導語(yu)：閉(bi)合電路和歐(ou)姆(mu)定律(lv)教案一文(wen)來源于(yu)網(wang)友上傳(chuan)，不代表本站觀點(dian)，若需要原創文(wen)章可(ke)咨(zi)詢客服老(lao)師，歡迎參考(kao)。

教學目標

(一)知識目標

1、知道電動勢的定義.

2、理解閉合電路歐姆定律的公式(shi)，理解各物(wu)理量及(ji)公式(shi)的物(wu)理意(yi)義，并能熟(shu)練地用來解決有關的電路問題.

3、知道電源的電動勢(shi)等于電源沒有接入(ru)電路時兩極間的電壓(ya)，電源的電動勢(shi)等于內、外(wai)電路上電勢(shi)降落之和.

4、理解路端電壓與電流(或外電阻)的關系，知道這種關系的公(gong)式(shi)表(biao)達(da)和(he)圖(tu)線表(biao)達(da)，并(bing)能(neng)用來分(fen)析、計(ji)算有(you)關問題.

5、理解閉合電路的(de)功率表達式.

6、理(li)解閉合(he)電(dian)路(lu)中能量轉化(hua)的情(qing)況.

(二)能力目標

1、培養學生分(fen)析(xi)解(jie)決問(wen)題能力(li)，會用閉合電(dian)(dian)路歐姆定律(lv)分(fen)析(xi)外電(dian)(dian)壓隨外電(dian)(dian)阻(zu)變化的規(gui)律(lv)

2、理解路端電壓與電流(liu)(或外(wai)電阻(zu))的關(guan)(guan)系，知道這(zhe)種關(guan)(guan)系的公式表達和圖線表達，并能用來分析、計(ji)算有關(guan)(guan)問題.

3、通過用(yong)公(gong)式(shi)、圖像分析(xi)外電(dian)壓隨外電(dian)阻改變規律(lv)，培養學生用(yong)多種方式(shi)分析(xi)問(wen)題能(neng)力.

(三)情感目標

1、通過外電阻改變(bian)引起電流、電壓的變(bian)化，樹(shu)立(li)學生(sheng)普遍聯系觀點

2、通(tong)過(guo)分析外(wai)電壓(ya)變化原因(yin)，了解(jie)內(nei)因(yin)與外(wai)因(yin)關(guan)系

3、通過對(dui)閉合電路的分析計算，培養(yang)學(xue)生能量守恒思想(xiang)

4、知(zhi)道用能(neng)量的(de)(de)觀點說明電動勢的(de)(de)意義

教學建議

1、電(dian)(dian)(dian)(dian)(dian)源(yuan)電(dian)(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)(shi)(shi)的(de)(de)(de)概(gai)念在高中(zhong)是個(ge)難點，是掌握閉(bi)合電(dian)(dian)(dian)(dian)(dian)路(lu)歐姆定(ding)律的(de)(de)(de)關鍵和基(ji)礎，在處理電(dian)(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)(shi)(shi)的(de)(de)(de)概(gai)念時(shi)，可以根據教材，采(cai)用不(bu)同的(de)(de)(de)講法.從理論(lun)上分析電(dian)(dian)(dian)(dian)(dian)源(yuan)中(zhong)非靜(jing)電(dian)(dian)(dian)(dian)(dian)力做(zuo)功(gong)從電(dian)(dian)(dian)(dian)(dian)源(yuan)的(de)(de)(de)負極將正電(dian)(dian)(dian)(dian)(dian)荷運送到正極，克服電(dian)(dian)(dian)(dian)(dian)場力做(zuo)功(gong)，非靜(jing)電(dian)(dian)(dian)(dian)(dian)力搬運電(dian)(dian)(dian)(dian)(dian)荷在兩(liang)極之間產生電(dian)(dian)(dian)(dian)(dian)勢(shi)(shi)(shi)(shi)差(cha)的(de)(de)(de)大小，反(fan)映了電(dian)(dian)(dian)(dian)(dian)源(yuan)做(zuo)功(gong)的(de)(de)(de)本領，由此引出(chu)電(dian)(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)(shi)(shi)的(de)(de)(de)概(gai)念;也可以按(an)本書(shu)采(cai)取討論(lun)閉(bi)合電(dian)(dian)(dian)(dian)(dian)路(lu)中(zhong)電(dian)(dian)(dian)(dian)(dian)勢(shi)(shi)(shi)(shi)升降的(de)(de)(de)方法，給出(chu)電(dian)(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)(shi)(shi)等于內、外電(dian)(dian)(dian)(dian)(dian)路(lu)上電(dian)(dian)(dian)(dian)(dian)勢(shi)(shi)(shi)(shi)降落(luo)之和的(de)(de)(de)結(jie)(jie)論(lun).教學中(zhong)不(bu)要求論(lun)證這個(ge)結(jie)(jie)論(lun).教材中(zhong)給出(chu)一個(ge)比喻(兒童滑(hua)梯)，幫助學生接受這個(ge)結(jie)(jie)論(lun).

需要強調的(de)(de)是電(dian)源的(de)(de)電(dian)動勢反映的(de)(de)電(dian)源做(zuo)功的(de)(de)能力，它與(yu)外電(dian)路無(wu)關，是由電(dian)源本生的(de)(de)特性決定(ding)的(de)(de).

電(dian)(dian)動勢(shi)是標量，沒有方向，這要給學生(sheng)說明(ming)，如果(guo)學生(sheng)程度(du)較好(hao)，可以向學生(sheng)說明(ming)，做為電(dian)(dian)源(yuan)，由正負(fu)極(ji)之(zhi)分，在電(dian)(dian)源(yuan)內部，電(dian)(dian)流(liu)從負(fu)極(ji)流(liu)向正極(ji)，為了說明(ming)問(wen)題方便(bian)，也給電(dian)(dian)動勢(shi)一個(ge)方向，人(ren)們規定電(dian)(dian)源(yuan)電(dian)(dian)動勢(shi)的方向為內電(dian)(dian)路的電(dian)(dian)流(liu)方向，即從負(fu)極(ji)指向正極(ji).

2、路端(duan)電(dian)(dian)(dian)壓(ya)與電(dian)(dian)(dian)流(liu)(或(huo)外電(dian)(dian)(dian)阻)的關(guan)系(xi)(xi)，是一個難(nan)點.希望作好(hao)演示實驗(yan)，使(shi)(shi)學生(sheng)(sheng)有明(ming)確的感性認識，然后用公式加以(yi)解(jie)釋.路端(duan)電(dian)(dian)(dian)壓(ya)與電(dian)(dian)(dian)流(liu)的關(guan)系(xi)(xi)圖線(xian)，可以(yi)直觀地表示出(chu)路端(duan)電(dian)(dian)(dian)壓(ya)與電(dian)(dian)(dian)流(liu)的關(guan)系(xi)(xi)，務(wu)必使(shi)(shi)學生(sheng)(sheng)熟(shu)悉這個圖線(xian).

學生(sheng)應(ying)該知道，斷(duan)路(lu)時的(de)(de)路(lu)端電(dian)(dian)(dian)(dian)壓(ya)(ya)等(deng)于電(dian)(dian)(dian)(dian)源(yuan)的(de)(de)電(dian)(dian)(dian)(dian)動(dong)勢.因此，用電(dian)(dian)(dian)(dian)壓(ya)(ya)表測出斷(duan)路(lu)時的(de)(de)路(lu)端電(dian)(dian)(dian)(dian)壓(ya)(ya)就(jiu)可以得(de)到電(dian)(dian)(dian)(dian)源(yuan)的(de)(de)電(dian)(dian)(dian)(dian)動(dong)勢.在考慮電(dian)(dian)(dian)(dian)壓(ya)(ya)表的(de)(de)內阻時，希望通過(guo)第五(wu)節的(de)(de)“思考與(yu)討(tao)論”，讓(rang)學生(sheng)自己(ji)解(jie)決這個問(wen)題(ti).

3、最后講(jiang)述閉合電(dian)(dian)(dian)(dian)路(lu)中(zhong)(zhong)的(de)功率，得出公式，.要從(cong)能(neng)量轉(zhuan)化的(de)觀點說明，公式左方的(de)表示單位時間(jian)內電(dian)(dian)(dian)(dian)源提(ti)供的(de)電(dian)(dian)(dian)(dian)能(neng).理解了(le)這一(yi)點，就(jiu)容(rong)易理解上式的(de)意義：電(dian)(dian)(dian)(dian)源提(ti)供的(de)電(dian)(dian)(dian)(dian)能(neng)，一(yi)部(bu)分(fen)消(xiao)耗在內阻上，其余部(bu)分(fen)輸出到外電(dian)(dian)(dian)(dian)路(lu)中(zhong)(zhong).

教學設計方案

閉合電路的歐姆定律

一、教學目標

1、在(zai)物理知識方面的要求：

(1)鞏固(gu)產生恒(heng)定電流的(de)條件;

(2)知道電(dian)動勢是表征(zheng)電(dian)源特性的物理量(liang)，它在數值上等于(yu)電(dian)源沒有接入電(dian)路時兩極間的電(dian)壓.

(3)明確在閉合回路中電(dian)動勢等(deng)于電(dian)路上內、外電(dian)壓之(zhi)和.

(4)掌握閉合電(dian)路的歐姆定律，理(li)解各(ge)物理(li)量及(ji)公式的物理(li)意義

(5)掌(zhang)握路(lu)端(duan)電壓、輸出功(gong)率(lv)、電源(yuan)效(xiao)率(lv)隨外(wai)電阻變化的規律.

2、在物理方(fang)法上的要求：

(1)通過電動勢等于電路上內、外電壓之(zhi)和的教學，使學生學會運用(yong)實驗探索(suo)物理規律的方(fang)法.

(2)從能(neng)量和能(neng)量轉化的(de)(de)角(jiao)度理(li)解(jie)電動勢的(de)(de)物理(li)意義.

(3)通過對路端(duan)電(dian)壓、輸(shu)出(chu)功(gong)率、電(dian)源效率隨外(wai)電(dian)阻變化的規律(lv)的討論培養學生的推理能力.

(4)通過用公式、圖像分析外(wai)(wai)電壓(ya)隨外(wai)(wai)電阻改變(bian)規律，培養(yang)學生用多種方式分析

二、重點、難點分析

1、重點：

(1)電動勢是(shi)表示(shi)電源特性(xing)的物(wu)理(li)量

(2)閉合電路歐姆定律的(de)內容(rong);

(3)應用(yong)定(ding)律討論路端電壓、輸出功率(lv)、電源效(xiao)率(lv)隨外(wai)電阻變化的規律.

2、難點：

(1)閉(bi)合回路中電源電動勢等(deng)于電路上內、外電壓之和.

(2)短路(lu)、斷路(lu)特征

(3)應用閉(bi)合電(dian)(dian)路(lu)(lu)歐姆定律討論電(dian)(dian)路(lu)(lu)中的路(lu)(lu)端電(dian)(dian)壓、電(dian)(dian)流強度隨外電(dian)(dian)阻變化的關系

三、教學過程設計

引入新課：

教師：同學們都知道，電(dian)荷(he)的(de)定向移(yi)動形成電(dian)流.那么，導體中形成電(dian)流的(de)條件是(shi)什么呢?(學生答：導體兩端有電(dian)勢差.)

演示：將(jiang)小燈泡(pao)接(jie)在充滿(man)電的電容(rong)器兩(liang)端，會看(kan)到(dao)什么(me)現(xian)象?(小燈泡(pao)閃亮一下就熄滅.)為(wei)什么(me)會出(chu)現(xian)這種(zhong)現(xian)象呢?

分析(xi)：當電(dian)(dian)(dian)容器充(chong)完(wan)電(dian)(dian)(dian)后，其上(shang)下兩(liang)極(ji)板(ban)分別(bie)帶上(shang)正(zheng)負電(dian)(dian)(dian)荷(he)，如(ru)圖1所(suo)(suo)示，兩(liang)板(ban)間形成(cheng)電(dian)(dian)(dian)勢差.當用導線(xian)把小(xiao)(xiao)燈泡和電(dian)(dian)(dian)容器兩(liang)極(ji)板(ban)連通后，電(dian)(dian)(dian)子就在電(dian)(dian)(dian)場力的(de)作用下通過導線(xian)產(chan)生定向移動而形成(cheng)電(dian)(dian)(dian)流，但這是(shi)一(yi)瞬間的(de)電(dian)(dian)(dian)流.因為兩(liang)極(ji)板(ban)上(shang)正(zheng)負電(dian)(dian)(dian)荷(he)逐漸中和而減(jian)少，兩(liang)極(ji)板(ban)間電(dian)(dian)(dian)勢差也逐漸減(jian)少為零，所(suo)(suo)以(yi)電(dian)(dian)(dian)流減(jian)小(xiao)(xiao)為零，因此(ci)只(zhi)有電(dian)(dian)(dian)場力的(de)作用是(shi)不能形成(cheng)持續電(dian)(dian)(dian)流的(de).

教師：為了(le)形成持(chi)續的電(dian)(dian)(dian)(dian)源，必(bi)須有(you)一種本(ben)質上(shang)完全不同于靜(jing)電(dian)(dian)(dian)(dian)性的力，能夠(gou)不斷地分離(li)正(zheng)負電(dian)(dian)(dian)(dian)荷來補充兩極板上(shang)減少的電(dian)(dian)(dian)(dian)荷.這才能使(shi)兩極板保持(chi)恒定的電(dian)(dian)(dian)(dian)勢(shi)(shi)差，從(cong)而在導線中維(wei)持(chi)恒定的電(dian)(dian)(dian)(dian)流(liu)(liu)，能夠(gou)提供(gong)這種非靜(jing)電(dian)(dian)(dian)(dian)力的裝(zhuang)置叫電(dian)(dian)(dian)(dian)源.電(dian)(dian)(dian)(dian)源在維(wei)持(chi)恒定電(dian)(dian)(dian)(dian)流(liu)(liu)時，電(dian)(dian)(dian)(dian)源中的非靜(jing)電(dian)(dian)(dian)(dian)力將(jiang)不斷做功，從(cong)而把已經流(liu)(liu)到低(di)電(dian)(dian)(dian)(dian)勢(shi)(shi)處的正(zheng)電(dian)(dian)(dian)(dian)荷不斷地送回到高電(dian)(dian)(dian)(dian)勢(shi)(shi)處.使(shi)它(ta)的電(dian)(dian)(dian)(dian)勢(shi)(shi)能增加.

板書：1、電(dian)源(yuan)：電(dian)源(yuan)是一種(zhong)能(neng)(neng)(neng)夠不(bu)斷地把其他形式的(de)能(neng)(neng)(neng)量轉變(bian)為電(dian)能(neng)(neng)(neng)的(de)裝置(zhi).它并不(bu)創(chuang)造(zao)能(neng)(neng)(neng)量，也不(bu)創(chuang)造(zao)電(dian)荷(he).例如：干電(dian)池是把化(hua)學能(neng)(neng)(neng)轉化(hua)為電(dian)能(neng)(neng)(neng)，發電(dian)機(ji)是把機(ji)械能(neng)(neng)(neng)、核(he)能(neng)(neng)(neng)等轉化(hua)為電(dian)能(neng)(neng)(neng)的(de)裝置(zhi).

教師：電(dian)(dian)(dian)(dian)(dian)源能(neng)(neng)夠不(bu)斷地把其他形(xing)式的(de)(de)(de)(de)能(neng)(neng)量(liang)轉(zhuan)變為電(dian)(dian)(dian)(dian)(dian)能(neng)(neng)，并且能(neng)(neng)夠提供(gong)恒定的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)，那么(me)(me)不(bu)同(tong)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)源，兩(liang)極間(jian)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)相同(tong)嗎(ma)?展示各種干電(dian)(dian)(dian)(dian)(dian)池(1號、2號、5號、7號)，請幾(ji)個同(tong)學觀察電(dian)(dian)(dian)(dian)(dian)池上面(mian)寫的(de)(de)(de)(de)規格，發現(xian)盡管電(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)(de)型號不(bu)同(tong)，但是都標有“1.5V”字樣.我們(men)(men)(men)把示教電(dian)(dian)(dian)(dian)(dian)壓(ya)表直接接在干電(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)(de)兩(liang)端(duan)進行測量(liang)，發現(xian)結果(guo)確實是1.5V.講臺上還(huan)擺放有手搖發電(dian)(dian)(dian)(dian)(dian)機、蓄電(dian)(dian)(dian)(dian)(dian)池、紐扣電(dian)(dian)(dian)(dian)(dian)池，它們(men)(men)(men)兩(liang)端(duan)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)是否也是1.5V呢?(學生回答(da)：不(bu)是)那么(me)(me)如(ru)何知道(dao)它們(men)(men)(men)兩(liang)端(duan)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)呢?(學生：用電(dian)(dian)(dian)(dian)(dian)壓(ya)表直接測量(liang))·

結論：電(dian)源(yuan)(yuan)兩(liang)極間的(de)(de)(de)電(dian)壓完全由電(dian)源(yuan)(yuan)本(ben)身的(de)(de)(de)性質(zhi)(如材料、工(gong)作方式等)決定，同(tong)種(zhong)電(dian)池(chi)用電(dian)壓表測量其兩(liang)極間的(de)(de)(de)電(dian)壓是相同(tong)的(de)(de)(de)，不同(tong)種(zhong)類(lei)的(de)(de)(de)電(dian)池(chi)用電(dian)壓表測量其兩(liang)極間的(de)(de)(de)電(dian)壓是不同(tong)的(de)(de)(de).為(wei)了表示(shi)電(dian)源(yuan)(yuan)本(ben)身的(de)(de)(de)這種(zhong)特性，物(wu)理學中(zhong)引入了電(dian)動勢的(de)(de)(de)概念(nian).

板書：2、電源電動勢(shi)

教師：從上面的演示和(he)分析可知(zhi)，電(dian)(dian)源的電(dian)(dian)動(dong)勢(shi)在數值上等(deng)于電(dian)(dian)源未接入電(dian)(dian)路時兩極間(jian)的電(dian)(dian)壓(ya).

板書：電源的(de)電動(dong)勢在(zai)數值上等于電源沒(mei)有接入電路(lu)時其兩極間的(de)電壓(ya).

例(li)如，各種型號的干電(dian)(dian)(dian)池的電(dian)(dian)(dian)動勢都是1.5V.那么把(ba)一(yi)節1號電(dian)(dian)(dian)池接入電(dian)(dian)(dian)路中，它兩極間的電(dian)(dian)(dian)壓(ya)是否還(huan)是1.5V呢?用示(shi)教板(ban)演示(shi)

，電路如(ru)圖所示，結論：開關閉合(he)前，電壓表(biao)示數(shu)是1.5V，開關閉合(he)后，電壓表(biao)示數(shu)變(bian)為(wei)1.4V.實(shi)驗表(biao)明，電路中有了電流后，電源兩極(ji)間的(de)電壓減少了.

教師：上面的實驗(yan)中(zhong)，開關閉(bi)合后，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源兩(liang)極間的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)降(jiang)為1.4V，那(nei)么減少的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)哪(na)去了呢?用投影儀展示實驗(yan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)，介紹閉(bi)合電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)可分為內(nei)、外電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)兩(liang)部(bu)(bu)分，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源內(nei)部(bu)(bu)的叫(jiao)內(nei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源外部(bu)(bu)的叫(jiao)外電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu).接在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源外電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)兩(liang)端(duan)的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)表測(ce)得(de)的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)叫(jiao)外電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya).在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源內(nei)部(bu)(bu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極附近的探針A、B上連接的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)表測(ce)得(de)的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)叫(jiao)內(nei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya).我們現在就通過實驗(yan)來研究閉(bi)合電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中(zhong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)動勢和內(nei)、外電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)之間的關系.

板書(shu)：3、內(nei)電壓(ya)和外電壓(ya)

教師：向學生介紹實(shi)驗(yan)裝置及電(dian)路連接(jie)方法，重點(dian)說明內電(dian)壓的(de)測(ce)(ce)量.實(shi)驗(yan)中接(jie)通電(dian)鍵(jian)，移動(dong)(dong)滑動(dong)(dong)變阻器的(de)滑動(dong)(dong)頭使其阻值減小(xiao)，由(you)兩(liang)個(ge)電(dian)壓表讀出(chu)若干(gan)組(zu)內、外電(dian)壓和(he)的(de)值.再斷(duan)開電(dian)鍵(jian)，由(you)電(dian)壓表測(ce)(ce)出(chu)電(dian)動(dong)(dong)勢(shi).分析實(shi)驗(yan)結果可以發現什么規(gui)律呢?

學生：在誤差許可(ke)的(de)范圍內，內、外(wai)電壓之和等于(yu)電源電動(dong)勢.

板(ban)書：在(zai)閉合電路中，電源(yuan)的(de)電動(dong)勢等(deng)于內(nei)、外(wai)電壓之和，即(ji).

下面我們(men)來分(fen)析在整個電(dian)路中電(dian)壓、電(dian)流、電(dian)阻之間的關(guan)系.

教師：我(wo)們來做一個實驗，電路(lu)圖如圖所示

觀察電鍵S先(xian)后接通1和2時小燈(deng)泡的亮度.

結(jie)論(lun)：把開(kai)關撥到(dao)(dao)2后，發現小燈泡的亮度比剛才(cai)接3V的電(dian)源(yuan)時還稍暗些.怎(zen)么解釋這個實驗現象(xiang)呢?這就要(yao)(yao)用到(dao)(dao)我(wo)們將(jiang)要(yao)(yao)學習的內容——閉合電(dian)路的歐姆(mu)定律.

板書：閉合電(dian)路的歐(ou)姆(mu)定律

教師：在圖(tu)1所示(shi)電(dian)(dian)路圖(tu)中，設電(dian)(dian)流(liu)(liu)為，根據歐姆(mu)定律(lv)，，，那么，電(dian)(dian)流(liu)(liu)強度(du)，這就是(shi)閉合(he)電(dian)(dian)路的歐姆(mu)定律(lv).

板書：4、閉合電(dian)(dian)路的歐姆定律的內(nei)容：閉合電(dian)(dian)路中的電(dian)(dian)流強(qiang)度和電(dian)(dian)源電(dian)(dian)動勢成正比，和電(dian)(dian)路的內(nei)外電(dian)(dian)阻之和成反比.表(biao)達式為.

同學們從這個表達式(shi)可以看(kan)出，在電(dian)(dian)(dian)源(yuan)恒定時(shi)，電(dian)(dian)(dian)路中的(de)電(dian)(dian)(dian)流強度隨(sui)電(dian)(dian)(dian)路的(de)外(wai)電(dian)(dian)(dian)阻變(bian)化(hua)而變(bian)化(hua);當外(wai)電(dian)(dian)(dian)路中的(de)電(dian)(dian)(dian)阻是定值電(dian)(dian)(dian)阻時(shi)，電(dian)(dian)(dian)路中的(de)電(dian)(dian)(dian)流強度和電(dian)(dian)(dian)源(yuan)有關.

教師：同學們能否用閉合電(dian)路的歐姆定律(lv)來解釋上一個實驗現(xian)象呢?

學生：9V的(de)(de)(de)電(dian)(dian)(dian)(dian)源如果內電(dian)(dian)(dian)(dian)阻很大，由閉合電(dian)(dian)(dian)(dian)路的(de)(de)(de)歐姆定律可知(zhi)，用它做電(dian)(dian)(dian)(dian)源，電(dian)(dian)(dian)(dian)路中的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)I可能(neng)較(jiao)小;而(er)電(dian)(dian)(dian)(dian)動(dong)勢3V的(de)(de)(de)電(dian)(dian)(dian)(dian)源內阻如果很小，電(dian)(dian)(dian)(dian)路中的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)可能(neng)比(bi)大，用這兩(liang)個(ge)電(dian)(dian)(dian)(dian)源分別給相同(tong)的(de)(de)(de)小燈泡供電(dian)(dian)(dian)(dian)，燈泡的(de)(de)(de)亮度取決于，那(nei)么就出現了(le)剛才的(de)(de)(de)實驗現象了(le).

教師(shi)：很好.一般電源的電動勢和(he)內電阻在短時間內可以(yi)認為是不(bu)變(bian)(bian)的.那么外電阻的變(bian)(bian)化，就會引(yin)起電路(lu)中電流的變(bian)(bian)化，繼而引(yin)起路(lu)端電壓、輸(shu)出功率、電源效(xiao)率等的變(bian)(bian)化.

幾個重要推論

(1)路(lu)端電壓隨外電阻(zu)變(bian)化的規律

板書：5幾個重要(yao)推論

(l)路端(duan)電(dian)壓隨外電(dian)阻變化(hua)的規律演示實驗,圖3所(suo)示電(dian)路，

4節1號電(dian)池和1個10Ω的定(ding)值電(dian)阻串(chuan)聯組成電(dian)源(yuan)(因為(wei)通常(chang)電(dian)源(yuan)內阻很(hen)小(xiao)，的變化(hua)也很(hen)小(xiao)，現象不明顯)移動滑動變阻器的滑動片，觀察電(dian)流表和電(dian)壓表的示數是如何隨(sui)變化(hua)?

教師：從實驗(yan)(yan)出(chu)發，隨著電(dian)阻的增大(da)(da)，電(dian)流逐(zhu)漸減小，路端電(dian)壓逐(zhu)漸增大(da)(da).大(da)(da)家(jia)能用閉(bi)合電(dian)路的歐(ou)姆定律(lv)來解釋這個實驗(yan)(yan)現象嗎(ma)?

學生：因為變大(da)(da)，閉合電路(lu)的(de)總電阻(zu)增大(da)(da)，根據閉合電路(lu)的(de)歐姆定律(lv)，，電路(lu)中的(de)總電流減小，又(you)因為，則路(lu)端電壓增大(da)(da).

教師：正(zheng)確.我們(men)得出(chu)結論，路(lu)(lu)端(duan)電(dian)(dian)(dian)(dian)壓隨外電(dian)(dian)(dian)(dian)阻增大而增大，隨外電(dian)(dian)(dian)(dian)阻減小而減小.一般認(ren)為(wei)電(dian)(dian)(dian)(dian)動勢和內(nei)電(dian)(dian)(dian)(dian)阻在(zai)短(duan)時(shi)間內(nei)是(shi)不變(bian)的，初中我們(men)認(ren)為(wei)電(dian)(dian)(dian)(dian)路(lu)(lu)兩端(duan)電(dian)(dian)(dian)(dian)壓是(shi)不變(bian)的，應(ying)該是(shi)有條件的，當→無窮大時(shi)，→0，外電(dian)(dian)(dian)(dian)路(lu)(lu)可視(shi)為(wei)斷路(lu)(lu)，→0，根據，則，即當外電(dian)(dian)(dian)(dian)路(lu)(lu)斷開時(shi)，用電(dian)(dian)(dian)(dian)壓表直接測量電(dian)(dian)(dian)(dian)源兩極(ji)電(dian)(dian)(dian)(dian)壓，數(shu)值等于電(dian)(dian)(dian)(dian)源的電(dian)(dian)(dian)(dian)動勢;當減小為(wei)0時(shi)，電(dian)(dian)(dian)(dian)路(lu)(lu)可視(shi)為(wei)短(duan)路(lu)(lu)，為(wei)短(duan)路(lu)(lu)電(dian)(dian)(dian)(dian)流，路(lu)(lu)端(duan)電(dian)(dian)(dian)(dian)壓.

板(ban)書5：路(lu)端電(dian)壓隨外電(dian)阻增(zeng)大而增(zeng)大，隨外電(dian)阻減小而減小.斷路(lu)時，→∞，→0，;短路(lu)時，，.

電(dian)路的路端電(dian)壓與(yu)電(dian)流的關系可以用圖像表示如下

(2)電源的輸出功率隨外(wai)電阻變(bian)化的規律.

教師：在純(chun)電阻電路中(zhong)，當用一個固定的電源(設、r是定值(zhi))向變化的外(wai)電阻供電時(shi)，輸出的功率，

又因為，

所以，

當時，電源有最(zui)大的輸(shu)出(chu)功率.我們可以(yi)畫出(chu)輸(shu)出(chu)功率隨外電阻變化的圖(tu)(tu)線，如圖(tu)(tu)所示.

板書6：在(zai)純(chun)電(dian)阻(zu)電(dian)路(lu)中，當用一個固定(ding)的(de)(de)電(dian)源(即、是定(ding)值)向變化的(de)(de)外電(dian)阻(zu)供電(dian)時，輸出的(de)(de)功率有最大值.

教師(shi)：當輸(shu)出功(gong)率(lv)最(zui)大(da)時，電源的效率(lv)是(shi)否也最(zui)大(da)呢?

板書7：電源的效率隨外電阻(zu)變化的規律(lv)

教師：在電(dian)路中電(dian)源的(de)(de)總功率(lv)為，輸出的(de)(de)功率(lv)為，內電(dian)路損耗的(de)(de)功率(lv)為，則電(dian)源的(de)(de)效(xiao)率(lv)為，當(dang)變大，也變大.而當(dang)時，即輸出功率(lv)最大時，電(dian)源的(de)(de)效(xiao)率(lv)=50%.

板書8：電源的效率隨(sui)外電阻(zu)的增(zeng)(zeng)大(da)而(er)增(zeng)(zeng)大(da).

四、講解例題

五、總結

探究活動

1、調查各(ge)種不同電源的性能特點(dian)。

(包括電動勢、內阻(zu)、能量轉化情(qing)況、工(gong)作原理、可否充電)

2、考察目前(qian)對廢(fei)舊電池的回(hui)收(shou)情況。

(1)化學電池的工作(zuo)原理;

(2)廢舊(jiu)電池對(dui)環境的污染主要表現在哪些(xie)方(fang)面(mian);

(3)當(dang)前(qian)社會對(dui)廢(fei)舊電池的(de)重(zhong)視程度;

(4)廢舊電池的(de)回(hui)收由哪些主要的(de)途徑(jing)和利用方式(shi);

(5)如何更好的(de)變(bian)廢為寶或使廢舊電池對環境的(de)污染減小(xiao)到最小(xiao)。

3、通過(guo)本章節的學習，根據全電(dian)路歐(ou)姆(mu)定律有關知識，可(ke)以得出(chu)結論：電(dian)源的輸(shu)出(chu)功(gong)率最大時，內外電(dian)阻應(ying)該相等(deng)，而此時電(dian)源的效率則(ze)只有50%;請(qing)你設計出(chu)一種方案，在實際(ji)應(ying)用中如何配置(zhi)電(dian)源和負(fu)載之間的關系，使電(dian)源的輸(shu)出(chu)功(gong)率和效率盡可(ke)能的達到較大。