高考物理經(jīng)典復(fù)習(xí)資料之恒定電流講義

1、專業(yè)好文檔 恒定電流一、概念薦入1.電流電流的定義式：，適用于任何電荷的定向移動(dòng)形成的電流。 對(duì)于金屬導(dǎo)體有i=nqvs（q是每個(gè)電荷的電荷量，n為單位體積內(nèi)的自由電子個(gè)數(shù)，s為導(dǎo)線的橫截面積，v為自由電子的定向移動(dòng)速率），這個(gè)公式只適用于金屬導(dǎo)體，千萬(wàn)不要到處套用。方向的規(guī)定：和正電荷的定向移動(dòng)的方向相同，和負(fù)電荷的定向移動(dòng)相反常用單位有：a安，ma毫安，微安，注意相互之間的進(jìn)級(jí)。形成電流的條件：內(nèi)因是有自由移動(dòng)的電荷，外因是導(dǎo)體兩端有電勢(shì)差。例1：在2s內(nèi)共有12.501018個(gè)電子通過(guò)導(dǎo)體某橫截面，則通過(guò)導(dǎo)體的電流強(qiáng)度大小是 a1.0a b6.251018a c6.251017a d0.

2、10a2.電阻定律導(dǎo)體的電阻r跟它的長(zhǎng)度l成正比，跟它的橫截面積s成反比。是反映材料導(dǎo)電性能的物理量，叫材料的電阻率（反映該材料的性質(zhì)，不是每根具體的導(dǎo)線的性質(zhì)）。純金屬的電阻率小，合金的電阻率大。材料的電阻率與溫度有關(guān)系：金屬的電阻率隨溫度的升高而增大。半導(dǎo)體的電阻率隨溫度的升高而減小。有些物質(zhì)當(dāng)溫度接近0 k時(shí)，電阻率突然減小到零這種現(xiàn)象叫超導(dǎo)現(xiàn)象。能夠發(fā)生超導(dǎo)現(xiàn)象的物體叫超導(dǎo)體。材料由正常狀態(tài)轉(zhuǎn)變?yōu)槌瑢?dǎo)狀態(tài)的溫度叫超導(dǎo)材料的轉(zhuǎn)變溫度tc。3. 歐姆定律： 公式：（適用于金屬導(dǎo)體和電解液，不適用于氣體導(dǎo)電）。 電阻的伏安特性曲線：注意i-u曲線和u-i曲線的區(qū)別（）。還要注意：當(dāng)考慮到電阻

3、率隨溫度的變化時(shí)，電阻的伏安特性曲線不再是過(guò)原點(diǎn)的直線（此時(shí)，也就是此時(shí)的電阻為圖線上的點(diǎn)與坐標(biāo)原點(diǎn)連線的斜率而不是切線的斜率） 例2 實(shí)驗(yàn)室用的小燈泡燈絲的i-u特性曲線可用以下哪個(gè)圖象來(lái)表示：a. b. c. d.i i i io u o u o u o u4. 電功和電功率 （1）電功和電功率：電流做功的實(shí)質(zhì)是電場(chǎng)力對(duì)電荷做功。電場(chǎng)力對(duì)電荷做功，電荷的電勢(shì)能降低，電勢(shì)能轉(zhuǎn)化為其他形式的的能量，因此電功，這是計(jì)算電功普遍適用的公式。（2） 電功率：?jiǎn)挝粫r(shí)間內(nèi)電流做的功，這是計(jì)算電功率普遍適用的公式。（3） 電熱和焦耳定律：焦耳定律，電熱q=i2rt對(duì)純電阻而言，電功等于電熱：w=q=uit

4、=i 2r t=對(duì)非純電阻電路（如電動(dòng)機(jī)和電解槽），由于電能除了轉(zhuǎn)化為電熱以外還同時(shí)轉(zhuǎn)化為機(jī)械能或化學(xué)能等其它能，所以電功必然大于電熱：wq，這時(shí)電功只能用w=uit計(jì)算，電熱只能用q=i 2rt計(jì)算，兩式不能通用。例3 某一電動(dòng)機(jī)，當(dāng)電壓u1=10v時(shí)帶不動(dòng)負(fù)載，因此不轉(zhuǎn)動(dòng)，這時(shí)電流為i1=2a。當(dāng)電壓為u2=36v時(shí)能帶動(dòng)負(fù)載正常運(yùn)轉(zhuǎn)，這時(shí)電流為i2=1a。求這時(shí)電動(dòng)機(jī)的機(jī)械功率是多大？例題 直流電動(dòng)機(jī)線圈的電阻為r，當(dāng)電動(dòng)機(jī)工作時(shí)通過(guò)線圈的電流是i，此時(shí)它兩端的電壓為u，則電動(dòng)機(jī)的輸出功率為（ ） aui；biui2r ciu一i2r； du2r5. 電阻的連接（簡(jiǎn)單說(shuō)明串聯(lián)和并聯(lián)電路的

5、電流電壓電阻電壓分配功率分配的關(guān)系） 此處單獨(dú)出題目直接考查以上知識(shí)的可能性不大，一般都是結(jié)合復(fù)雜電路跟電功率最大最小啊，額定功率啊來(lái)出題，做這類題目的最根本還是電路分析，畫(huà)等效電路圖一般用等勢(shì)點(diǎn)法。已知如圖，兩只燈泡l1、l2分別標(biāo)有“110v，60w”和“110v，100w”，另外有一只滑動(dòng)例題：變阻器r，將它們連接后接入220v的電路中，要求兩燈泡都正常發(fā)光，并使整個(gè)電路消耗的總功率最小，應(yīng)使用下面哪個(gè)電路？l1 l2 l1 l2 l1 l2 l1r r r l2ra. b. c. d. 例題： 圖中的 a為理想電流表，v1和v2為理想電壓表，r1為定值電阻，r2為可變電阻，電池e內(nèi)阻不

6、計(jì)，下列說(shuō)法中不正確的是 ar2不變時(shí)，v2讀數(shù)與a讀數(shù)之比等于r1 br2不變時(shí)，v1讀數(shù)與a讀數(shù)之比等于r1 cr2改變一定量時(shí)，v2讀數(shù)的變化量與v1讀數(shù)變化量之和一定為零 dr2改變一定量時(shí)，v2讀數(shù)的變化量與v1讀數(shù)變化量之和不一定為零6. 閉合電路的歐姆定律 ueor內(nèi)容：閉合電路的電流跟電源的電動(dòng)勢(shì)成正比，與內(nèi)、外電路的電阻之和成反比，即i=e/（r+r）由euir可知，電源電勢(shì)等于內(nèi)外電壓之和；適用條件：純電阻電路(3)路端電壓跟負(fù)載的關(guān)系路端電壓：根據(jù)可知，當(dāng)r增大時(shí)，u增大；當(dāng)r減小時(shí)，u減小。u一i關(guān)系圖由閉合電路歐姆定律知：ueir , 路端電壓隨著電路中電流的增大而減

7、小；當(dāng)電路斷路即i0時(shí)，縱坐標(biāo)的截距為電動(dòng)勢(shì)e；當(dāng)外電路電壓為u0時(shí)，橫坐標(biāo)的截距i短=e/r為短路電流；圖線的斜率的絕對(duì)值為電源的內(nèi)電阻（做題的時(shí)候需要注意的是縱坐標(biāo)數(shù)值在坐標(biāo)原點(diǎn)是否是從零開(kāi)始的）(4).閉合電路的輸出功率功率關(guān)系：電源的輸出功率與外電路電阻的關(guān)系:當(dāng)rr時(shí)也即i=e/2r時(shí),電源的輸出功率最大, 由圖象可知，對(duì)應(yīng)于電源的非最大輸出功率p可以有兩個(gè)不同的外電阻rl和r2，不難證明由圖象還可以看出：當(dāng)rr時(shí)，若r增大，則p出減小應(yīng)注意:對(duì)于內(nèi)外電路上的固定電阻，其消耗的功率僅取決于電路中的電流大小電源的供電效率（說(shuō)明：電源的效率總小于1，不能過(guò)分追求效率大）e rr2r1例題

8、：已知如圖，e =6v，r =4，r1=2，r2的變化范圍是010。求：電源的最大輸出功率；r1上消耗的最大功率；r2上消耗的最大功率。r1r2r3r4e r閉合電路中只要有一只電阻的阻值發(fā)生變化，就會(huì)影響整個(gè)電路，使總電路和每一部分的電流、電壓都發(fā)生變化。討論依據(jù)是：閉合電路歐姆定律、部分電路歐姆定律、串聯(lián)電路的電壓關(guān)系、并聯(lián)電路的電流關(guān)系。以右圖電路為例：設(shè)r1增大，總電阻一定增大；由，i一定減??；由u=e-ir，u一定增大；因此u4、i4一定增大；由i3= i-i4，i3、u3一定減??；由u2=u-u3，u2、i2一定增大；由i1=i3 -i2，i1一定減小?？偨Y(jié)規(guī)律如下：總電路上r增大

9、時(shí)總電流i減小，路端電壓u增大；變化電阻本身和總電路變化規(guī)律相同；和變化電阻有串聯(lián)關(guān)系（通過(guò)變化電阻的電流也通過(guò)該電阻）的看電流（即總電流減小時(shí)，該電阻的電流、電壓都減?。缓妥兓娮栌胁⒙?lián)關(guān)系的（通過(guò)變化電阻的電流不通過(guò)該電阻）看電壓（即路端電壓增大時(shí)，該電阻的電流、電壓都增大）。 例題 如圖所示，電源電動(dòng)勢(shì)為e，內(nèi)電阻為r當(dāng)滑動(dòng)變阻器的觸片p從右端滑到左端時(shí)，發(fā)現(xiàn)電壓表v1、v2示數(shù)變化的絕對(duì)值分別為u1和u2，下列說(shuō)法中正確的是 v2v1l1l2l3pa.小燈泡l1、l3變暗，l2變亮 b.小燈泡l3變暗，l1、l2變亮c.u1u2winger tuivasa-sheck, who sc

10、ored two tries in the kiwis 20-18 semi-final win over england, has been passed fit after a lower-leg injury, while slater has been named at full-back but is still recovering from a knee injury aggravated against usa.both sides boast 100% records heading into the encounter but australia have not conc

11、eded a try since josh charnleys effort in their first pool match against england on the opening day.aussie winger jarryd hayne is the competitions top try scorer with nine, closely followed by tuivasa-sheck with eight.but it is recently named rugby league international federation player of the year

12、sonny bill williams who has attracted the most interest in the tournament so far.the kiwi - with a tournament high 17 offloads - has the chance of becoming the first player to win the world cup in both rugby league and rugby union after triumphing with the all blacks in 2011.id give every award back

13、 in a heartbeat just to get across the line this weekend, said williams.the (lack of) air up there watch mcayman islands-based webb, the head of fifas anti-racism taskforce, is in london for the football associations 150th anniversary celebrations and will attend citys premier league match at chelse

14、a on sunday.i am going to be at the match tomorrow and i have asked to meet yaya toure, he told bbc sport.for me its about how he felt and i would like to speak to him first to find out what his experience was.uefa hasopened disciplinary proceedings against cskafor the racist behaviour of their fans

15、 duringcitys 2-1 win.michel platini, president of european footballs governing body, has also ordered an immediate investigation into the referees actions.cska said they were surprised and disappointed by toures complaint. in a statement the russian side added: we found no racist insults from fans o

16、f cska. baumgartner the disappointing news: mission aborted.the supersonic descent could happen as early as sunda.the weather plays an important role in this mission. starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud

17、cover. the balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. it will climb higher than the tip of mount everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners

18、 (5.6 miles/9.17 kilometers) and into the stratosphere. as he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.the balloon will slowly drift to the edge of space at 120,000 feet ( then, i would assume, he will slowly step out onto something resembling an olympic di

19、ving platform.they blew it in 2008 when they got caught cold in the final and they will not make the same mistake against the kiwis in manchester.five years ago they cruised through to the final and so far history has repeated itself here - the last try they conceded was scored by englands josh char

20、nley in the opening game of the tournament.that could be classed as a weakness, a team under-cooked - but i have been impressed by the kangaroos focus in their games since then.they have been concentrating on the sort of stuff that wins you tough, even contests - strong defence, especially on their own goal-line, completing sets and a good kick-chase. theyve been great at all