问题 填空题

用电压U和kU通过同距离相同材料的输电线向用户输送同样的总功率,若要求输电线损失的功率相同,则前后两种情况输电线的横截面积之比为______,若采用同样粗细的导线输电,则前后两种情况输电线损失的功率之比为______.

答案

根据P=UI得,输送电流之比等于输电电压之比,则

I1
I2
=
U2
U1
=K.

①若输电线损失的功率相同,根据△P=I2R得,输电线上的电阻R=

P
I2
,则电阻之比
R1
R2
=
I22
I21
=
1
k2
,由R=ρ
L
S
知,导线长度和材料也相同,
S1
S2
=
R2
R1
=K2

②若采用同样粗细的导线输电,则两次电阻相同,根据△P=I2R得,前后两种情况输电线损失的功率之比为:

P1
P2
=(
I1
I2
)2=k2

故答案为:k2,k2

阅读理解

If you go into the forest with friends, stay with them .If you don’t, you may get lost. If you do get lost, this is what you should do. Sit down and stay where you are. Don’t try to find your friends. Let them find you. You can help them to find you by stay in one place.

There is another way to help your friends or other people to find you. Give them a signal(信号)outing or whistling (吹口哨) three times. Stop. Then shout or whistle three times again. Any signal given three times is a call for help.

Keep on shouting or whistling, always three times together. When people hear you, they will give two shouts or two whistles. When a signal is given twice, it is an answer to a call for help.

If you don’t think that you will get help before night comes, try to make a small room with branches.(树枝)

What should you do if you get hungry or need drinking water? You would have to leave your little branch room to look for something to eat and drink. Don’t just walk far away. Pick up small branches and drop them as you walk so that you can find your way back.

The most important thing you need to do when you are lost ---stay in one place.

小题1:When you hear two shouts or whistles, what does that mean?

A.Some animals are coming.

B.Someone is calling for help.

C.People will come to help you

D.Someone is lost.小题2:What shouldn’t you do, if you get lost in the forest?

A.Walk everywhere to find your friends.

B.Sit down and stay where you are.

C.Make a small room with branches.

D.Try to find something to eat and drink小题3:Which of the following is wrong according to the passage?

A.You can leave your branch room to find some food.

B.You can’t go anywhere even when you feel thirsty or hungry.

C.You should leave some branches as you walk.

D.Don’t go far away from your room.

判断题