高二物理選修電勢能和電勢

第四節(jié)電勢能和電勢Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

1、功的計算：2、重力做功的特點(diǎn)：(1)與路徑無關(guān),由初末位置的高度差來決定.(2)重力做功WG與重力勢能變化的關(guān)系(3)重力勢能是相對的,須選定零勢能參考面功是能量轉(zhuǎn)化的量度！重力做正功，重力勢能減少重力做負(fù)功，重力勢能增加復(fù)習(xí)：Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

一個試探電荷在電場中某點(diǎn)由靜止釋放,將如何運(yùn)動?E在電場力作用下電荷做加速運(yùn)動,一段時間后獲得一定的速度,試探電荷的動能增加.+F什么能轉(zhuǎn)換為動能的呢?Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.一、電場力做功的特點(diǎn)

在勻強(qiáng)電場中移動電荷時，電場力做的功與電荷經(jīng)過的路徑無關(guān),只與電荷的起始位置和終止位置有關(guān)。對于非勻強(qiáng)電場也適用B+MAF?q+

WAB=F·|AB|cosθ=qE·|AM|

把正電荷沿不同路徑從A點(diǎn)移到B電場力做功？Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

將+q由A移到B的過程電場力對電荷做功為：將-q由A移到B的過程，電場力對電荷做功為：dAB+qEL做功的過程是一個能量轉(zhuǎn)化的過程，那么電場力做功的過程是什么能轉(zhuǎn)化為什么能呢？Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.二、電勢能

電場中的電荷由于受到電場力作用而具有的勢能。Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

正電荷從A點(diǎn)移到B點(diǎn)時,電場力做正功,電荷的電勢能減少.

正電荷從B點(diǎn)移到A點(diǎn)時,電場力做負(fù)功,即電荷克服靜電力做功,電荷的電勢能增加.ABE+vABE+v如果是負(fù)電荷情況相同Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.二、電勢能1、電荷在電場中具有與其位置有關(guān)的能量叫電勢能。2、電場力對電荷做正功，電荷的電勢能減小3、電場力對電荷做負(fù)功，電荷的電勢能增加電場力做的功等于電勢能變化量的負(fù)值Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例1.如圖,在場強(qiáng)的勻強(qiáng)電場中,點(diǎn)電荷q=+1c從A移動到B,AB相距L=1m,電場力做功為多少?電勢能如何變化?電荷在B點(diǎn)具有的電勢能多大？電勢能減少EqFAB設(shè)A點(diǎn)的電勢能WAB=EPA－EPB=103J電勢能為負(fù)表示的物理意義是什么？+Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

通常把電荷在離場源電荷無限遠(yuǎn)處的電勢能規(guī)定為零，或把電荷在大地表面上電勢能規(guī)定為零。

電荷在某點(diǎn)的電勢能，等于電場力把它從該處移動到零勢能位置所做的功Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.電勢能為負(fù)表示電荷在該處的電勢能比零還要小.電勢能是標(biāo)量電勢能是相對的，與零勢能面的選擇有關(guān)。電勢能是電荷和電場所共有的，具有系統(tǒng)性Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.電場力做正功，電勢能減少；電勢能與重力勢能類比電場力做負(fù)功，電勢能增加。與重力做功跟重力勢能變化關(guān)系相類比Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.重力勢能和電勢能類比

重力場中，重力：地球和物體之間存在的吸引力電場中，電場力：電荷之間的作用力有重力就有重力勢能Ep=mgh有電場力就有相應(yīng)的能，叫電勢能EPEp由物體和地面間的相對位置決定EP由電荷間的相對位置決定電場力做正功，電勢能就減少電場力做負(fù)功，電勢能就增加電場力作功，電勢能改變重力做功，重力勢能改變重力做正功，重力勢能就減少重力做負(fù)功，重力勢能就增加W重=EP1-EP2W電=EPA-EPBEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.1、下列說法中，正確的是：A、把兩個同種點(diǎn)電荷間的距離增大一些，電荷的電勢能一定增加B、把兩個同種點(diǎn)電荷間的距離減小一些，電荷的電勢能一定增加C、把兩個異種點(diǎn)電荷間的距離增大一些，電荷的電勢能一定增加D、把兩個異種點(diǎn)電荷間的距離減小一些，電荷的電勢能一定增加課堂訓(xùn)練：Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.2、把電量q1=5×10-5C的電荷從電場中的A點(diǎn)移到B點(diǎn)，電場力做功W1=10-3J，則電荷q1在A、B兩點(diǎn)的電勢能EA1與EB1之差EA1-EB1=________J；若把q2=-1×10-4C的電荷從A點(diǎn)移到B點(diǎn)，電場力做功W2=_______

J，電荷q2從A到B電勢能的變化量為多少？10-3J-2×10-3JEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.3、將帶電量為6×10-6C的負(fù)電荷從電場中的A點(diǎn)移到B點(diǎn)，克服電場力做了3×10-5J的功，再從B移到C，電場力做了1.2×10-5J的功，則（1）電荷從A移到B，再從B移到C的過程中電勢能共改變了多少？（2）如果規(guī)定A點(diǎn)的電勢能為零，則該電荷在B點(diǎn)和C點(diǎn)的電勢能分別為多少？（3）如果規(guī)定B點(diǎn)的電勢能為零，則該電荷在A點(diǎn)和C點(diǎn)的電勢能分別為多少？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+該比值是一個與q無關(guān)，僅與電場及A點(diǎn)的位置有關(guān)的量。三、電勢設(shè)EP0=0該比值稱為A點(diǎn)的電勢零勢能參考點(diǎn)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.電荷電場中在某一點(diǎn)的電勢能與它的電荷量的比值1V=1J/C1.定義:2.公式:3.單位:伏特(V)

(計算時要代入正負(fù)號)4.標(biāo)量(有正負(fù))5.電勢具有相對性：確定電勢,應(yīng)先規(guī)定電場中某處的電勢為零.(通常規(guī)定離場源電荷無限遠(yuǎn)處或大地的電勢為零.)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例1.試探電荷q1放在電場中的A點(diǎn),電勢能為E(規(guī)定無限遠(yuǎn)處電勢能為零),A點(diǎn)的電勢為________,若把試探電荷q1換成試探電荷q2放在電場中的A點(diǎn),A點(diǎn)的電勢為___________(仍規(guī)定無限遠(yuǎn)處電勢能為零).Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

例2.將一電量為的點(diǎn)電荷從電場外一點(diǎn)P移至電場中某點(diǎn)A,電場力做功,求A點(diǎn)的電勢.解:

場外一點(diǎn)P的電勢為從PA,電場力做的功

PAEW-=注意:計算電勢首先要規(guī)定零電勢處,且電勢可以為負(fù).Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.沿著電場線方向，電勢如何變化呢？4、沿著電場線的方向，電勢越來越低如何確定電場中某點(diǎn)的電勢呢？5、電勢具有相對性，先規(guī)定電場中某處的電勢為零，然后才能確定電場中其他各點(diǎn)的電勢。（通常取離場源電荷無限遠(yuǎn)處或大地的電勢為零）Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.四、等勢面電場中電勢相同的各點(diǎn)構(gòu)成的面等勢面與電場線的關(guān)系：（1）電場線與等勢面垂直，并且由電勢高的等勢面指向電勢低的等勢面問題：等勢面能相交、相切嗎？（2）等差等勢面越密的地方，電場強(qiáng)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.如圖所示，虛線a、b和c是某靜電場中的三個等勢面，它們的電勢分別為、和，一帶正電粒子射入電場中，其運(yùn)動軌跡如實線KLMN所示。由圖可知：A、粒子從K到L的過程中，電場力做負(fù)功B、粒子從L到M的過程中，電場力做負(fù)功C、粒子從K到L的過程中，電勢能增加D、粒子從L到M的過程中，動能減少Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.如圖所示