《八年级物理电与热》由会员分享，可在线阅读，更多相关《八年级物理电与热（9页珍藏版）》请在金锄头文库上搜索。
1、电与热,一、电流的热效应,电流通过导体时电能转化为热，这个现象叫做电流的热效应。,演示实验：,实验结论： 在电流、通电时间相同的情况下，电阻越大，产生的热量越多。 在通电时间、电阻相同的情况下，电流越大，产生的热量越多。,二、焦耳定律,电流通过导体产生的热量跟电流的二次方成正比， 跟导体的电阻成正比，跟通电时间成正比。,Q热量焦耳（J）. I电流安培（A） R电阻欧姆() t时间秒(s),Q=I2Rt,焦耳,焦耳（18181889）Joule,James Prescott ，英国物理学家。1818 年 12月24日生于曼彻斯特附近的索尔福德，1889年10月11日卒于塞尔。,思考：,电炉丝通过
2、导线接到电路里，电炉丝和导线通过的电流相同。为什么电炉丝热得发红，而导线几乎不发热？,三电热的利用和防止,如：电熨斗、电烙铁、电炉等,电热器: 主要部分发热体 工作原理：电流热效应,用电来加热的设备,电视机加散热窗 计算机、电动机等还要加风扇,利用：,防止：,http:/ 上海外围招聘 -day where there are so many little foreigners not born in this country or whose parents havent been long here. She”dreamily soliloquizing, with a glance at
3、 that lavender-smocked figure“said that, last year, she and the other members of the Akiyuhapi Camp Fire in that Pennsylvanian milling town, where she became a Camp Fire Girl, did so much voluntary work upon the public playground, largely among the little immigrants, teaching them American songs, Am
4、erican games, telling them stories, settling their squabbles. Well! I guess Im not going to bother her with questions about her Morning-Glory name just now. Over there where shes standing”flashing another glance at the gray auto, with two girls in it and one leaning against its silver door-knob“Id h
5、ave to bray like a jackass to be heard above the music of that absurd piano, perched upon a low cart. Goody!” with a sudden, excited movement of her vivid shoulders. “I shouldnt like to be that perched-up pianist. Just suppose the playground horse should take it into his head to popto danceto chase
6、the weasel, too?”Was it any suddenly restless movement on the part of that four-footed servant of the city which drew the strolling piano upon a low cart from playground to playground to thresh out music for the childrens danceswas it that which flashed the thought backward over his flicking tail, o
7、ver the head of the pounding pianist seated upon a light cane chair before the lashed piano, flashed it into Sallys brain? That, or the elfin dance of sunbeams upon his stamping hoofs which, together with the popping dance-cries of the children and the louder popping of the musical instrument behind
8、 himdeliriously out of tune, toomust surely infect the staidest horse?Sally did not know which launched the apprehension, the tickling sunbeams or the restless hoofs and head. But she was used to horses. She found herself mechanically straightening up, controlling the giddy dance-spirit in her own s
9、oles, moving nearernearerto the low cart as if she could not help it.A brilliant orange streak in the sunlight she, flecked oriole-like with black, from the velvet ribbon that lent tone to that saucy little Tam, to the black needlework stars upon the heaving girlish breast.Then all at once this huma
10、n flame-bird weaving its way in and out between sets of dancing children was halted by a musical crash, brought up short on tiptoe by a screaming commotion through which rang a nightmare of treble chords wildly sustained by the pianists right hand blundering among the shrieking keys of the elevated
11、piano, while her left arm waved on high, imploring help, the whole seeming a premature, mad finale to the popping music, to which every voice upon the playground, animate and inanimate, lent a crydiscordantly at that!The effect was so feverishly funny that Sally, who had the orioles gay spirit withi
12、n her orange-smocked breast, vented a shriek as loud as any, to swell the confusion, automatically clapping her fingers to her ears.The voices of some fourscore children had popped explosively from song and shout to scare-note and shriek, a conglomerate shriek, strengthened by every foreign accent u
13、nder the sun (any cry ever hurled from the crumbling Tower of Babel was nothing to it!), a shriek that hung, sustained, in air together with the rasping, squelching notes of that unfinished musical measure which seemed to tatter the air itself.“Ouch! My s-soul!” murmured Sally under her breath. “The
14、 horse! Its thehorse. He is bolting, with the piano lashed to the cart behind him. And thepoorpianist!”It needed no more. She saw the girl-musicians left arm waving, imploring, saw her rock upon the light cane chair before the instrument that was not lashed to the rocking cart; she heard the horses
15、mutinous snort, heard it strangely echoed in dumb fashion by a pair of parted childish lips near her; crowning all, she caught the terrified shriek of a small boy who clutched at his raven-black hair and what English he could muster as he started toward a sand-pile ahead, yelling, “Mine babeemine ba
16、bee! Horse he go kill her; sheshe go alldeaded!”And like the flame from the cloud leaped the answering fire in Sesoolittle Camp Fire Girl!“The driverthe boy driverhe ought to be shot; hes umpiring a baseball game,” was the first distinct thought that leaped to her mind as, like an oriole on the wing
17、, she sped across the sunlit grass in the wake of the still rocking cart, the fiendishly howling piano, the screaming, swaying pianist. The second lightning conviction was: “Its up-hill and the horse cant really run very fast with that absurd piano behind him! Hes dancing all over the place, rather
18、than wildly running, now!. Rolie showed mehas told me so oftenhow to stop a runaway!”Rolie was her Boy Scout brother and that gallant fourteen-year-old Scout seemed to run neck and neck with her in this crisis, whispering heart into her, advising her movements.The firefly in her eyes, soaring, golde
19、n, above consternation, has lit now upon the horses quivering haunchon his black mane.“After all, hes only a horse; Ive not alone ridden one, but, as a Camp Fire Girl, have saddled and bridled and fed an currycombed it, too, every day for the past month!” whizzed thought, darting ahead of her as wit
20、h another springy step or two her right hand has seized the carts shaft to hold on and prevent herself from falling in the supreme effort she is about to make.Her left hand, attached to a strong little wrist for a girl not yet sixteen, has snatched at the dragging reins, holding them short, is tryin
21、g to pull the horses head down, turn it toward her!Only a horse! And a brother-horse was such a friend of hers! The firefly bore that thought upon its wings as it wheeled above doubt, resistance, wrenching strain that was tugging her soft young arms from their socketsher feet from the solid earth.Only a horse! But a maddened horse, distracted by the shrieking ivories behind him!Her girls strength against his!Yet his rebel-crest was lowering. His lifted forelegs were uncurling, the waving hoofs that cared not what they smashed returning sanely to the sod.And,
