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九年级物理电功课件2


电功和电能

会师中学 物理组

常见的用电器

电流通过电灯时,灯丝灼热发 光,电能转化为内能和光能

电吹风利用电流做功,把电 能转化成机械能和内能。

电热水瓶,利用电能把水烧 开。电能转化为内能

洗衣机利用电能,把衣物洗 净。电能转化为机械能。

观察结论
? 1、以上用电器工作时,能是怎样 转化的? ? 答、把电能转化为其他形式的能。 ? 2、做功的实质什么? ? 答、做功的实质是实现能的转化 (能的转化是靠做功实现的),能 转化的过程就是做功。

? 3、电流流动时能做功吗? ? 电流流过用电器,把电能转化为其 他形式能的过程,也就是电流做功 的过程,所以,电流能够做功。

电功:电流所做的功。
表示:W

实质:电流做功的过程实际上就是 电能转化为其他形式能量的过程, 电流作了多少功就有多少电能转化 为其他形式的能。也就是消耗了多 少电能;获得了多少其他形式的能。

想一想:下面几种情况电流做功了没 有?如果做了电路中的能量是如何转 化的?

1、正在转动的电动机。
2、家中取暖的电热毯。

3、教室里亮着的电灯。
4、热饭用的微波炉。

你能猜想电流做功的多少 与哪些因素有关吗?
? ? ? ? 电压、电流、电阻中你想否绝什么? 还想添加什么? 确定电流做功与哪些因素有关? 电压、电流、通电时间。

交流论证:

1、电路中电流电压一定,通电时间越长, 钩码升高的越高,表明电流做功越多。 2、在通电时间一定时,电压电流越大,钩 码升高的越高,表明电流做功越多。

结论: 实验表明:电路中电压越大,电流越 大,通电时间越长,电流所作的功越 多。

精确试验数据表明: 电流所作的功跟电压、电流和 通电时间成正比。电功等于电压、 电流和通电时间的乘积。

U:电压——伏特
t :时间——秒 耳

I :电流——安培
W:电功——焦

电功的单位

电功的单位是焦耳 ( J ) 生活中常用千瓦时又叫“度” , 符号是 KW· h

1

6 KW· h=3.6?10 J

小明的书桌上有一盏台灯, 正常使用时通过灯丝的电流为 150mA,他每晚要在灯下面学 习30min,那么每晚流经台灯的 电流做多少功?

220 V —— 电能标应在220V的电路中使用。

2.5(10)A —— 电能表的允许通过的最大电流 为2.5A,短时间允许通过的最大电流是10A。
50 Hz —— 电能表在50 Hz的电路中使用。 1440 r/kWh —— 每消耗1 KWh的电能,电能 表转盘转过1440转。

6 1kWh=1度=3.6×10 J

3、技术参数和示数说明: 额定电压 220V

持续工作的 最大电流5A, 短时间可通 过电流5A— 10A

1KW· h电 能表转盘 转动圈数 为1200转

IC卡电能表

? 其他:电子式单向电能表 ? 重要参数imp/(KW· h): 表示用 电1KW· h指示灯闪烁的次数,如 6400imp/(KW· h)

计数特点:累计



月末的数字减去月初的数字:

561.6-478.2=83.4kWh

三.电能表
1、电能表的作用:测电功 2、根据计数器读数:
月初 月末

3246.8 kW· h

3265.4 kW· h
KW· h

本月用电18.6

3760,上月末的示数是3646,这个月他家 用去多少电能?如果电费为0.45元/度,这 个月他家应交多少电费? 解:W=W1-W2=3760度-3646度 =114度

1、小明家的电能表本月末的示数是

114度×0.45元/度=51.3元
答:这个月他家应交51.3元电费。

? 2、家用电灯在一次使用时通过的电流 是0.2A,消耗的电能是2.64×104J,这 个电灯这次用了多长时间? ? 已知:I=0.2A,W= 2.64×104J, ? U=220V,求:t=? W ? 解:由W=UIt得,t= UI = 2.64×104J/(220V×O.2A)=600S

例1:

一电能表表盘上标明“1200 r / (KW· h)”,将某用电器单独接在该表上。 当用电器工作4min后,电能表转盘转过55r。 若用电器额定电压是220V,求通过该用电 器的电流。

55 ? KW ? h 解:W= 1200 r /( KW ? h) 1200 55 6 5 ? ? 3.6 ?10 J ? 1.65?10 J 1200

55r

W=1.65×105J

∵W=U I t

W 1.65?10 J ?I ? ? ? 3.125A Ut 220 V ? 240s
5

答:通过该用电器的电流为3.125A。

例2、阻值分别是4Ω 和6Ω 的两个电阻, 并联在4V 的电路上时,通电1 min ,通过 每只电阻的电流做的功分别 是多少?串联 在4 V 的电路上,通电1min ,通过每只电 R =4Ω I =1A 阻的电流做的功分别是多少? 1
1

分析:

第一种情况:并联

R2=6Ω I2=2/3A

W1=U1· I1· t= 4V×1A×60s=240J

U=4V

U1=U2=U=4V

W2=U2· I2· t= 4V×2/3A×60s=160J

第二种情况:串联

U1=1.6V
R1=4Ω

U2=2.4V
R2=6Ω

I=U / R串= 4V/10Ω=0.4A
W1=U1· I· t= 1.6V×0.4A×60s= 38.4J W2=U2· I· t= 2.4V×0.4A×60s=57.6J
U=4V

思考: 1、如果让你测量你家中电路中 电流所做的功,你有哪些方法? 2、电功的单位为焦耳,那么除 了焦耳以外还有哪些是电功的单位?

http://www.sciave.com/ 最准的免费时彩计划软件
CHAPTER I A STROLLING PIANO“Why did she choose ‘Morning -Glory’ as her tribe name?” asked M?nkw?n the Rainbow of Sesooā the Flame , as Rainbow and Flame, with girlish arms entwining, stood beneath the shelter of the Silver Twins, two kingly birch -trees, so identical in stature even to their topmost jeweled crowns of leaves flashing in the July sun, so alike in the silver symmetry of each fair limb as to be named the Twins .These silver kings were one-hearted, too, in their benevolent purpose in life, which was to unite in casting a brotherly shade over a certain corner of the broad city playground, dotted with children from every clime, and incidentally to fan the flushed cheeks of the two girls directly benea th them, bound together by a girdling rainbow that played about their waists, woven by the sun’s shuttle amid the quivering birch -leaves, fit symbol of their binding Camp Fire sisterhood.Sesooā’s eyes danced, lit by a tiny golden flame that uncurled itself in their demure hazel like a firefly alighti ng on a brown leaf. She caught her lower lip between the pretty incisors that decorated the front of her mouth as she scrutinized the semi -distant figure of a sixteen-year-old girl—perhaps nearer to seventeen—clad in a loose lavender smock to her knees, whence to her ankles there was a gleam of white sk irt, with the most bewitching, frilled summer “Tam” of lavender, matching her smock, shielding her brown head, sheltering her face, like the hoo d of a flower. This floral figure leaned against the open door of a handsome automobile which was standing upon the playground avenue.“I’m sure i t’s beyond me to tell why Jessica Holley (Jessica Dee Holley; she always likes to bring the unusual little middle name in, because it was her mothe r’s, I suppose), why she chose Welatáwesit, which is the only Indian equivalent she could find for Morning -Glory—literally meaning ‘Climbing Plant’ or ‘Pretty Flower’ for her Camp Fire name. But I believe there’s a story attached to the choice, some ‘cunning’ little anecdote of her childhood. Wi sh I could ferret it out! She seems, always, to have been called ‘Glory,’ nearly as much as Jessica,” answered Sesooā racily, she who in every -day life bore no flaming cognomen, but was plain little, gay little, Sally Davenport, as full of quips and quirks, of lightning impulses and sudden tu rns as the wheeling firefly in her eyes.“Goody! I’d like to hear the anecdote, too. The Morning -Glory name suits her so well that I thought she must have dreamed it—that it came to her in sleep—as I dreamed mine,” laughed the Rainbow, whose rightful name when she was not clad in a leather -fringed robe of khaki, in moccasins and head-band, and seated by a Council Fire, was Arline Champion. “But I call it absurd, meanly absurd, that if there’s any stor y about her and her name, we should not hear it, we who have named our Camp Fire (and it’s the best in the city, too, though I say it myself!), o ur whole group or tribe of fourteen girls, after her,” she went on with a stamp of her foot on the playground sod and with rainbowed emphasis; she was t he shell-tinted, demurely shining kind of fifteen-year-old girl who unconsciously aims at carrying a rainbow in her pocket, to brighten the dull or tear -wet day.“Oh! we didn’t exactly ‘name it after her,’” demurred Sally. “She happened to come here last winter to visit those rich girls, the Deerings, who are all fluff an’ stuff; that exactly describes them, Olive and Sybil ——” There was the least little green tinge of the spitfire about Sesooā’s fla me now as she shot a glance toward two girls seated in the waiting automobile together with an older woman, evidently chaperon to the band of girl s. “Oh! I say, pinch me; I shouldn’t have said that, should I, seeing that they brought us here in their car? But ’twas the first time they ever did it, though my father is head-bookkeeper in their father’s office at the Works; and I’ll engage ’twas Morning -Glory—Jessica—who suggested it, as we all wanted to visit this playground where there are so many foreign children, to see them dance their folk dances,” she ran on, speech flitting away f rom its starting-point in the wake of her firefly dance, which vivaciously hovered from one object or group of objects to another.Arline waited for it to alight again on Jessica, as it presently did.“Well! as I was saying,” reverted Sally, “you remember how she came here last February just when we were beginning to organize our Camp Fire group, when we had secured Miss Darina Dewey as Guardian (I think she’s a love of a Guardian and I like her unu sual first name, too, though some of the girls don’t!) but before we had applied for our Charter, when we were searching for a name for our new Cam p Fire circle, raking over Indian names like leaves until —goodness! we seemed half-smothered in them.” Sally paused for breath, breathlessly smothered, ind eed, by the sunlit torrent of her own words, which had a trick of inundating a listener.“It was at our second meeting, I think, at Miss Dewey’s house,” she went on, “that Jessica came in, all snow an’ sparkle from her eyes to her toes, and introduced herself by showing a transfer card sign



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