问题 填空题

有一根导线,如果在它两端加上12V电压,通过它的电流是0.4A,那么它的电阻是______Ω;如果在它两端加15V电压,通过它的电流是______A,它的电阻是______Ω;如果这根导线两端电压为零,则通过它的电流为______A,它的电阻为______Ω.

答案

∵U=12V,I=0.4A,

∴导线的电阻为:R=

U
I
=
12V
0.4A
=30Ω;

因为电阻与它两端的电压无关,所以导体的电阻为30欧,

而U′=15V,

∴此时电路中的电流为:I′=

U
R
=
15V
30Ω
=0.5A;

当导体两端的电压为0V时,由公式I=

U
R
可知,通过导体的电流也为0A,电阻不变,仍为30Ω.

故答案为:30,0.5,30,0,30.

阅读理解

Some people say, “Extreme(极限的) sports are so dangerous. Why would anyone want to do them?” But the danger is what makes them so excited. Keep reading to find out the most popular ones.

Bungee jumping: People on some South Pacific islands have been bungee jumping for hundreds of years. In 1979, some people bungee jumped off a bridge in England and made the sport popular around the world. Bungee jumping came to China in 1996.

Skateboarding: At first, this sport was called “sidewalk surfing”. Skateboarders often jump high into the air, go down steps or slide(滑行) down rails on their boards. They wear baggy trousers(宽松裤) so they can move more freely.

Skydiving: This is probably the scariest(最惊险的) extreme sport. Skydivers jump out of airplanes about four kilometers up in the sky. They usually free-fall for a while and then open a parachute to slow down for a safe landing.

Snowboarding: This became a winter Olympic sport in 1998. People tie their feet to the board with special shoes. They don’t use any ski sticks. Then, they ride down hills, jump high and do tricks.

小题1:What made bungee jumping popular all over the world?

A.A history of hundreds of years.

B.Jumping over a river in England.

C.Coming to China in 1996.

D.Jumping off a bridge in Britain.小题2:In which sport do people jump from a plane?

A.Bungee jumping.

B.Skydiving.

C.Skateboarding.

D.Snowboarding.小题3:Why do people wear “baggy trousers”?

A.They have many bags on them.

B.They can hold many things.

C.They make people move freely.

D.People wearing them can move fast.小题4:What’s the meaning of the underlined word “parachute”?

A.直升飞机

B.降落伞

C.滑板

D.飞艇小题5:Why do some people like to do the extreme sports?

A.It’s dangerous but exciting.

B.It’s exciting and safe.

C.It’s scary but safe.

D.It’s easy to learn.

单项选择题