全電路歐姆定律及焦耳定律(lv)介(jie)紹

全電(dian)(dian)(dian)路(lu)(lu)是(shi)指含有電(dian)(dian)(dian)源和負(fu)載的閉(bi)合回路(lu)(lu)。全電(dian)(dian)(dian)路(lu)(lu)歐(ou)姆(mu)定律又稱閉(bi)合電(dian)(dian)(dian)路(lu)(lu)歐(ou)姆(mu)定律，其內容是(shi)：閉(bi)合電(dian)(dian)(dian)路(lu)(lu)中的電(dian)(dian)(dian)流(liu)與電(dian)(dian)(dian)源的電(dian)(dian)(dian)動勢成正比，與電(dian)(dian)(dian)路(lu)(lu)的內、外電(dian)(dian)(dian)阻之和成反比，即

全電路歐姆定律應用如圖所(suo)示。

圖(tu)全電路歐姆定律(lv)應用說明圖(tu)

圖(tu)中點畫線框(kuang)內(nei)為電(dian)(dian)(dian)源，ro表示電(dian)(dian)(dian)源的(de)(de)內(nei)阻，e表示電(dian)(dian)(dian)源的(de)(de)電(dian)(dian)(dian)動勢。當開關(guan)s閉合后，電(dian)(dian)(dian)路中有電(dian)(dian)(dian)流i流過，根據

全電路歐姆定律可求得。電(dian)(dian)源(yuan)輸(shu)出(chu)電(dian)(dian)壓(也(ye)即電(dian)(dian)阻(zu)r兩(liang)端的(de)電(dian)(dian)壓)u=ir=1×10v=10v,內阻(zu)ro兩(liang)端的(de)電(dian)(dian)壓u=iro=1×2v=2v。如果將(jiang)開關s斷開，電(dian)(dian)路中的(de)電(dian)(dian)流i=0a,那么內阻(zu)ro上消(xiao)耗(hao)的(de)電(dian)(dian)壓u=0v,電(dian)(dian)源(yuan)輸(shu)出(chu)電(dian)(dian)壓u與電(dian)(dian)源(yuan)電(dian)(dian)動(dong)勢相等，即u=e=12v。

根據全(quan)電(dian)(dian)(dian)路歐姆(mu)定律不難看出以(yi)下幾(ji)點。①在電(dian)(dian)(dian)源(yuan)未接(jie)負載時，不管電(dian)(dian)(dian)源(yuan)內阻(zu)多大(da)，內阻(zu)消耗(hao)的(de)電(dian)(dian)(dian)壓始終(zhong)為0v,電(dian)(dian)(dian)源(yuan)兩(liang)端電(dian)(dian)(dian)壓與電(dian)(dian)(dian)動勢相等。

②當電源(yuan)與負(fu)載構成閉合(he)電路(lu)后，由于有(you)電流流過(guo)內(nei)阻，內(nei)阻會消(xiao)耗(hao)電壓(ya)(ya)，從而使電源(yuan)輸(shu)出(chu)電壓(ya)(ya)降低。內(nei)阻越(yue)大(da)(da)，內(nei)阻消(xiao)耗(hao)的電壓(ya)(ya)越(yue)大(da)(da)，電源(yuan)輸(shu)出(chu)電壓(ya)(ya)越(yue)低。

③在電(dian)(dian)源內阻(zu)不變(bian)的情況(kuang)下(xia)，如(ru)果(guo)外阻(zu)越小，電(dian)(dian)路中的電(dian)(dian)流越大，內阻(zu)消耗的電(dian)(dian)壓(ya)也(ye)越大，電(dian)(dian)源輸出電(dian)(dian)壓(ya)也(ye)會降低。

由于正常電(dian)(dian)源的內(nei)(nei)阻(zu)很小，內(nei)(nei)阻(zu)消耗的電(dian)(dian)壓很低，故一般情況下可認為(wei)電(dian)(dian)源的輸出電(dian)(dian)壓與電(dian)(dian)源電(dian)(dian)動勢相(xiang)等(deng)。

利用(yong)全電(dian)(dian)(dian)路歐姆定律可以解釋很多(duo)現(xian)象。比如(ru)，舊電(dian)(dian)(dian)池(chi)兩端電(dian)(dian)(dian)壓(ya)(ya)與(yu)正常電(dian)(dian)(dian)壓(ya)(ya)相同，但將舊電(dian)(dian)(dian)池(chi)與(yu)電(dian)(dian)(dian)路連(lian)接后除了輸(shu)出電(dian)(dian)(dian)流(liu)很小外(wai)，電(dian)(dian)(dian)池(chi)的(de)輸(shu)出電(dian)(dian)(dian)壓(ya)(ya)也會急劇下降，這是因為(wei)舊電(dian)(dian)(dian)池(chi)內阻變大的(de)緣故；又如(ru)，將電(dian)(dian)(dian)源(yuan)(yuan)正、負極(ji)直接短(duan)路時，電(dian)(dian)(dian)源(yuan)(yuan)會發(fa)熱(re)甚至燒壞，這是因為(wei)短(duan)路時流(liu)過(guo)電(dian)(dian)(dian)源(yuan)(yuan)內阻的(de)電(dian)(dian)(dian)流(liu)很大，內阻消(xiao)(xiao)耗的(de)電(dian)(dian)(dian)壓(ya)(ya)與(yu)電(dian)(dian)(dian)源(yuan)(yuan)電(dian)(dian)(dian)動勢(shi)相等，大量(liang)的(de)電(dian)(dian)(dian)能在電(dian)(dian)(dian)源(yuan)(yuan)內阻上消(xiao)(xiao)耗并(bing)轉換成熱(re)能，故電(dian)(dian)(dian)源(yuan)(yuan)會發(fa)熱(re)。

焦耳定律

電(dian)流(liu)(liu)流(liu)(liu)過導(dao)(dao)體時(shi)導(dao)(dao)體會發熱(re)，這種現(xian)象稱為電(dian)流(liu)(liu)的熱(re)效(xiao)應(ying)(ying)。電(dian)熱(re)鍋、電(dian)飯(fan)煲和(he)電(dian)熱(re)水器等都(dou)是(shi)利用電(dian)流(liu)(liu)的熱(re)效(xiao)應(ying)(ying)來工作的。

英國(guo)物理學家焦耳通(tong)過實驗(yan)發現：電(dian)流流過導(dao)體(ti)，導(dao)體(ti)發出(chu)的熱量與導(dao)體(ti)流過的電(dian)流、導(dao)體(ti)的電(dian)阻和通(tong)電(dian)的時間(jian)有(you)關。這個關系(xi)用公式表(biao)示(shi)就是

式中(zhong)，q表示熱量(liang)，單位是焦耳(er)（j）；t表示時(shi)間，單位是秒（s）。

焦耳定(ding)(ding)律(lv)(lv)說明：電(dian)流流過導體(ti)產(chan)生的(de)熱量，與(yu)電(dian)流的(de)平方及導體(ti)的(de)電(dian)阻(zu)成正比(bi)，與(yu)通(tong)電(dian)時間也(ye)成正比(bi)。由于這個定(ding)(ding)律(lv)(lv)除了由焦耳發現(xian)外(wai)，科學家楞次也(ye)通(tong)過實(shi)驗(yan)獨(du)立(li)發現(xian)，故該定(ding)(ding)律(lv)(lv)又稱(cheng)焦耳-楞次定(ding)(ding)律(lv)(lv)。

舉例(li)：某(mou)臺(tai)電動機額定電壓(ya)是220v，線圈的(de)(de)電阻為0.4ω，當電動機接(jie)220v的(de)(de)電壓(ya)時，流過的(de)(de)電流是3a，求電動機的(de)(de)功(gong)率和線圈每秒發出的(de)(de)熱量(liang)。