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巧解初中物理作图题
发布时间:2021-06-15 12:58  来源:未知  点击量:

  作图题,是每年初中升高中考试中必考的题型,虽然作图题部分占的分数不算多(6分左右),但是,对于要升上重点高中的考生来说,作图题是要稳拿满分的。而初中物理课本中很少提到作图题的做法,对于初教物理的新教师来说,如何教会学生解答作图题,确是一个值得研究的问题。初中物理作图题主要有光学作图、热学作图、力学作图及电学作图几个部分,对于不同类型的作图题,解答方法及技巧也有不同。下面就一部分作图题的教学,谈谈我个人的见解。

  一、光学作图

  光学作图包括光的反射作图、平面镜作图、光的折射作图及透镜作图,光学作图常常要借助三角板、量角器或圆规进行作图。光学作图注意:(1)必须作出法线,法线用虚线表示;(2)必须为光线添上箭头表示光传播的方向;(3)光线和实物用实线表示,光线的反向延长线和虚像用虚线表示;(4)透镜作图要分清透镜的种类,正确判断光线是会聚还是发散。其中透镜的作图,初中阶段的要求是较简单的,题型主要分为:加透镜和透镜折射光路两种,往往是根据“三条特殊光线”和“透镜对光线的作用”及“凸透镜成像规律”三个规律进行,在教学中,要求学生熟练这三个规律即可顺利解答。

  光的反射作图依据的是光的反射定律,而光的反射定律只阐明了“三线两角”的关系,没有作光的反射的具体例子,例如:如何确定入射光线所对应的反射光线呢?在教学中,我把具体做法归纳为三步:(1)找到入射点O,过入射点O作法线,这时要充分利用三角板的直角及两条直角边,让三角板的其中一条直角边与界面重合且三角板的直角正对准入射点O,则用虚线连结三角板的另一条直角边即为法线ON;(2)根据“反射光线与入射光线分居法线两侧,反射角等于入射角”确定反射光线,这一步最好借助量角器来完成:让量角器的圆心与入射点重合,The normal line coincides with the 90 degree line of the protractor,Then the degree of incident angle can be read from the protractor,According to "the angle of reflection equals the angle of incidence",Reflected light can be made on the other side of the normal; (3) According to the meaning of the title, mark the arrow, the angle and the degree.such,The optical path diagram of the reflected light corresponding to the incident light is determined.According to the principle of light path reversibility,Determine the incident light corresponding to the reflected light,The steps and methods are the same as above.

  The drawing of light refraction follows the law of light refraction,And under what circumstances the angle of refraction is smaller than the angle of incidence,Under what circumstances the angle of refraction is greater than the angle of incidence is a problem that students are prone to confusion.In order to facilitate students' memory,Together with my students, I concluded from the law of light refraction: when light is refracted,Whether light enters other media from the air or enters the air from other media,The optical angle in the air is always larger than the optical angle in other media (that is, the angle in the air is larger),The "angle" here can refer to the angle of incidence,It can also be the angle of refraction.I also summarized the steps of light refraction mapping as follows: (1) Draw a normal line (that is, the perpendicular line of the interface) through the incident point,The normal should pass through two media; (2) Correctly determine the plane where the refracted light lies,It can be determined by extending the incident light (the refracted light is in the plane where the extension of the incident light is located); (3) Determine whether the direction of the refracted light is away from the normal or close to the normal according to the "large angle in the air",Make refracted rays,And mark the refraction angle.

  Common test question types for plane mirror mapping are: plane mirror imaging mapping and adding mirror to change the optical path for mapping.

  Among them, the imaging and mapping of the plane mirror must be completed according to the law of imaging of the plane mirror.At this time, it should be noted that the object and the image are in axial symmetry with respect to the plane mirror.Therefore, when determining the corresponding key point position of the image,The key points of the real object should be determined by an auxiliary line perpendicular to the mirror surface (the auxiliary line is indicated by a dashed line); the solid line is used to indicate the object,The virtual image is represented by a dashed line.Plane mirror imaging and mapping can be summarized into the following three steps: (1) Make the vertical lines to the mirror surface through the key points of the object; (2) According to "the distance between the image and the object to the mirror surface is equal",Find the image point corresponding to the key point of the real object on the vertical line passing through the mirror; (3) Connect each image point with a dotted line,Draw the image of the object.The same reason,Knowing the position of the image and the plane mirror, the method of making the corresponding object is similar to this,It's just that the object is represented by a solid line.And add a mirror to make a picture,That is, the reflected light and the incident light are known,Make a map of the position of the plane mirror,The key is to use a ruler to make the bisector of the angle between the reflected light and the incident light.Which is the normal,再过入射点作出法线的垂线即为平面镜的位置。

  二、力学作图

  力学作图包括作力的示意图、杠杆作图及滑轮组的绕线。其中力的示意图的做法较简单,具体做法可归纳为三步:(1)在受力物体上找出力的作用点;(2)从力的作用点起,沿力的方向画一条线段,并在线段的末端加上箭头表示力的方向;(3)标出力的符号及大小。对于重力则强调重力的作用点必须在物体的重心,且重力的方向总是竖直向下的(即垂直于水平面向下)。

  杠杆作图,主要是作杠杆的力臂,而对于力臂的概念,学生往往会误解为“从支点到力的作用点的距离”,把从支点到力的作用点的线段连结起来就当作力臂,故作杠杆的力臂是学生常出错的题目。为了让学生能正确、方便地作出杠杆的力臂,本人总结出如下的方法:(1)充分利用直角三角板的两条直角边,让三角板的其中一条直角边与力的作用线重合;(2)向支点平移三角板(三角板的直角边始终与力的作用线重合,力的作用线可延长);(3)当三角板的另一条直角边到达支点时,沿该直角边连结从支点到力的作用线的线段,就是这个力的力臂了。学生用这个方法作力臂,不会把支点到力的作用点的距离误认作力臂,且简单方便,效果很好。

  对于滑轮组的绕线,很多学生难以判断是从定滑轮开始绕线,还是从动滑轮开始绕线,为了便于学生掌握,本人引导学生从多种滑轮组绕线中总结出规律:当承担动滑轮与重物的绳子的段数n为偶数时,从定滑轮绕起;当承担动滑轮与重物的绳子的段数n为奇数时,从动滑轮绕起。这样,对于滑轮组的绕线这类题目,学生做起来就得心应手了。

  三、电学作图

  电学作图中较常见且学生颇感困难的是根据实物连接画出对应的电路图和根据电路图连接实物。而对于串联电路,由于电流只有一条路径,只要把各元件顺次连接起来或画在电路图上就可以了,一般学生是不会感到困难的。但对于并联电路,由于电流有多条路径,难度加大了,学生都感到比较棘手。为了便于学生掌握,在讲并联电路的按电路图连接实物时,我要求学生先只看电路图的干路及其中的一条支路,其它的支路暂时放下,这样电路图就变成了只有一条路径的了,并且按这个只有一条路径的电路图连接好实物,再在电路图和已连接好的实物图上顺着电流的方向分别找出电流的分支点和汇合点,把余下的支路并接上,这样整个并联电路就连接好了。本人用这个方法教学生,难度降低了,学生连接电路的兴趣调动起来了,收效甚好。

  对于并联电路中根据实物连接画出对应的电路图的题目,我是这样要求学生的:(1)先在实物图上顺着电流的方向用不同颜色的笔画出电路的干路和不同的支路,(2)再弄清干路和各支路上各有什么元件,这样整个电路的框架就出来了,(3)把电路图设计成扁的长方形,若实物图中电路有两条支路,则在长方形上加一条长边,若有三条支路,则在长方形上加两条长边,依此类推,(4)把其中一条长边作为干路,其余的长边作为支路,按照实物图中元件的顺序,顺着电流的方向分别把干路及各支路上的元件画上。这样条理分明,思路清晰,学生做起来就不会手忙脚乱了,效果很好。

  以上这些是本人在教学过程中总结出来的关于光学作图、力学作图、电学作图方面的一些见解。总之,在解答初中物理作图题时,不仅要注意学科之间的联系,Transfer mathematics knowledge flexibly and effectively to physical mapping,It is also necessary to cleverly use drawing tools such as triangles, protractors, and compasses to simplify more complex drawings.Good at straightening out the ideas of problem solving,Summarize the law,You will be able to get twice the result with half the effort,The effect of analogy.

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