问题 实验题

图是小明自制电子秤的原理示意图,托盘、轻弹簧上端和金属滑片P固定在一起(弹簧的电阻不计).

(1)电路设计:要使电子秤能正常工作.下列电路连接正确的是        

A.点“1”与点“2”连接

B.点“1”与点“3”连接

C.点“2”与点“3”连接

(2)故障排除:小明在正确完成电路连接后,闭合开关S.改变托盘中物体的质量,发现电压表有示数,但始终不变,发生故障的原因可能是         

A.R0断路

B.R1短路

C.电压表短路

D.金属滑片P断路

(3)刻度调试:

①调零:在托盘中无物体时,使金属滑片P处于R1最上端.此时电压表的示数为零.

②标度:每次向托盘中增加100 g的砝码,在电压表指针所指位置上标出相应的质量值,当将1 kg的物体放在托盘中.金属滑片P刚好指在距R1上端1/3处(如图所示),请计算回答“1 kg”应标在电压表多少伏的位置上?(电阻R0 =5Ω,电阻R1=15 Ω,电源电压U=3.0 V,电压表的量程为0-3.0 V)

(4)继续改进:小明在标示电子秤刻度时,发现电子秤的刻度与电压表的刻度有不同特点,请说出不同之处:______________________________________________________________于是,他又开始了新的研究.

答案

(1)C  

(2)D  

(3)已知接人电路的电阻为  R1/3 =5 Ω,

电源电压U=3V,电阻R0 =5 Ω,

根据Ι= U/R得,电路中的电流Ι= U/( R0+ R1/3)=3 V/(5Ω+5Ω)=0.3 A,

电压表示数为U=ΙR =ΙR1/3 =0.3A×5Ω=1.5V.

(4)电压表的刻度均匀,电子秤的刻度不均匀

阅读理解

根据短文判断下列句子的正确(T)与错误(F)。

     Being safe at school and in your everyday life needs knowledge. If you remember the following information, your life will be much safer.

     ●Always notice the environment around you. You shouldn't walk alone outside. Make sure where the public

phones are. If anything dangerous happens, you can find them quickly.

     ●Don't use earphones (耳机) when running in the schoolyard or on the street. You will keep yourself from

the outside world and can be easily surprised.

     ●School bags should be carried towards the front of your body instead of putting them on your back. When buses are crowded, it is easy enough for thieves to steal the things in your bags on your back.

     ●If you are followed by a stranger, cross the street and go in the other direction (方向), let the stranger

know that you know he or she is there. Next, go and get help from others if it is necessary. Don't go home

directly. You are safer on the street than you are in your home or in a lift.

     ●If you have to take a bus to a place far away, try to get to the stop a few minutes before the buses leave.

This prevents a stranger from studying you. On buses, don't sit alone, sit behind the driver or with you friends. Don't sleep.

     ●Don't play with matches (火柴) or candles, because it may cause fire. If your house catches fire and you have to escape through smoke, move on your hands and knees with your head 30 to 60 centimetres (厘米)

above the floor. Please remember there is cleaner, cooler air near the floor during the fire.

( )1. If you find a stranger following you, go home right away.

( )2. You'd better sit after the driver when you are alone to take a bus.

( )3. It's good for you to use earphones when you are running outside. 

( )4. You shouldn't sleep all the way although it's a long bus trip.

( )5. During the fire, the cleaner and cooler air is far from the floor.

( )6. When you escape from the house on fire, you should move on your

          hands and knees with your head 300 to 600 centimetres above the floor.

单项选择题