问题 问答题

如图所示是测磁感应强度的一种装置.把一个很小的测量线圈放在待测处,测量线圈平面与该处磁场方向垂直,将线圈跟冲击电流计G串联(冲击电流计是一种测量电量的仪器).当用反向开关K使螺线管里的电流反向时,测量线圈中就产生感应电动势,从而有电流流过G.该测量线圈的匝数为N,线圈面积为S,测量线圈电阻为R,其余电阻不计.

(1)若已知开关K反向后,冲击电流计G测得的电量大小为q,则此时穿过每匝测量线圈的磁通量的变化量为△φ=______(用已知量的符号表示).

(2)待测处的磁感应强度的大小为B=______.

答案

当用反向开关S使螺线管里的电流反向流动时,穿过小线圈的磁通量的变化量是:

△Φ=2BS

产生的感应电动势:E=n

△?
△t

感应电流:I=

E
R
与I=
q
t

可得:q═

N?△?
R

解得:△?=

qR
N
 

而△Φ=2BS

所以:B=

qR
2NS

故答案为:(1)

qR
N
,(2)
qR
2NS

选择题
阅读理解

阅读理解。

     Even if you are an experienced backpacker or climber, you need to know what to do if something goes

wrong. It is not that Alaska is that much more dangerous than other places. It is no more hazardous than

elsewhere, as long as the proper precautions (预防措施) are taken. So, being prepared is important to a safe

trip.

      First of all, make sure you pack everything you will need for your trip. Even if you are traveling by car,

pack clothers and gear (用具) for backpacking. Because of the extreme weather, cars break down frequently.

Pack a coat, gloves, a hat, snow-pants, sleeping bags, candles, a flashlight, and high-energy food.

     If you are traveling by car, also pack extra belts, a spade, and a chain (铁链). If you are camping or

hiking, make sure you know how to avoid bears. If you are lost in the forest, there is no doubt that you will

encounter (遇到) them.

     However, there is no need to be nervous. Just take these precautions: make noise as you walk, bears will

avoid you; cook your food away from where you sleep, and store your food away from where you sleep, so

the bears are not attracted. Both of these should keep the bears away. However, if you do encounter a bear,

there are two different schools of thought. One is to make noise and scare the bear away. The other is to

stand still and speak softly to get the bear to leave. If you are dealing with a brown bear, you can climb a tree,

since they cannot follow. But black bears can climb. If for some reason the bear attacks, play dead. Try to

protect your head and neck, and hopefully the bear will leave its “dead” prey (捕获物).

1. The underlined word “ hazardous ”(in Paragraph 1) most probably means “______ ”.

A. interesting

B. dangerous

C. wonderful

D. important

2. The author mainly tells us in Paragraph 2 that ______ .

A. it is dangerous to travel by car

B. the weather is always bad in Alaska

C. you should take what you need for your trip

D. cars break down often while traveling

3. All the following can protect you from being attacked by bears EXCEPT that ______ .

A. trying to make noise as you walk

B. saying something to the bear in a soft voice

C. using a club to scare the bear away

D. pretending to be dead when you are in danger

4. The passage is mainly about ______.

A. the tips on a safe trip in Alaska

B. the very dangerous bears in Alaska

C. how to prevent your cars from being attacked by a bear

D. how to protect yourself when in trouble