问题 计算题

图是由“控制电路”和“工作电路”两部分组成的光控电路。“控制电路”由光敏电阻R、磁控开关L、定值电阻R0、电源U1等组成,当线圈中的电流大于或等于20mA时,磁控开关的磁性弹片相护吸合。“工作电路”由工作电源=20V)、发热电阻=5)和(=15)组成。问:

(1)图是光敏电阻R的阻值随照射在光敏电阻上的光强E(表示光照射的强弱的物理量,单位cd)之间的变化关系图。如果照射在光敏电阻R上的光强____,则光敏电阻R的阻值增大,磁控开关产生的磁性将____(均选填“变强”、“变弱”、“不变”)。

(2)当“控制电路”中的电流小于20mA时,求“工作电路”中的R1两端的电压是和消耗的功率各是多少?

(3)当“控制电路”中的电流等于30mA时,求“工作电路”中R1两端的电压?

(4)如果“控制电路”中的线圈阻值=1=140=3V,请通过计算分析,照射在光敏电阻上的光强在什么范围内时,磁控开关的磁性弹片相护吸合?

答案

(1)变弱,变弱(2分)(2)5,5(2分)(3)20(1分)(4)由题意可知,要使遥控开关的磁性弹片相护吸合,则控制电路的电流要大于或等于20mA。(1分)由欧姆定律,R总=U/I=3V/0.02A=150可知控制电路的总电阻要小于或等于150。(1分)此时光敏电阻R=R总-R0-RL=150-140-1=9(1分)右图可知,对应的光照强度为2cd。因此,当光照强度大于或等于2cd时,磁控开关的磁性然片相护吸合。(1分)

(1)由图可知:光照强越强时,光敏电阻R的阻值变弱;

故照射在光敏电阻R上的光强变弱时,则光敏电阻R的阻值增大,电路中的总电阻变大;

根据欧姆定律可知,电路中的电流变小;

则磁控开关产生磁性将变弱.

故答案为:变弱;变弱.    

(2)当“控制电路”中的电流小于20mA时,控制电路L断开,则工作电路由R1和R2串联组成;

电路中的电流:

I= U/R1+R2 =20V/20Ω=1A;

R1两端的电压:

U1=IR1=1A×5Ω=5V;

R1消耗的电功率为:

P1=I2R1=(1A)2×5Ω=5W.

答:R1两端的电压是5V,消耗的功率为5W.

(3)当“控制电路”中的电流等于30mA时,控制电路L闭合,则工作电路R2短路,工作电路只有R1

故R1两端电压等于电压电压,即U1=U=20V.

答:R1两端的电压为20V.

(4)由题意可知,要使遥控开关的磁性弹片相互吸合,则控制电路的电流要大于或等于20mA.

由欧姆定律,R=U/I=3V/0.02A=150Ω,

可知控制电路的总电阻要小于或等于150Ω.

此时光敏电阻R=R-R0-RL=150Ω-140Ω-1Ω=9Ω,

由图可知,对应的光照强度为2cd.

因此,当光照强度大于或等于2cd时,磁控开关的磁性然片相互吸合.

阅读理解

Last summer vacation I went to Taiwan with my parents.Taiwan is really beautiful! The weather is always warm and humid because the sea is around Taiwan.In the north,it’s cold in winter,but in the south,it is still warm in winter and you can enjoy sunshine every day.The typhoon(台风)season is from June to October.

There are many mountains in Taiwan.(A)A-li Mountain is one of the biggest mountain there.We went to climb A-1i Mountain one afternoon.There are many trees in the forest there. They are very tall and many of them are really old.

Sun Moon Lake is the largest natural lake in Taiwan.In the lake there are many kinds of fishes.

 (B)We also went to the beach and enjoyed the sunshineThere are many kinds of food in the small towns near the sea.Dumplings with shrimp(虾) in them are my favorite.

Taiwan, a beautiful island,I love it! I took many photos there .Would you love to see them?

根据短文内容,完成下列句子。

小题1:The writer spent the summer vacation with            .

根据短文内容,选择正确答案。

小题2:The trees in the forest in A-li Mountain are       .

A.tall and old

B.short

C.new

D.short and old小题3:将画线(B)处的句子翻译成汉语。

                                                             

小题4:画线(A)处句子有一处错误,请指出并改正。

                                                              

小题5:回答下列问题。

(1)What’s the weather like in Taiwan?

                                                                

(2)Where did the writer spend his/her vacation?

                                                                

单项选择题