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九年级物理电功和电功率


第十三章 电功和电功率
电功率计算(习题课)

教学过程:

重点:电功率公式的运用.
难点:灵活运用电功率、欧姆定律公 式解决问题

先复习上节的习题:

有一个标有“8V、2W”的小灯泡, 如果连接在12伏特的电源上必须 16 欧姆的电 串 联一个电阻值为____ ____ 阻才能使小灯泡正常发光,此时在 1 瓦 这个电阻上消耗的电功率是____ 特。(点拨:学会画图分析)
A U R B

教学过程:

复习: 电功率计算公式:

W P? t P ? UI
U2 P? R

P?I R
2

教学过程:

例题1
? 灯L1(PZ220-25)和灯L2(PZ220-60) ? (1)、将它们并联接在220V的电压上; ? (2)、将它们串联接在220伏的电路上, ? 两种情况下哪盏灯泡亮些?为什么? ? 分析与解

例题2

教学过程:

例题2
? 标有“6V 3W”的小灯泡能不能 直接接到9伏的电源上?若要使 小灯泡接上后正常发光,应怎 么办? ? 分析与解

例题3

生活物理社会

例题3
? ?全解?P133 ? 一教室里有4盏标有“PZ220-100”字样 的灯泡,问: 答案:(1)0.45A,484Ω ? (1)每盏灯泡在正常工作时,通过灯丝电 流多大?灯丝电阻多大? ? (2)每天每灯开5h,一个月(30天)消 答案:(2)60度 耗电能多少度? ? (3)如果让其中一盏灯泡接入380 V的电路后能正常工作,应串联一个多 大电阻?这电阻的实际功率为多大? 例题4 答案:352Ω,72.7W

生活物理社会

? ?全解?P127 ? 为估算家中功率为1.5×10-3W的石英钟使 用一节新5号干电池能工作多少天,小亮同 学将阻值为1Ω 的小电珠连接在一节同样 新干进行了放电放电实验,结果他发现累 计放电96min后基本失效.若电池在工作过 程中电压保持不变,则根据实验,估算该 石英钟使用这种新5号干电池能工作的时 间最接近于( C ) ? A.2400天 B.360天 ? C.100天 D.40天 例题5

例题4

生活物理社会

例题5

? ?全解?P136 ? 现有2个灯泡灯L1“10V,10W” 和灯L2 “9V,5.4W”字样,当 将它们串联使用时,电路两端允许 15V;当将它们 加的最大电压是_____ 并联使用时,电路两端允许加的最 1.5 大电流是_____ A,电路消耗的最大 功率是______ W. 13.5

练习

下课了!

金山中学资源中心制作
2006.4

结束

教学过程:

分析 ? 要判断两灯的亮与暗,只要比 较二灯的实际功率大小就可以 了.
? (笔记):比较两灯的亮

与暗,看实际功率.

教学过程:

例1解答:
解:并联时,每盏灯的实际电压均为 220V , 则其实际功率就等于灯的额定功率,因 此可直接判断出灯L2比灯L1亮. 串联时,因每盏灯两端的电压均小于 220V ,所以两灯均不能正常发光,根据 计算结果知,

R1>R2,又因为两盏灯串联,所以通过 它们的电流一样大.因此可根据P=I2R判断 出P1>P2,L1这盏灯亮些.

小结

教学过程:

小结1
? 在并联电路中(各用电器两端电压相 等),电阻大的用电器消耗电功率小; 2 U ? P? 2 U R 并联时比较用 P?
R

? 在串联电路中(各用电器的通过电流相 等),电阻大的用电器消耗的电功率大.

P?I R
2

串联时比较用 P?I R
2

教学过程:

例2分析:
? 根据已有的知识不难判断,因 为9V已大于灯泡的额定电压6V,如 果直接接上去,因实际功率比额定 功率大得多,灯泡会烧坏,所以不 能直接接入.若要接,应和灯泡串联 一个电阻R再接入,让电阻R分担3V 的电压.

教学过程:

例2解答:
解:不能直接接入 .应和一个阻值是 R 的电阻串联后再接入. (画图分析) P额 3W I ? I额 ? ? ? 0.5( A) U 额 6V

U ? U 额 9V ? 6V ?R ? ? ?( 6 ?) I 0.5 A
讨论此题还有哪些方法求R.

小结2

教学过程:

小结2 当加在用电器两端的实际电压比 额定电压大许多时,用电器可能会烧 坏,应和它串联一个电阻分去多余的 电压再接入. ? 返回
?

面向全体双基巩固

练习1:
? 将一只标有“12V 6W”字样的灯 泡接在6V的电源上,求它的实际 功率. ? (你有几种计算方法)
思维 拓展

返回例5

生活物理社会

练习2:
2.一台额定电压为220V的电烤箱,由 4条合金丝并联组成,当电烤箱正常工 作时,它的电功率是多少?这台电烤箱 正常工作0.5h,消耗的电能是多少度? 答案:4400W,2.2KW· h
返回

思维 拓展

探究学习:

练习3

? ?全解?P135 ? 电阻为R1=10Ω ,开关S由断开到闭合, 电流表的示数从0.6A变为1A若电源电 压保持不变,求: 答案:(1)15Ω;(2)6W ? (1)电阻R2的阻值; ? 开关S闭合时,电路消耗的总功率.
R1 R2
A

S

思维 拓展

灯L1(PZ220-25)和灯L2(PZ220-60)

? 例1计算结果:
U 2202 R1 ? ? (?) P 25 1额
2 1额

U 220 2 R2 ? ? (? ) P2 额 60

2 2额

? R1 ? R2

教学过程:

例2另解(分析):
先与方法1相同计算出总电流, 然后计算出总电阻,再计算小灯 泡的电阻(R灯=U额2/P额),最后 用串联电阻规律求出串联电阻的 阻值.

思维拓展:

? 将一只标有“12V 6W”字样的灯泡 接在6V的电源上,求它的实际功率. (你有几种计算方法)
解法1:P额 ? 6W,U 额 ? 12V,U 实 ? 6V U2 由公式P ? 得 R 2 2 U额 ( 12V) R? ? ? 24(?) P额 6W ( 12V) ? P实 ? ? ? 1.( 5 W) R 24? U实
2 2

返回

思维拓展:

? 将一只标有“12V 6W”字样的灯泡接 在6V的电源上,求它的实际功率. (你有几种计算方法)
解法2:P额 ? 6W,U 额 ? 12V,U 实 ? 6V U2 由公式P ? 得 R 2 2 U额 ( 12V) R? ? ? 24(?) P额 6W 6V 根据欧姆定律? I 实 ? ? ? 0.25 (A) R 24? ? P ? U 实 I 实 ? 6V ? 0.25A ? 1.( 5 W)或 P?I
2 实

U实

R? (0.25A)? 24? ? 1.( 5 W)
2

返回

思维拓展:

? 将一只标有“12V 6W”字样的灯泡接 在6V的电源上,求它的实际功率. (你有几种计算方法)
解法3:P额 ? 6W,U 额 ? 12V,U 实 ? 6V 因为电阻不变,由公式 R?
2 2

U实

P实

2 U额 ? 得 P额

2 U 实 ? P额 (6V) ? 6W P实 ? ? ? 1.5(W ) 2 2 U额 ( 12V)

返回

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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