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2011-2012学年第二学期八年级欧姆定律单元测试(精品)

2011-2012学年第二学期八年级欧姆定律单元测试(精品)
2011-2012学年第二学期八年级欧姆定律单元测试(精品)

第七章欧姆定律

第四节欧姆定律和安全用电

学习目标

1.会用欧姆定律的知识理解安全用电的道理。

2.通过本节课的学习,了解安全用电的常识。

3.知道安全电压,理解安全电压为什么安全。

4.通过联系实际生活中安全用电的教育,进一步培养学生学习物理的兴趣,培养学生热爱科学的精神。

学习重点

1.应用已学到的欧姆定律的知识,分析实际生活中的问题。

2.了解安全用电常识。

学习方法

观察法、比较法、讨论法

教具准备

安全用电挂图、投影仪、录像资料

学习过程

一、复习:

1、干电池的电压是 V,家庭照明电路的电压是 V

2、欧姆定律的内容和表达:

二、引入新课

教师利用投影仪投影各种警示标志,请学生辨认,并能说出这些标志都在什么地方或说明哪里有这些标志,你在什么地方看到过这些标志。不需要同学们对所有的标志都认识,或说清楚,重点放在对“高压危险”的标志认识上。

1、投影“高压危险”“有电危险”的警示标志。同学们认识这是什么标志吗?

2、你知道这些标志的意思吗?

3、同学们都在生活中或电视里的哪些地方见过这种标志呢?

4、投影:高压线、变压器、交通路口靠近高压线的地方、配电房等地方的警示标志,明确凡是容易引起人们触电事故的地方,都应有这种明显的警示标志。

三、进行新课

1、为什么电压越高越危险(让学生自学课本P

31

(1)触电是什么回事?

(2)电流对人体的危害与哪些因素有关?

(3)从下表中你能了解到什么?

8 ~10 有麻痹感,但可以摆脱热感觉增强

20 ~25

手迅速麻痹,不能摆脱,

呼吸困难

热感觉大大增强,手部肌肉不

强烈收缩

50 ~80 呼吸麻痹,心房开始震颤

强烈的热感觉,手部肌肉收缩、痉挛,呼吸困难

呼吸麻痹,延续3s就会

(4) 为什么电压越高越危险?

由欧姆定律:I=U/R知,当R一定时:U增大,I随着增大

人体是导体,电压越高,通过人体的电流就越大。经验证明,只有不高于36V的电压才是安全的。

(5)想一想:为什么不能用湿手触摸电器?电吹风为什么不可在浴室中使用?

请记:安全用电的原则:不能接触低电压不要靠近高电压

2、电路的三种状态:

(1)通路

如图所示,处处接通的电路叫通路

(2)断路

如图所示,某处断开的电路叫断路

(3)短路

①如图所示,电路中电源两极或用电器两端被导线直接连在一起的现象叫短

②根据欧姆定律可知,由于导线的电阻很小,短路时电路中的电流会很大,

这样大的电流,电源或其它用电器都不能承受,会造成电源损坏,更为严重的是,因为电流太大,会使导线发热升温,严重时有可能引起火灾。因

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