问题 多选题

在如图(a)所示的电路中,R1为定值电阻,R2为滑动变阻器.闭合电键S,将滑动变阻器的滑动触头P从最右端滑到最左端,两个电压表的示数随电路中电流变化的完整过程图线如图(b)所示.则(  )

A.图线甲是电压表V2示数随电流变化的图线

B.电源内电阻的阻值为10Ω

C.电源的最大输出功率为1.8W

D.滑动变阻器R2的最大功率为0.9W

答案

A、当滑片左移时,滑动变阻器接入电阻减小,则电路中总电阻减小,由闭合电路欧姆定律可知,电路中电流增大;而R1两端的电压增大,故乙表示是V1示数的变化;甲表示V2示数的变化;故A正确;

B、由图可知,当只有R1接入电路时,电路中电流为0.6A,电压为3V,则由E=U+Ir可得:E=3+0.6r;

当滑动变阻器全部接入时,两电压表示数之比为

1
4
,故
R1
R2
=
1
4
;由闭合电路欧姆定律可得E=5+0.2r

解得r=5Ω,E=6V,故B错误;

C、因当内阻等于外阻时,电源的输出功率最大,故当外阻等于5Ω时,电源的输出功率最大,故此时电流I=

E
2×r
=0.6A,故电源的最大输出功率P=UI=1.8W;故C正确;

D、由C的分析可知,R1的阻值为5Ω,R2电阻为20Ω;当R1等效为内阻,则当滑动变阻器的阻值等于R+r时,滑动变阻器消耗的功率最大,故当滑动变阻器阻值为10Ω时,滑动变阻器消耗的功率最大,由闭合电路欧姆定律可得,电路中的电流I′=

6
20
A=0.3A,则滑动变阻器消耗的总功率P′=I'2R′=0.9W;故D正确;

故选ACD.

阅读理解

A long walk in the desert or the mountains can be exciting. And it’s a great way to keep fit. If you carry a tent and some food, you can camp overnight. But wild places can be dangerous. Here’s how you can enjoy walking and camping safely.

Before your trip

☆ Tell someone where you are going and when you expect to return. If you don’t return on time, the people at home will know that you are lost or in trouble. They will know where to look for you.

☆ Make sure you’ve got a good map of the area that you’re going to.

☆ Make sure you’ve packed the right equipment for your activities. The desert is often hot, but nights can be very cold, and sometimes there are big rainstorms. In the mountains, the weather can change suddenly. A sunny day can turn cold and cloudy quickly.

The tools for survival(生存)

Sunglasses, sun hat, sun cream, windproof jacket, good walking shoes or boots, food, water, maps, compass, first-aid kit, matches, rope, knife, ground sheet,

the pack to carry your equipment.

What to do if you get lost

☆ If you’re in a car and it has broken down, stay with your car.

☆ If you are not traveling by car, make yourself easy to see. A signal fire can be seen at night. During the day, smoke may be used to show where you are. A brightly-colored blanket(毯子) may help rescuers to find you.

☆ Stay where you are.

☆ If you walk for any reason, avoid walking during the hottest part of the day. Morning and evening walking is better for saving your energy water.

☆ Keep yourself away from the rain, wind and sun, but remember: try to make yourself be in sight.

小题1:According to the passage, we should make a fire to _________. 

A.show where we are

B.keep warm at night

C.cook our meal

D.drive animals away小题2:If we get injured, we can choose ________from the following tools.

A.a ground sheet

B.a first aid kit

C.matches

D.sun cream小题3:What can we know from the passage?

A.A map and a compass will show the right direction to us.

B.It is a secret for us to tell where to go or when to return.

C.We should leave our car when it breaks down in the desert.

D.We have to keep walking while the sun is shining brightly.

单项选择题