与EB1之差EA1-EB1=________J；若把q2=-1×10-4C的电荷从A点移到B点，电场力做功W2=_______

J，电荷q2从A到B电势能的变化量为多少？10-3J-2×10-3JEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.3、将带电量为6×10-6C的负电荷从电场中的A点移到B点，克服电场力做了3×10-5J的功，再从B移到C，电场力做了1.2×10-5J的功，则（1）电荷从A移到B，再从B移到C的过程中电势能共改变了多少？（2）如果规定A点的电势能为零，则该电荷在B点和C点的电势能分别为多少？（3）如果规定B点的电势能为零，则该电荷在A点和C点的电势能分别为多少？增加了1.8×10-5J.EPC=1.8×10-5J

EPC=-1.2×10-5JEPB=3×10-5J

EPA=-3×10-5J

Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.A0lθE+该比值是一个与q无关，仅与电场及A点的位置有关的量。三、电势设EP0=0该比值称为A点的电势零势能参考点Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.电荷电场中在某一点的电势能与它的电荷量的比值1V=1J/C1.定义:2.公式:3.单位:伏特(V)

(计算时要代入正负号)4.标量(有正负)5.电势具有相对性：确定电势,应先规定电场中某处的电势为零.(通常规定离场源电荷无限远处或大地的电势为零.)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例1.试探电荷q1放在电场中的A点,电势能为E(规定无限远处电势能为零),A点的电势为________,若把试探电荷q1换成试探电荷q2放在电场中的A点,A点的电势为___________(仍规定无限远处电势能为零).Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例2.将一电量为的点电荷从电场外一点P移至电场中某点A,电场力做功,求A点的电势.解:

场外一点P的电势为从PA,电场力做的功

PAEW-=注意:计算电势首先要规定零电势处,且电势可以为负.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.沿着电场线方向，电势如何变化呢？4、沿着电场线的方向，电势越来越低如何确定电场中某点的电势呢？5、电势具有相对性，先规定电场中某处的电势为零，然后才能确定电场中其他各点的电势。（通常取离场源电荷无限远处或大地的电势为零）Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.四、等势面电场中电势相同的各点构成的面等势面与电场线的关系：（1）电场线与等势面垂直，并且由电势高的等势面指向电势低的等势面问题：等势面能相交、相切吗？（2）等差等势面越密的地方，电场强度E越大Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.如图所示，虚线a、b和c是某静电场中的三个等势面，它们的电势分别为、和，一带正电粒子射入电场中，其运动轨迹如实线KLMN所示。由图可知：A、粒子从K到L的过程中，电场力做负功B、粒子从L到M的过程中，电场力做负功C、粒子从K到L的过程中，电势能增加D、粒子从L到M的过程中，动能减少Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.如图所示